Metabolism of Minerals in Living Organisms | Nutrition | Biology

Elements such as iron, copper, cobalt, manganese, iodine, selenium, etc., are required by plants and animals in very small quantities and are termed trace elements. The importance of the inorganic salts will be understood from the fact that salt starvation causes death much earlier than food starvation. Although the salts give no energy yet they are essential for our life.

To get a better conception as to why the salts should be of so much importance to us, one has to take a very long view and remember the old story that life was first born in the sea-water. If that is a fact, then it is only natural to expect that the sea salts will be intimately concerned with the structure and function of cells.

In course of evolution, although the human beings have departed a long way from their marine ancestors, yet the primitive aquatic life has left a permanent impression on our body. Each salt present in the sea is also found in the human body. Blood plasma is a typical example where the various inorganic salts are present almost in the same proportions as they are in the sea-water.

Each one of these salts serves some essential functions. Even the minute quantity of iodine found in the sea-water is utilised for the synthesis of different thyroid hormones.From these considerations and from various other experimental facts it is seen that these inorganic salts and elements-individually and collectively- are so intimately blended with our life processes that life becomes impossible without them.

If, thus, we remember our marine genesis and the fact that the inorganic elements are still carrying on the same intimate relationship with the life of a cell, then the fundamental principles of mineral metabolism in higher animals will not be very difficult to grasp.

1. Copper:

Sources:

It is widely distributed in the sea-water, plants, vegetables, milk, brain tissue, liver, blood, etc. Daily requirement: In the adult about 2 mgm. Normal mixed diet is quite adequate.

Storage:

Liver is the main store. Minute amount (0.1-0.5 mgm) is present in blood. In normal serum, a copper- containing globulin ceruloplasmin, has been found to occur. Copper content of the brain and liver of foetus and infant is much greater than that of adult. This high storage is useful to prevent deficiency in the sucking period as found in the case of iron. Total amount in adult body is 0.1 gm.

Functions:

i. Normochromic microcytic anaemia occurs during copper deficiency which is due to failure in proper absorption of iron. The anaemia can be cured by parenteral administration of iron. It is claimed that the therapeutic effect of iron in the cure of hypochromic anaemia is enhanced in presence of traces of copper. It is held that best effect is obtained if Cu and Fe are administered together in the ratio of 1:100. Most iron salts do not contain more than 0.02 mgm of copper per gram. Since, this is less than the optimum daily requirement; traces of copper should be added for best therapeutic effect.

ii. In certain marine animals the blood contains a copper compound-haemocyanin (with 0.38% of Cu)-instead of haemoglobin. There it serves the same function as haemoglobin.

iii. Copper is present in a number of enzymes like tyrosinase, cytochrome oxidase, uricase, etc., in copper deficiency the marrow of red bones markedly decreased in cytochrome oxidase. In human red blood cor­puscles, a copper-containing protein erythrocuprein is present, the function of which is not known as yet.

iv. Cu in some way is related to development of bone.

v. In female rats, copper deficiency causes continued anestrus.

Wilson’s Disease:

It is due to disorder of copper metabolism. There is a fall in plasma ceruloplasmin and increase in copper content of brain and liver leading to neurological disturbances and hepatic damage.

2. Manganese:

Daily Requirement:

Traces:

Ordinary mixed diet contains about 2 mgm, which is enough. Widely distributed in the body especially in the intestinal tract, liver and reproductive organs.

Functions:

i. It exerts a similar but less pronounced effect like copper in the synthesis of haemoglobin. Some workers believe that hypochromic anaemia is best treated by a combination of Fe, Cu and Mn. In its deficiency growth is poor, bone formation abnormal.

ii. It has some role in normal reproductive physiology. Experimental deficiency of manganese in rats disturbs oestrous cycle and lactation in females and degeneration of germinal epithelium in males.

iii. In the lamelli branch (Pinna squamosa) the blood pigment, instead of haemoglobin, is a manganese compound, called Pinna globulin. It serves the same functions as haemoglobin.

iv. Mn ions activate many enzymes, viz., arginase, phosphoglucomutase, muscle adenosinetriphosphatase, choline esterase, hexokinase, isocitric dehydrogenase pyrophosphatase and various decarboxylases.

Transmanganin:

It is β₁-globulin which is different from transferrin and responsible for keeping Mn bound.

3. Sodium:

Sources:

Taken as common salt. Present in milk, water and all ordinary articles of diet.

