Term Paper on Lysosomes | Cell Organelle | Cell Biology

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Term Paper on Lysosomes

Term Paper Contents:

1. Term Paper on the Introduction to Lysosomes
2. Term Paper on the Cytochemical Definition of Lysosomes
3. Term Paper on the Origin of Lysosomes
4. Term Paper on the Occurrence of Lysosomes
5. Term Paper on the Morphology of Lysosomes
6. Term Paper on the Extraction of Lysosomes
7. Term Paper on the Chemistry of Lysosomes
8. Term Paper on the Formation of Lysosomes
9. Term Paper on the Functions of Lysosomes
10. Term Paper on the Peroxisomes, Glyoxysomes and Other Microbodies

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Term Paper # 1. Introduction to Lysosomes:

The concept of the lysosome originated from the development of cell fractionation techniques by which different subcellular components are isolated. By 1949, a class of particles having centrifugal properties somewhat intermediate between those of mitochondria and microsomes was isolated by de Duve and found to have a high content of acid phosphatase and other hydrolytic enzymes.

Because of their enzymatic properties they were named lysosomes (Gr., Lysis = disolution, soma = body). According to Golian (1972) membrane bounded storage granules containing digestive enzymes are considered lysosomes of plant cells. Hence spherosomes, aleuronic granules and vacuoles of plant cells are supposed to have lysosome like functions.

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Term Paper # 2. Cytochemical Definition of Lysosomes:

The cytochemical definition of lysosomes based on the presence of a single unit membrane and a positive staining reaction for acid phosphatase and some related enzymes, may be considered for most practical purposes equivalent to the biochemical definition.

As our knowledge of the significance of lysosomes in cell physiology has progressed, it has become evident that the term lysosome covers a variety of different forms which can be distinguished on the basis of morphological and functional criteria.

The following are the terms commonly used in the literature:

(a) Autophagic Vacuoles- It is a membrane lined vacuole containing morphologically recognisable cytoplasmic components.

(b) Cytolysome- Same as Autophagic vacuoles.

(c) Cytosome- Particles referred to cytosomes are usually lysosomes. Some workers include the un-related micro-bodies under this term.

(d) Cytosegresome- Same as Autophagic vacuoles.

(e) Micro body- A particle found in liver and kidney, bounded by a single unit membrane and containing a finely granulated material. According to de Duve, they are definitely not lysosomes.

(f) Multivesicular Bodies- Structures lined by a single membrane and containing inner vesicles resembling Golgi complex and are considered to be lysosomes.

(g) Resfdual bodies- Membrane lined inclusions characterised by undigested residues comprises telo-lysosomes and hypothetical post-lysosomes.

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Term Paper # 3. Origin of Lysosomes:

They have multiple origins depending upon the tissue in which they are located or on their function in a specific cell.

Extracellular Origin:

Lysosomes may be the vacuoles absorbed into cell by the process, pinocytosis. The pinocytic vacuole may later become cytoplasmic particle and thereon enzymatic activity becomes developed.

Origin from Golgi Complex:

There are evidences that lysosomes originate from the Golgi complex and represent zymogen granules. Their similar function and structure with Golgi complex support this view. Recent studies have shown that accumulation of secretory products within Golgi vacuoles leads to the formation of lysosomes, and membranes surrounding the products are derived from Golgi membrane.

Origin from ER:

Novikoff reported that the lysosomes originate directly from granular endoplasmic reticulum, by a process of blabbing.

Extracellular Digestion by Hydrolytic Enzymes:

In addition to digesting materials contained within the cell, hydrolytic enzymes may in some instances be secreted into the extracellular space. During fertilisation, for example, penetration of the sperm head through the outer layers surrounding the egg surface is aided by the release of hydrolytic enzymes derived from the Golgi complex of the sperm cell. Situations also occur, however, where release of hydrolytic enzymes from the cell has detrimental effects.

A case in point is the lysosomal enzyme release that often follows cell damage caused by physical trauma or microbial infection. In such instances the released enzymes may trigger additional tissue damage and inflammation. The idea that release of lysosomal enzymes can contribute to tissue inflammation is supported by the discovery that arthritis like disease can be produced by injecting drugs known to disrupt lysosomes.

This finding explains why drugs that inhibit lysosomal enzyme -lease, such as the steroid hormones cortisone and hydrocortisone, are effective anti-inflammatory agents. Vitamin A, on the other hand, increases the ability of the lysosomal membrane and promotes discharge of hydrolases from the cell. This explains why connective tissue damage and spontaneous one fracture often occur in individuals consuming excess quantities of vitamin A.

Lysosomal Storage Diseases:

Several dozen genetic diseases afflicting young children are known to be caused by an excessive intracellular accumulation of polysaccharides or lipids. The quantity of polysaccharide or lipid stored is often massive enough to interfere with and even destroy the cells involved. Depending on the particular cell types affected symptoms such as muscle weakness, skeletal deformities, and mental retardation may result.

The first of these so-called lysosomal storage diseases to have is underlying mechanism unraveled was type II glycogenesis, an illness whose victims die at an early age with abnormally large amounts of glycogen in the liver, heart, and muscles. In 1963 H. Hers discovered that type II glycogenesis is caused by a severe deficiency of the lysosomal enzyme P-glucosidase, which catalyses hydrolysis of glycogen to oligosaccharides and glucose. In the absence of this enzyme, undigested glycogen accumulates within lysosomes.

