Metabolism of Pigments in Human Body | Biology

A number of pigments are found in the human body, some of which serve important physiological function, while others represent simple waste products or that that their functions are yet obscure. It is difficult to classify pigments on a single scientific basis.

They may be classified in many ways, for instance:

i. According to the body substance in which they are present, such as, blood pigments, bile pigments, urine pig­ments, skin pigments, intracellular pigments, etc.,

ii. According to their functions.

iii. According to their chemical structure. The following classification has been attempted with all the three views taken together.

Pyrrole Derivatives:

In these pigments four pyrrole rings remain joined together through a methinyl (= CH) or methylene (-CH₂-) group to form either a chain compound or a ring compound [Table 10.4].

Bilirubin, biliverdin and the other bile pigments are derivatives of haemoglobin. Urobilinogen and urobilin are derived from bile pigments. Stercobilin is believed to be identical with urobilin.

Pyrrole compounds are widely distributed throughout animal and plant life. The porphyrins have the property of combining with proteins and various metals producing a number of coloured compounds known as metalloporphyrins. These compounds, wherever found, are responsible for transport of oxygen or actual tissue oxidation. For instance chlorophyll, the green-colouring matter of plants, is a metalloporphyrin containing Mg.

Haemoglobin is an iron porphyrin which is the red pigment of blood. Myoglobin is an iron-containing-red pigment found in the red muscles. Haemocyanin is a blue-coloured copper porphyrin present in the blood of molluscs and crustacea, and replaces haemoglobin in them.

In certain pathological conditions known as porphyrinuria, large quantities of porphyrins are passed out in the urine. Such people are highly sensitive to sunlight. Porphyrins are believed to be highly photosensitive and are related to the production of light sensitiveness of these patients.

Tyrosine Compounds:

1. Melanin:

This is the chief pigment of the body and is derived from tyrosine or phenylalanine.

Distribution:

It is very widely distributed in the body, but peculiarly enough, it is limited only to those structures which have got an ectodermal origin, for instance, skin, hair, choroid coat of retina, substantia nigra, etc.

Function:

In the skin and hair it is probable that melanin serve a protective action against the harmful effects of sunlight. Prolonged exposure to sunlight stimulates melanin formation and causes the so-called sun-tanning. The function of melanin in the choroid coat is mainly to convert the eyeball into a perfect dark chamber. Since, nervous tissue is derived from ectoderm, the melanin in the substantia nigra may represent the vestigial remnants of the melanin forming properties.

Synthesis:

Melanin, whenever it is found, is formed in the local cells by the enzyme—tyrosinase or melanase. The mother substance, upon which the enzyme acts, is a tyrosine derivative (DOPA) believed to be formed in the adrenals.

The broad steps of melanin synthesis from the oxidation of phenylalanine or tyrosine are as follows:

Phenylalanine tyrosine → Dihydroxyphenylalanine (DOPA) → Melanin.

Melanin formation in both human and amphibian skin is augmented by the hormone known as intermedin or melanocyte-stimulating hormone- (MSH) secreted by the pars intermedia of the pituitary gland. Adrenocorticotrophic hormone (ACTH) secreted by anterior pituitary has melanocyte-stimulating activity similar to MSH although to a much lower degree. In Addison’s disease, ACTH is secreted in a large amount and there is brownish-black pigmentation of the exposed parts of the skin, e.g., hands, feet, etc., and mucous membrane.

Melatonin, extract from bovine pineal gland, causes concentration of melanin near the nuclei of melanocyte in frog, and as a result of this the skin becomes paler. Its role in human is not known. In melanotic sarcoma, melanin may be found in the urine.

2. Homogentisic Acid:

It is a metabolic product of tyrosine and is normally oxidised further to acetoacetic acid and fumaric acid. In one of the ‘in born errors of metabolism’ known as alcaptonuria, homogentisic acid accumulates as it cannot be oxidised. The urine if made slightly alkaline produces a dark coloured melanin-like product.

Lipochromes (Carotenoids):

These are fat-soluble pigments having different shades of colour, ranging from light orange to yellow. Chemi­cally, they are unsaturated hydrocarbons containing 40 carbon atoms. They may be of different types, such as the carotenes, present in the carrots and plant leaves, the xanthophylls and lycopenes in the tomatoes, etc.

The lipochromes of the human body present in the red nucleus of the brain are probably derived from vegetable lipochromes. The most rich sources are carrots, vegetables and plant leaves. 25 gm of carotene is obtained from 1,000 kg of carrot. The carotenes are of immense physiological importance, because they act as provitamin A, converted into vitamin A in the liver.

Lutein:

It, produced in the corpus luteum, is also a xanthophyll compound serving an essential role in the normal sex life of females. Carotenes and xanthophylls have been isolated from the body fat, milk fat, nervous tissue, etc. The yellow colour of fat and butter (from cow’s milk) is due to these lipochromes. The grey colour of the nerve cells and the red colour of the so-called red nucleus are believed to be due to lipochromes. Lycopene has also been demonstrated in the human fat.

The reddish colour, sometimes seen in human hair, the bright red colour on the scales of the gold fish and the bright pigments of Canary’s feather are all due to carotenoid compounds.

Flavins (Lyochromes):

These are water-soluble, nitrogenous pigments widely distributed in nature having a yellow fluorescence. As a rule the flavins remain combined with phosphoric acid, one specific protein radicle and a pentose molecule. The enzymes Flavin mononucleotide (FMN) and Flavin adenine dinucleotide (FAD) present in different cell, lactoflavin of milk, ovoflavin of eggs, etc., are the examples. Riboflavin is a component of vitamin B group.

All these flavins take an important part in the oxidative processes inside the cells. Although very widely distribut­ed, yet they are found in very low concentrations. From the whites of 10,000 eggs only 180 mgm of ovoflovins are obtained. 5,400 litres of whey yield only 1 mgm of lactoflavin.

Visual Pigment:

Several visual pigments have been isolated which are mentioned below:

i. Visual Purple:

Visual purple or rhodopsin is a chromoprotein and is present in the rods of the retina. In presence of light, the rhodopsin is bleached into retinene₁ and opsin. Retinene is the derivative of vitamin A. In the dark, the rhodopsin is efficiently resynthesized. For details vide under Eye.

ii. Porphyropsin:

Like rhodopsin, there is also another pigment present in the fresh water vertebrates which is known as por­phyropsin. In the light, it is bleached into retinene and scotopsin. Retinene₂ is the derivative of vitamin A₂ (retinol₂). In the dark, the porphyropsin is resynthesized.

iii. Lodopsin:

It is a visual pigment of the cone of the retina. It has been isolated from the cone of the chicken. In presence of light, it is also bleached into retinene₁ and photopsin. Retinene₁ is the derivative of vitamin A₁ (retionl₁). In the dark, the iodopsin is re-synthesised.

iv. Cyanopsin:

This pigment has been synthesised in the laboratory and has not been isolated in the retina. It is also bleached in the light into retinene₂ and cone opsin. Retinene₂ is the derivative of vitamin A₂ and is also resynthesized in the dark.

Urochrome:

It is a sulphur-containing yellow pigment, being the chief colouring matter of the urine. Its amount does not depend upon food and is constant under normal conditions. It depends upon endogenous metabolism and increases in all conditions where tissue damage is going on. It should be considered as an end product of en­dogenous protein metabolism (neutral sulphur).

Uroerythrin:

Reddish colour, nature unknown

Coproporphyrin:

It is derivative of haemoglobin.

Faecal Pigments:

Stercobilin:

Same as urobilin. Various other pigments may be found in the stool derived directly from diet, drug and prob­ably bacterial products.