Daily Requirement:

As NaCl-minimum 5-10 gm. The average intake is much above this, 8-10 gm and even more, hence natural sodium deficiency is rare.

Distribution:

Total-0.1 % of body weight.

i. In Blood (as NaCl):

Whole blood contains 350-550 mgm per 100 ml. Plasma contains 560-630 mgm per 100 ml. As Na Plasma contains 340 mgm%, corpuscle 65 mgm%, and whole blood 200 mgm%.

ii. In the Cerebrospial Fluid:

As NaCl-650-750 mgm%, as Na-330 mgm%, slightly higher than in plasma.

It is to be noted that the distribution of Na and K in the cells and extracellular fluids are in reverse order. Na is chiefly found in the extracellular fluids and very little inside the cells, whereas, K is chiefly found inside the cells and very little in the extracellular fluids.

Metabolism:

Controlled by adrenal cortex.

Excretion:

Daily excretion is generally same as daily intake. Excreted chiefly through urine and partly through sweat and stool. The average daily excretion in urine as chlorides is about 10-15 gm (i.e., 1%).

Functions:

The sodium in plasma provides over 90% of the total base of the body. Sodium works in the body in two capacities.

Either:

(a) As sodium ions, or

(b) As sodium compounds.

(a) As Sodium Ions:

1. Initiates and maintains contraction of heart.

2. Essential for the normal functions of cells.

3. Essential for the contraction of voluntary and involuntary muscles.

4. Excites nerves- Na ions are neuroexcitatory as opposed to calcium ions.

(b) As Sodium Compounds:

Such compounds are bicarbonates, phosphates, chlorides, proteinates, etc.

Their functions are as follows:

i. Maintain Blood Reaction:

This is done in many ways.

For instance:

a. Sodium bicarbonate is the chief buffer of blood and other body fluids.

b. The acid and alkaline phosphates also constitute an important buffer system.

c. Sodium which remains combined with plasma proteins (sodium proteintes) can also act as a buffer.

d. Sodium of NaCl can also fix acids with the help of the phenomenon known as chloride shift.

ii. Controls Reaction of Urine:

Kidneys regulate the urine reaction by altering the proportion of acid and alkaline phosphates in the urine.

iii. The Reaction of Pancreatic Juice and Bile:

It is due to the presence of sodium carbonate.

iv. Maintain Osmotic Pressure:

NaCl is the chief regulator of the osmotic pressure of the body fluids.

v. Help in the Formation of HCL of Gastric Juice:

NaCl takes part in the series of reactions as a result of which HCl is manufactured by the stomach.

vi. Fat Absorption:

In adrenalectomised animals, administration of Na salts rectifies defective fat absorption.

vii. Maintains water balance.

Effects of Deficiency:

Na deficiency shows reduction in fat deposit, atrophy of muscle and testis, lung infection, retarded bone growth, reduction in osteoid tissue, and a definite eye changes in animals.

4. Potassium:

Sources:

In all foodstuffs, specially in the animal cell.

Daily Requirement:

4 gm approximately.

Distribution:

Total- 0.1% of body weight i.e., same as sodium. Its chiefly intracellular as opposed to sodium.

i. In Blood:

Whole blood, 200 mgm; plasma, 15-20 mgm; cell, 180 mgm per 100 ml.

ii. Cerebrospinal Fluid:

Cerebrospinal fluid 12-17 mgm per 100 ml.

It is to be noted that when whole blood is stored at 2-4°C., as for the purpose of blood transfusion, po­tassium from corpuscles migrates into plasma in which the potassium concentration rises. Such sample of blood, if administered may produce serious effects.

Metabolism:

Controlled probably by adrenal cortex, either directly or indirectly by regulating sodium me­tabolism.

Excretion:

2-3 gm excreted through urine per day which varies with the amount of intake. In potassium defi­ciency kidneys reabsorb more. With high meat diet and in the later stages of fasting, when tissues are breaking down, potassium excretion rises.

Functions:

i. Maintains intracellular osmotic pressure as KCl.

ii. Maintains intracelluar reaction. As bicarbontes, phosphates, and proteinates – potassium constitutes a very important buffer system in the cell.

iii. CO₂ carriage. Potassium of KHb in the red cells helps CO₂ transport.

iv. Potassium ions inhibit cardiac contraction and prolong relaxation.

v. Potassium ions inhibit muscular contraction in general.

vi. Potassium exerts important effects upon the function of nervous system.