Not only did this discovery provide an explanation for glycogen storage disease, but it also led Hers to postulate the existence of a wide spectrum of other diseases corresponding to genetic defects in particular lysosomal enzymes.

He predicted that in each case, the disease symptoms are caused by an abnormal accumulation of the undigested substrates of the defective enzyme. This unifying theory has led to an understanding of the causes of what was once a bewildering array of mysterious diseases.

Although in some instances excessive storage of simple polysacch­arides or muco-polysaccharides is involved most of these storage diseases result from an abnormal accumulation of glycolipids.

Because glycolipids are highly concentrated in brain tissue and are important constituents of the myelin sheath surrounding the axon, the symptoms of these storage diseases usually include severe mental retardation. The glycolipid accumulated in brain cell lysosomes can often be recognised morpholo­gically by its tendency to form unusual layered structures known as zebra bodies.

The discovery of the molecular basis of the lysosomal storage dise­ases has led to the suggestion that these illnesses might be alleviated by replacing the defective enzymes. Direct administration of missing lysosomal enzymes, however, is impractical for several reasons. To begin with, destruction of the administered enzymes by serum proteases or by the individual’s immune system would minimise the effectiveness of treatment. Furthermore, many cells do not take up foreign molecules efficiently.

An alternative approach is to encapsulate the required enzyme in artificial lipid vesicles (liposomes). This encapsulation process protects the enzymes from destruction and has the added advantage that liposomes are known to be actively taken up by cells and fused with lysosomes. In this way a missing lysosomal enzyme might be delivered directly to its appropriate site of action.

Encouraging results from animal studies suggest that this approach may ultimately be useful for the treatment of inherited enzyme deficiencies in humans. An alternative therapeutic approach is to attempt to correct or replace the gene coding for the defective Iysosomal enzyme.

Killing and Digestion:

Simple enzymatic digestion by hydrolases is not the only process that occurs in secondary lysosomes. Many microorganisms, while they are viable, resist attack by hydrolases. Hence, mechanisms have evolved specifically to kill ingested organisms prior to their breakdown.

Lysosomes typically contain lysozyme, especially the simple gram-positive bacteria. The lysosomes of neutrophils also contain lactoferrin, which binds iron and other metal ions tightly, thus inhibiting growth of the ingested organisms. The contents of the digestive vacuoles are usually very acidic (less than pH 4 in neutrophils) which by itself inhibits growth of many bacteria.

And finally, oxidases, thought to be present at the surface of the cell membrane and thus also on the inside surface of digestion vacuoles produce hydrogen peroxide from molecular oxygen. Hydrogen peroxide is microbicidal in its own right, but in neutrophils and most macrophages it is also utilised by an enzyme called myeloperoxidase to covalently fix Cl^– or I^– onto organic targets such as bacteria. This halogenation greatly facilitates killing of the organism.

Autophagy:

Although lysosomes are present in probably all nucleated cells of both animals and plants, we have described their activities only in the major phagocytic cells. Their function in other cells is much the same. That is, they fuse with endocytic (including pinocytic) vesicles to destroy the vesicle and its contents. Lysosomes are involved not only in this process of digesting extracellular materials, known as heterophagy, but also in the digestion of intracellular substances, an activity known as autophagy.

In plants, for example, the cytoplasm frequently has large vacuoles containing reserve food supplies in a stored form. These storage Dolvmers can be broken down to useful size by fusion of the vacuole with lysosomes.

Lysosomes are similarly responsible for turnover of other organelles in plants and animals, a process that is accelerated during involution states— in a muscle that is not being used, during starvation, after injury, or during the remodeling that occurs with cellular differentiation. Even in unstressed normal cells, the turnover of organelles by autophagy is relatively rapid. The half-life of liver cell mitochondria, for ample, is five to six days, for peroxisomes it is one to two days, and for ribosomes about five days.

The autophagy of cytoplasmic organelles starts with an isolation envelope, which is a sack of smooth-surfaced membrane that wraps around the material to be removed and separates it into a sort of phagocytic vacuole known as an isolation body. The source of the isolation membrane at least in damaged liver and kidney seems to be the endoplasmic reticulum.

However, recent work by Michael Locke and his associates at the University of Western Ontario indicates that the isolation membrane in insect cells during metamorphosis, and in some other animal cells, is derived instead from the Golgi apparatus. In any case, lysosomes can fuse with the isolation body to create an autophagic digestive vacuole and release breakdown products to the cytoplasm to be used as an emergency source of food or as raw material for cellular remodeling.

Residual Bodies:

Although killing and breakdown in the lysosomes is efficient, some materials are simply not digestible. These substances may be released from the cell by fusion of the digestive vacuole with the cell membrane (exocytosis). This process is very efficient in protozoa, but not so efficient in higher animals. Hence, inactive lysosomes containing debris may accumulate.

These debris-filled organelles, called residual bodies, are of no consequence to cells like macrophages or neutrophils, because the cells don’t live long enough for significant amounts to accumulate. In cells that do not divide, and are not replaced, substantial quantities of this material may accumulate.

In neurons and muscle cells, especially, there is a steady increase in the number of these bodies, which are called in these context lipofuscin granules. The age of an animal can often be estimated from their concentration, but whether enough ever accumulates in a normal lifespan to compromise the function of the cell is unclear.

Spherosomes and Related Vacuoles in Plants:

The isolation of lysosomes from plant tissues has been a difficult undertaking because methods developed for animal tissues cannot be directly applied to plant lysosome isolation.

Using modified procedures, however, it is now possible to recover acid hydrolase activity in particulate fractions of plant cells. The purified fractions containing these hydrolases are enriched in small, membrane- enclosed vesicles.

These structures resemble spherosomes, which are highly refractive spherical particles, approximately 0.5-1.0 μm in diameter, originally identified by light microscopists in the plant cell cytoplasm. Cytochemical staining for acid phosphatase in tissue sections has confirmed that spherosomes, as well as larger cytoplastic vacuoles, contain lysosomal enzymes.

A principal feature distinguishing animal lysosomes from plant spherosomes is that the latter stain intensely with fat soluble dyes, indicating high lipid content. The accumulation of lipid in vesicles containing hydrolytic enzymes suggests that the spherosome functions in storing and mobilising reserve lipid. In addition to its role in lipid metabolism, the spherosome and other hydrolase-containing plant vacuo­les are thought to be involved in digesting and recycling intracellular constituents in a manner analogous to the animal cell lysosome.

Digestion of foreign particulate matter is probably not a major function of the spherosome, however, because the presence of the plant cell wall generally prevents phagocytosis from bringing in foreign particulate matter.

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Term Paper # 4. Occurrence of Lysosomes:

With the exception of mammalian R.B.C., the lysosomes have been reported practically from all the animal cells. The presence of lysosomal particles have also been suspected (and in some cases established) in protista (protozoan, slime-moulds, fungi, algae and prokaryotic protista). In plant cells, considering the evidences as a whole, there now seems little doubt about their presence. Further they have strong affinities with the lysosomes of animals and protista.

In plants, further they should not be confused with spherosomes in function. According to Pitt, lysosomes and spherosomes are two different organelles and the latter are comparable to lipid droplets of animals. Yatsu and Jack have clearly shown that spherosomes are morphologically distinct organelles. Gahm reviewed the occurrence and histochemistry of plant lysosomes.

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Term Paper # 5. Morphology of Lysosomes:

Shape and Size:

The shape and size of lysosome is variable. Morphologically they can be compared with Amoeba and white blood cells (W.B.C.). Due to their changing habit they cannot be accurately identified as the basis of their shape. Normally lysosomes vary in size from 0.4 to 0.8μ but they may be as large as 5μ in mammalian kidney cells and are exceedingly large in phagocytosis.

Structure of Lysosomes:

Like other cytoplasmic complexes, lysosomes are like round tiny bags filled with dense material and digestive enzymes.

They consist of two parts:

(1) Limiting membrane and

(2) Inner dense mass.

(1) Limiting Membrane:

This membrane is single, unlike that of mitochondria and composed of lipoprotein. The chemical structure is homologous with the unit membrane of plasma lemma consisting of bimolecular layer as suggested by Robertson.

(2) Inner Dense Mass:

This enclosed mass may be solid or of very dense contents. Some lysosomes have a very dense outer zone and less dense inner zone. Some others have cavities of vacuoles within the granular material. Usually they are supposed to possess denser contents than mitochondria.

They show their polymorphic nature and their contents vary with the stage of digestion, as they help in intracellular digestion.

Permeability of Lysosomal Membrane:

The lysosomal membrane is impermeable to substrates of the enzymes contained in the lysosome. Certain substances, called labializes, causes instability of the lysosomal membrane, leading to release of enzymes from the lysosome. Other substances, called stabilisers, have a stabilising action on the membrane. A list of some labializes and stabilisers is given in Table.

A labiliser might increase the permeability of the lysosomal membrane to small solutes like sucrose. The osmotic swelling results might completely disrupt the membrane.

The limited permeability of lysosomal membrane explains why lysosomal hydrolases do not have a direct access to cellular components. This prevents uncontrolled digestion of the cell contents by the lysosomal enzymes.

Polymorphism:

Lysosomes are polymorphic in nature. The polymorphic nature is due to variation in contents of lysosomes with different stages of digestion.

Generally lysosomes can be traced in four forms given below:

(1) Primary Lysosomes:

These are also called the true, pure or original lysosomes, having a single unit membrane containing enzymes in the inactive forms.

(2) Secondary Lysosomes:

These are also called the phagosomes as they contain the engulfed material and enzymes. The fused mass is called the secondary lysosomes. The enzyme present in such lysosomes gradually digests the engulfed material.

(3) Residual or Post Lysosomes:

Lysosomal membrane characterised by the presence of undigested material like myelin figures is called residual body.

(4) Autophagic Vacuoles:

The autophagic vacuoles are also known as aulophogosomes or cytolysosomes. The autophagic vacuoles are formed when the cell feeds on its intracellular organelles such as the mitochondria and endoplasmic reticulum by the process of autophagy. In such cases, the primary lysosomes are concentrated around the intracellular organelles and digest them ultimately.

The autophagic vacuoles are formed in special pathological and physiological conditions. C. de Duve and Allison have observed that during starvation of the organisms many autophagic vacuoles developed in the liver cells which feed on the cellular components.

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Term Paper # 6. Extraction of Lysosomes:

The phenomenon of centrifugation has played a great role in the study of cytoplasmic inclusion. The technique about the separation of the lysosomes has been developed in laboratory of de Duve. For the extraction of lysosome, first of all the cells are homogenated in sucrose solution.

Rapid mechanical rotation of the pestle ruptures the cells, setting the intracellular particles free in the medium. Then successive centrifugation of the resulting homogenate produces fractions, which can be separated by micro-needle. The entire process of centrifugation for lysosome can be studied.

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Term Paper # 7. Chemistry of Lysosomes:

Lysosomes contain variety of enzymes, upto the present time about 40 enzymes have been isolated in variety of tissue type. Some common enzymes, are β -galactosidase, β -glucuronidase, β -N-acetyl- glucosaminidase, α -glucosidase, α -mannosidase; cathepsin A (Acid protease), cathepsin B (Acid protease), aryl sulphatase A, aryl sulphatase B, acid ribonuclease, acid deoxyribonuclease, acid phosphatase, acid lipase, phospholipase A, phosphotidic acid phosphatase byaluronidase, phosphoprotein phosphatase, amino peptidase A, dextranase, saccbarase, lysozyme (muramidase), Mg⁺⁺ activated ATPase, indoxy, lacetate, esterase and plasminogen activator.

All these enzymes of the lysosomoes are enclosed within the single lipoprotein membrane. Most of these enzymes function more efficiently under slightly acidic medium, pH optima around 5.0, as such they are collectively called as acid hydrolases.

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Term Paper # 8. Formation of Lysosomes:

Most of the hydrolytic enzymes present in lysosomes are glycopr­oteins synthesized on ribosomes bound to the endoplasmic reticulum. Like other proteins synthesized by this mechanism, insertion of the newly forming polypeptide chain through the endoplasmic reticulum is guided by a signal sequence that is subsequently removed, and oligosaccharide chains are transferred from the dolichol-phosphate carrier system to the polypeptide chain as it emerges into the lumen of the endoplasmic reticulum.

The major distinction between lysosomal enzymes and other glycoproteins manufactured in this way is that mannose residues situated within the core oligosaccharide chains lysosomal enzymes become phosphorylated shortly after the oligosaccharide chain has been introduced.

The resulting mannose-6 phosphate groups serve to distinguish presumptive lysosomal enzymes from other glycoproteins as the various proteins synthesized within the rough endoplasmic reticulum migrate to the Golgi complex.

Their recognition of lysosomal enzymes by virtue of their phosphorylated mannose groups is made possible by the fact that some Golgi membranes contain specific receptors that recognise phosphorylated mannose, causing the lysosomal enzymes to become selectively bound to those regions of the Golgi complex.

The unique presence of mannose-6-phosphate in lysosomal enzymes thus allows the Golgi complex to distinguish these molecules from other glycoproteins and package them into specialised vesicles destined to become lysosomes. After these vesicles have budded off from the Golgi complex, the internal pH of the vesicles is lowered by the action of an active transport system that pumps protons into the interior of the vesicle.

As the pH within the lysosome is lowered, the hydrolytic enzymes are released from their binding sites on the membrane because the interaction between the phosphorylated mannose groups and their membrane receptors is disrupted at low pH. After release into the lysosome interior, the free hydrolytic enzymes tend to lose their phosphorylated mannose groups. At this point the stage is set for the normal functioning of the lysosome.

The realisation that all lysosomel enzymes catalyse hydrolysis reactions has naturally fostered the theory that lysosomes serve a degradative or digestive role within the cell.

There are at least four distinct ways in which this digestive function is utilised by cell.

These include:

(1) Degradation of foreign matter taken up by endocytosis,

(2) Destruction of worn-out organelles (autophagy),

(3) Breakdown of cellular structures associated with cell death (autolysis), and

(4) Digestion of extracellular materials.

Endocytosis:

One of the most important functions of the lysosomes is its ability to catalyse the breakdown of complex macromolecules brought into the cell by the process of endocytosis. Endocytosis refers to the uptake of extracellular materials trapped in membrane vesicles that pinch off from the plasma membrane.

When the materials taken up by such vesicles consist of large particles (i.e., those visible by light microscopy), the term phagocytosis is applied. Pinocytosis refers to the uptake of all other matter, including small particles and water-soluble macro molecules such as antibodies, enzymes, hormones, and toxins.

Endocytosis usually involves an interaction between the substance being ingested and plasma membrane binding sites. Some of the earliest evidence for the importance of such binding was provided by Ralph Steinman and Zanvil Cohn, who investigated the uptake of soluble and insoluble forms of the protein horse-radish peroxidase. These studies were carried out using specialised scavenger cells, termed macrophages, which are highly active in carrying out phagocytosis.

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Term Paper # 9. Functions of Lysosomes:

i. Lysosomal Digestion of External Particles:

Large molecules are taken into the cell by the process called phagocytosis. This is a perfectly adequate and accurate term that implies a cell that eats. But recently the new term endocytosis has won favour. The first clear indication of a relationship between lysosomes and engulfment of extracellar material was provided by Stians. As shown in the figure, the cell engulfs the particles and then forms and invagination that become pinched off from the cell membrane to become an internal sac or body.

It is referred to as a phagosome. A phagosome then moves towards the lysosome. Exposure of the material to the lysosomal hydrolases occurs through fusion of the phagosome with a lysosome. This results in the formation of a secondary lysosome or digestive vacuole. The lysosome involved in the process may be primary or secondary, depending on the relative sizes of the two partners.

The process may appear to an observer as a lysosome discharging enzymes into a phagosome or as a phagosome shedding its contents into a lysosome, as may be the case in hepatic parenchymal cells; or simply as a mutual sharing of the contents of the two vacuoles, if they are of comparable sizes. Now the enzymes from the lysosome can come into contact with the molecules brought into the cell in the phagosome and digestion occurs.

Once the molecules are digested, the digested products can diffuse out of the so called digestive vacuole into the cytoplasm of the cell leaving the residue in the digestive vacuole. The digestive vacuole now moves on to the cell membrane where the so called reverse phagocytosis or defecation occurs.

ii. Digestion of Intracellular Substance:

In certain cases portions of the cell, somehow, find their way inside the cell’s own lysosomes and are broken down. This process is termed as cellular autophagy. How do they get in, is not clear, and the role which autophagy plays in cell function- can only be summarised.

Proteins, fats and polysaccharides can all be synthesized, and stored in the cell. During the starvation of cell these stored food materials are digested by lysosomes to give energy. What stimulates autophagy to take place and how do the large molecules get into the lysosome is not clear.

iii. Cellular Digestion:

When a cell dies, the lysosomal membrane ruptures. The liberated enzymes become free in the cell, which then quickly digest the entire cell. The hypothesis has been advanced that this is a built in mechanism for removing dead cells. In multicellular animals, many cells are constantly being formed, live for a short period of time, and then die.

The self-digestion may occur as a pathological mechanism just for example, if a cell is cut off from its oxygen supply or poisoned, the lysosomal membrane may rupture, thereby permitting the enzymes to dissolve the cell. Therefore, they are also considered as suicide bags of the cells.

iv. Extracellular Digestion:

A cell can discharge lysosomal enzymes to destroy the surrounding structures. This function is performed by reverse phagocytosis. A pocket of enzymes from a lysosome is released outside the cell where it then digests contagious structures.

This is thought to explain, how sperms penetrate the protective coating of the ovum during fertilisation. It may also explain how osteoblast cells that destroy bones, function. This may also be the explanation for the well-known ability of white blood cells to pass quickly out of the blood vessels and into the tissues spaces at the site of an infection.

v. Role in Secretion:

In recent years evidences have started accumulating suggesting the role of lysosomes in the formation of secretory products in secretory cells. The phenomenon of lysosomes-mediated thyroid hormones secretion is the best known example of direct lysosomes involvement in secretory process.

Lysosomes also play possible role in the regulation of hormone secretion. It is believed that mammotrophic hormones of the anterior pituitary is synthesized on the ribosomes of the RER and is packaged into secretory granules by passage through the Golgi.

The epithelial cells of the thyroid also contain lysosomes rich in lysosomal enzymes. The follicles of the thyroid gland contain high molecular weight protein thyroglobulin, which is stored as colloid in the lumen. The thyroid hormones thyoxine and tri-iodothyroxin are linked with this protein.

The colloid containing thyroglobulin enters the epithelial cell by pinocytosis. The colloid droplets fuse with primary lysosomes to form secondary lysosomes or digestive vacuoles. The thyroid hormones are split from the thyroglobulin and released into the blood stream. Thus the thyroid hormones are released by hydrolysis of thyroglobulin.

vi. Chromosome Breaks:

Lysosomes contain the enzyme deoxyribonuclease (DNAse). This enzyme causes chromosomal breaks and their rearrangement. DNAase has two active sites and breaks down both the strands of DNA. The breaks have been produced experimentally in isolated chromosomes incubated in DNAase. These breaks lead to various syndromes.

vii. Role in Development and Metamorphosis:

Lysosomes are important in development. Good evidences lave accumulated on the role of lysosomes in involution of uterus and mammary glands immediately in postpartum, purine metamorphosis. The process of resorption of the tadpole tail and regression of the various larval tissues, including the fat body and the salivary gland, are accompanied by increased lysosomal acid hydrolase activity (Weber).

viii. Osteogenesis:

During conversion of cartilage into bone, the special osteoblast cells produce lytic substances which erode the matrix of the cartilage and help in the formation of bone.

ix. Role of Lysosomes during Cell Division:

During the cell division, the lysosomes of that particular dividing cell move towards the periphery instead of near the nucleus, as in usual cases they are seen. During the cytokinesis roughly equal number of them move towards opposite poles. Sometimes during cell-division certain repressors in cytoplasm inhibit cell-division. Lysosomes secrete certain repressors which destroy the repressor and results in cell division.

x. Help in Protein Synthesis:

Novikoff and Essner have suggested the possible role of lysosomes in protein synthesis. Recently, Singh has correlated lysosomal activity with the protein synthesis. In the liver and pancreas of some birds, lysosomes seem to be more fictive and developed showing possible relationship with cell metabolism.

xi. Lysosomes and Cancer:

Malignant cells are found to contain abnormal chromosomes. It is presumed that the chromosomal abnormality is caused by chromosomal breakage presumably produced by the lysosomal enzymes. The partial deletion of chromosome 21 in man is associated with the chronic myeloid leukemia (blood cancer).

xii. Removal of Dead Cells:

Hirsch and Colin suggested that lysosomes help in the removal of dead cells in tissue. The lysosomal membrane ruptures in these cells, releasing the enzyme into body of cell, so that whole cell may be digested. This process of tissue, degeneration (necrosis) is due to this lysosomal activity.

xiii. Fertilisation:

During fertilisation the sperm releases hydrolytic enzymes from the acrosome vesicle. These enzymes help in the penetration of the sperm through the envelopes of the egg.

Fluorescence microscopy studies of acridine-stained spermatozoa of the guinea pig show that the acrosome vesicles contain several enzymes, including byaluronidase and proteases, which are also found in lysosomes. In fact the acrosome vesicle has been looked upon as a giant lysosome. The acrosome vesicle enzymes also apparently activate the egg by breaking down its cortical granules.

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Term Paper # 10. Peroxisomes, Glyoxysomes and Other Microbodies:

Among the enzymes monitored by de Duve in his early idles on lysosomes was the purine-catabolising enzyme, urate idase. In spite of its high concentration in lysosomal fractions isolated by differential centrifugation, some minor differences in its distribution pattern relative to that of other osomal enzymes suggested that it might be localised in some other organelle.

When isodensity gradient centrifugation was deployed to enhance the resolution obtained. Shortly thereafter the enzymes amino acid oxidase and catalase, involved in the formation d breakdown of hydrogen peroxide, respectively, were found to behave the same way as urate oxidase. Because of the involvement of these latter enzymes with hydrogen peroxide metabolism, the organelle containing them came to be called the peroxisome.

The conclusion that peroxisomes and lysosomes are distinct organelles received subsequent support from experiments in which the rate of release of acid phosphatase and catalase from detergent-treated subcellular fractions was compared.

If these two enzymes were located in the same particle, one would expect them to be released simultaneously as one adds increasing concentrations of detergent to rupture the vesicles in which they are contained. In fact ten times as much detergent is required to liberate catalase as compared to acid phosphatase, indicating that these two enzymes are localised in particles with differing properties.

Because of the relatively small difference in density between peroxisomes and lysosomes, isodensity centrifugation does not normally achieve a complete separation of these particles from each other.

Better separation can be accomplished, however, by isolating subcellular fractions from animals that have been first injected with the detergent Triton WR 1339. This detergent is selectively accumulated within lysos­omes and causes their density to decrease dramatically making possible the large-scale isolation of peroxisomes for structural and functional studies.

Electron microscopic examination of isolated subcellular fractions enriched in peroxisomes has confirmed the unique identity of these organelles. The small, roughly spherical vesicles present in such fractions resemble structures seen in tissue sections by early electron microscopists and given the general name microbodies. Microbodies vary from 0.2 to 2 μm in diameter and consist of a finely granular matrix surrounded by a single limiting membrane. These structures often contain elaborate crystalloid cores containing the enzyme urate oxidase.

Microbodies in which such cores are present can clearly be identified as peroxisomes, but in cases where distinctive cores are not present, peroxisomes may be difficult to distinguish from lysosomes and other cytoplasmic vesicles, Unequivocal identification can be made, however, by using the cytochemical staining reaction for catalase.

This reaction, based on the ability of catalase to oxidise diaminobenzidine (DAB) to an electron-dense reaction product, permits the unequivocal identification of peroxisomes. It should be emphasized that the term microbody is only a morphological label employed by electron microscopists when they see a “small body.” Unless these structures have a urate oxidase core or are stained for catalase, they cannot be clearly stated to be peroxisomes. Other types of microbodies exist in addition to peroxisomes.

Peroxisomes occur primarily in the liver and kidney cells of vertebrates, in the leaves and seeds of plants, and in eukaryotic microorganisms such as yeast, protozoa, and fungi. The spectrum of enzymes present in peroxisomes varies considerably among tissue sources. The only enzyme common to all peroxisomes is catalase which accounts for up to 15 percent of its protein. In addition to catalase, hydrogen-peroxide producing oxidases such as urate oxidase, amino acid oxidase, and glycolate oxidase are usually present.

These enzymes employ molecular oxygen as oxidising agent and generate hydrogen peroxide (H₂O₂) according to the following general reaction:

RH₂ + O₂ → R+H₂O₂

The H₂O₂ formed by reactions of this type can subsequently be hydrolysed by catalase acting in one of two alternative ways. In the so- called catalatic mode, one molecule of H₂O₂ donates its electrons to another molecule of H₂O₂, forming a molecule of water and a molecule of oxygen catalase.

H₂O₂ + H₂O₂ → O₂+ 2H₂O

In the peroxidatic mode the electron donor is a substrate other than H₂O₂:

RH₂ + H₂O₂ → R + 2H₂O

In either case, the net result is the breakdown of hydrogen peroxide.

The hallmark of peroxisomes is therefore the association of H₂O_2- ­producing oxidases with the H₂O₂-degrading enzyme, catalase. Although the exact functional significance of this association is not certain, some of the more likely possibilities will be discussed below.

Metabolic Functions of Peroxisomes:

Because hydrogen peroxide is toxic, it might seem logical to conclude that the peroxisome functions to protect cells from exposure to this substance by compartmentalizing the enzymes that produce and destroy it. The flaw in this argument, however, is that many H2O,-producing oxidases occur in the cell sap and in mitochondria, suggesting that the peroxisome contributes little to the protection of the cell from the effects of H₂O₂.

This conclusion is further reinforced by the fact that many cells producing H₂O₂ do not even contain peroxisomes. It therefore appears that the need for peroxisomes must be related to the metabolic pathways these organelles carry out rather than with the simple need to protect cells from H₂O₂.

Examination of the various enzymes present in peroxisomes has led to consideration of the following possibilities:

1. Inactivation of Toxic Substances:

One possible role of peroxisomes may involve the coupling of H₂O₂ degradation to the inactivation of toxic compounds. Catalase is the most active enzyme present in the peroxisome, and therefore degrades hydrogen peroxide at a much faster rate than it is formed by other peroxisomal reactions.

The resulting scarcity of hydrogen peroxide favours the peroxidatic mode of catalase action, in which an alternative electron donor is substituted for the second molecule of H₂O₂. The electron donors employed, for this purpose include methanol, ethanol, phenols, nitrites, Formaldehyde, and formate. Oxidation of these compounds driven by, the breakdown of H₂O₂ results in the detoxification of what are otherwise noxious substances.

2. Regulation of Oxygen Tension:

Because peroxisomal oxidases employ molecular oxygen as an oxidising agent, the reactions catalysed by these enzymes may have a significant effect on oxygen levels within the cell. In liver cells, for example, as much as 20 percent of the total oxygen consumption is accounted for by peroxisomes. Most of the remaining oxygen consumption occurs in mitochondria, where energy released during the oxidation of organic fuels by molecular oxygen is used to drive formation of the energy-conserving molecule, ATP.

Hence respiration, a term referring to the oxidation of organic fuels by molecular oxygen, occurs in both mitochondria and peroxisomes. The major difference is that a portion of the energy released during respiration in mitochondria is trapped as ATP, while in peroxisomes the energy is all lost as heat.

Peroxisomal respiration also differs from mitochondrial respiration in its sensitivity to oxygen concentration. Mitochondrial respiration occurs at a maximal rate when the oxygen concentration is around 2 percent, with further increases in oxygen producing no further increase in respiratory rate. The rate of peroxisomal respiration, on the other band, increases in roughly direct proportion to oxygen tension.

This disparity gives mitochondria an advantage over peroxisomes in utilising small amounts of oxygen, but also allows peroxisomes to exceed the respiratory ability of mitochondria at high oxygen concentrations. This property of peroxisomes may help to protect cells from the toxic effects of high concentrations of oxygen. Support for this idea has been obtained from studies carried out on plant cells exposed to bright light to enhance photosynthetic activity. This results increase in oxygen production.

Although photorespiration per se does not occur in animal tissues, an analogous enhancement of peroxisomal respiration takes place when oxygen tension rises in the cell because of a reduction in the mitochondrial respiration rate.

If the depression in mitochondrial respiration rate has been caused by a lack of oxidisable substrates, some of these substrates may be regenerated by the peroxisomal oxidation reactions stimulated by the rise in oxygen tension. Such a feedback system permits regulation of mitochondrial respiration by peroxisomal respiration, and vice versa.

3. Regeneration of Cytoplasmic NAD⁺:

Many of the oxidised compounds generated by peroxisomal respiration can be converted back to their reduced forms by NADH-dependent enzymes present in the cell sap. For example, pyruvate produced in peroxisomes by the oxidation of lactate can be reduced in peroxisomes by the oxidation of lactate can be reduced back to lactate in the cell sap, accompanied by the conversion of NADH to NAD⁺. This conversion of NADH to NAD⁺ is useful in cases where NADH is being produced in the cell sap by glycolysis faster than it can be regenerated back to NAD⁺.

4. Metabolism of Nitrogenous Bases, Lipids, and Carbohydrates:

Because uric acid is a degradation product of the purine bases present in DNA and RNA, the localisation of urate oxidase and other enzymes of purine metabolism in peroxisomes suggest that this organelle plays a role in the breakdown of nitrogenous bases derived from nucleic acids.

Enzymes involved in the breakdown of fatty acids, and in the metabolism of various carbohydrates and organic acids, have also been identified in peroxisomes. The particular combination of enzymes present suggests the possibility that peroxisomes are involved in gluconeogenesis, the synthesis of carbohydrate from fats and other non-carbohydrate materials.

This speculation is reinforced by the fact that the only vertebrate cell types with significant numbers of peroxisomes are liver and kidney, tissues known to be major sites of gluconeogenesis. However, the role of peroxisomes in gluconeogenesis has been most firmly established in plant seedlings where a special type of peroxisome known as a glyoxysome is involved.

Glyoxysomes and the Glyoxylate Cycle:

Many plants store large amounts of lipid in their seeds for subsequent use as an energy source during germination. At the appropriate time, this lipid is converted to carbohydrate by a pathway that can be divided into two major parts. The first part involves degradation of fatty acids by fl-oxidation, a process in which two-carbon fragments derived from the fatty acid chain are successively released in the form of a molecule called acetyl-CoA.

These two-carbon units are then converted to carbohydrate by the glyoxylate cycle, a modified version of the Krebs tricarboxylic acid cycle. In essence, the glyoxylate cycle by-passes the CO₂ evolving steps of the Krebs cycle with the aid of two enzymes not present in animal cells, isocitrate lyase and malate synthase. The net result of glyoxylate cycle is the conversion of two molecules of acetyl- CoA to one molecule of succinate, a precursor for the synthesis of various carbohydrates.

In the late 1960s Harry Beevers and his associates discovered that in plant seedlings, the enzymes of the glyoxylate cycle and fatty acid β -oxidation are localised in particles that can be separated by isodensity centrifugation from both mitochondria and chloroplasts. These particles, logically named glyoxysomes, also contain catalase and several H₂O-producing oxidases. Hence they represent a special type of peroxisome in which the complete set of glyoxylate cycle enzymes is present along with the normal H₂O₂-associated enzymes.

True glyoxysomes bearing the complete set of enzymes required for gluconeogenesis via the glyoxylate cycle have thus far been identified only in plant seedlings. Protozoa and yeast have peroxisomes containing some, but not all, of the glyoxylate cycle enzymes.

In such cases the glyoxylate cycle may be carried out by cooperation between mitochondria and peroxisomes, each executing a portion of the cycle. In higher animal’s fatty acid β -oxidation has been observed in peroxisomes, but the products of this pathway cannot be used for gluconeogenesis by the glyoxylate cycle because this cycle does not occur in higher animals.

Biogenesis of Peroxisomes:

Two theories have been proposed to explain how peroxisomes are manufactured by cells. The first model suggested that peroxisomal proteins, like secretory proteins, are synthesized on ribosomes bound to the endoplasmic reticulum these newly formed peroxisomal proteins were though to pass into the cisternae of the ER and into outpocketings that pinch off to form peroxisomes.

A more recent model proposes that peroxisomal proteins are synthesized on free ribosomes, released into the cell sap, and then taken up by peroxisomes. According to this latter view, new peroxisomes are formed by expansion and budding of existing peroxisomes.

Early support for the first model was obtained from electron micrographs in which peroxisomes were seen to exhibit “tails” believed to represent connections to the endoplasmic reticulum. However, peroxisomal and ER membranes are difficult to distinguish from one another in electron micrographs, and it can be just as easily argued that the “tails” represent connections between an interconnected network of peroxisomes.

Hence in order to clearly distinguish between the alternative models of peroxisome biogenesis, it has been necessary to carry out biochemical studies designed to determine where peroxisomal enzymes are synthesized within the cell. Subcellular fraction studies have therefore been carried out on tissues briefly incubated in the presence of radioactive amino acids.

According to the first model, the newly formed radioactive peroxisomal proteins should first appear associated with the rough ER of the microsomal fraction. The experimental data have failed to support this prediction, however. Instead, peroxisomal proteins such as catalase first appear in the cytosol fraction and only later become concentrated in peroxisomes, suggesting that they are being synthesized on free ribosomes.

Additional support for this conclusion has come from the demonst­ration that peroxisomal enzymes are synthesized by messenger RNAs isolated from free, but not membrane-bound, ribosomes. The notion that peroxisomal proteins are not synthesized like secretory proteins is further supported by the discovery that the major peroxisomal proteins are neither glycosylated nor synthesized as precursors containing a signal sequence.

The above findings raise the significant question of how peroxisomal proteins made in the cell sap are selectively transported into peroxisomes. Although the answer to this question is not completely understood, it is though that specific protein localised within the peroxisomal membrane aid in the recognition and uptake process.

Evolutionary Origin of Peroxisomes:

Although several possible functions of the peroxisome have been mentioned, there are reasons for questioning whether any of these are important enough to justify the existence of such an organelle, especially in the cells of higher animals. To begin with, most vertebrate cells get by perfectly well without peroxisomes, and even when they are present their necessity is questionable.

Humans inheriting genetic deficiencies of the enzyme catalase, for example, exhibit no obvious, disease symptoms. The peroxisomal enzyme urate oxidase is absent in many organisms, including humans, and the function of the peroxisomal enzyme that oxidises D-amino acids is somewhat puzzling because it is the L- amino acids that are normally found in protein molecules. Finally, most of the metabolic events supposedly occurring in peroxisomes.

The above considerations have spawned the speculation that the present-day peroxisome is a fossil organelle derived from an ancestral particle that performed critical functions hundreds of millions of years ago, but is no longer needed and is in the process of dying out.

According to this theory, the original peroxisome formed when oxygen first appeared on earth. Oxygen can be toxic because it tends to react with biological molecules to form hydrogen peroxide. Adaptation to the appearance of oxygen in the atmosphere therefore required the evolution of a system for disposing of hydrogen peroxide.

The peroxisome was especially well suited to this task because it uses the oxygen-induced production of hydrogen peroxide to facilitate the oxidation of intermediates involved in cellular carbohydrate metabolism. The major disadvantage of peroxisomal respiration, however, is that the energy released during this carbohydrate oxidation is not – conserved in a useful chemical form, but is dissipated as heat.

When mitochondria appeared later in evolution, their ability to link respiration to the formation of ATP permitted them to prevail over peroxisomes. Hence the peroxisome gradually lost many of its enzymes and became less and less important.

Some biologists believe that mitochondria evolved from ancient bacteria that were engulfed by primitive nonbacterial -cells hundreds of millions of years ago. Such a notion can be combined with our picture of the ancestral peroxisome to form an integrated theory of the evolution of respiration. This theory asserts that when oxygen first appeared on the earth, cellular life diverged in two directions.

One resulted in relatively small cells that linked respiration to ATP formation, while the other produced larger cells that depended on peroxisomes for respiration. At a later time, some of the smaller bacteria-like cells were engulfed by the larger, peroxisome-containing cells. The engulfed bacteria ultimately evolved into mitochondria, displacing the peroxisomes in importance.

In spite of the apparent superiority of mitochondria in respiration, one should not overlook the fact that peroxisomes have persisted in the presence of mitochondria for hundreds of millions of years, albeit only in certain cell types.

This persistence must reflect some useful function, though we are not certain what it may be. In plant cells the glycoxylate cycle certainly falls in this category, but the enzymes of this cycle were lost during the course of animal evolution, and are lacking in the leaves of green plants. Thus the reason for the evolutionary persistence of peroxisomes in most cell types remains somewhat of a mystery.