Some facts are given below:

a. Synaptic transmission. Potassium is intimately related to the formation and action of acetylcholine at the synapses upon which the transmission of nerve impulses depends.

b. Potassium is intimately related to the excitability of nerve cells.

c. It is intimately concerned with the development of potential difference with the help of which an impulse is conducted through a nerve.

vii. Hastens ciliary movement. Taking everything together, the action of potassium is generally opposed to that of calcium. In adrenal deficiency serum potassium rises, due to less excretion by the kidneys and more leakage from the tissue cells. In intestinal obstruction serum potassium rises as a terminal event. In shock, the same picture is seen.

Effects of Deficiency:

The heart rate is slow. There are scarring of heart muscle, hypertrophy of kidneys, paralysis of muscle and retarded bone growth. Bones become fragile excessively. Both sexes may become sterile.

5. Magnesium:

Sources:

Green vegetables, meat, bread, etc.

Daily Requirement:

Not definitely known. Its presence in normal diet is generally enough for its requirement in the body. Recent recommendation is 350 mgm per day for adult males and 300 mgm per day for adult females.

Absorption:

From the upper part of the small intestine, Mg ions are absorbed with great difficulty. It forms insoluble phosphates like calcium. Hence the principles, that apply for Ca absorption, are also applicable for Mg.

Distribution:

i. Blood:

Whole blood 3 mgm, plasma 2.5 mgm, corpuscles 3.5 mgm per 100 ml. Distribution of Ca and Mg in blood is generally opposite. Calcium remains almost wholly in plasma and very little in the cells, while Mg is mostly found in cells and less in plasma.

ii. Bone:

70% of the total Mg of the body is present in the bones as phosphates. Bones contain 2% of the Mg₃(PO₄)₂.

iii. Voluntary Muscles:

About 0.02%. (Ca—0.007%).

iv. Chlorophyll:

It is the green pigment of plants. It is a porphyrin derivative of magnaesium.

Excretion:

In urine, 0.1—0.2 gm in 24 hours. It is generally not excreted through the large gut. Mg content of faeces largely represents unabsorbed Mg of diet. Metabolism of Mg is probably controlled by adrenal cortex directly or indirectly.

Functions:

i. Forms bone and tooth.

ii. Activator of Enzymes:

Mg ions activate the enzyme phosphatase. It accelerates the action of bone phos­phatase.

Iii. Takes Part in the Chemistry of Muscular Contraction:

Mg is a member of a complex coenzyme system which takes part in the chemical changes underlying muscular contraction. It may be that here also it helps the action of muscle phosphatase.

iv. Antagonistic to Calcium Ion:

The distribution and function of Mg ion is generally antagonistic to calcium ion.

Effects of Deficiency:

In man vascular disturbances, hyperexcitability, convulsions and ultimately death. Something like tetany develops and is known as Mg-tetany. In rats Mg—tetany is fatal in about 18 days. In this case blood magnesium falls, blood cholesterol rises and the calcium content in the soft tissues, specially in the muscles and the kidneys, becomes very high. If Ca intake is increased the effect of Mg deficiency becomes very severe.

6. Chlorine:

Sources:

Present as sodium chloride in milk, water, other articles of diet, etc.

Daily Requirement:

10—12 gm per day (about 4 gm of sodium).

Excretion:

(1) Mainly in the urine, about 15 gm in 24 hours. In starvation or in deprivation of salts, chlorides are retained in the body and less than 1 gm per day is excreted. Under all such conditions blood chlorides remain unchanged. This low excretion is due to more reabsorption by the renal tubules. Chlorides are high threshold substances.

(2) In the sweat-Usually the chloride content of sweat is very low—0.15%, but in profuse sweating, a man may lose 10 gm or more in the course of three hours only.

Distribution:

Mostly present in the form of inorganic chlorides both inside the cells as well as in the extracellular fluids. Whole blood contains about 0.25%; plasma, 0.36%; corpuscles, 0.19%. CO₂ tension of blood determines the proportion of chlorine between cell and plasma. They are readily absorbed from the intestine. They cannot be replaced by other halides.

Storage:

Large amounts of chlorides can remain stored in the skin and subcutaneous tissue.

Control of Metabolism:

I. Adrenal Cortex:

Due to its influence on sodium metabolism.

II. Posterior Pituitary:

Posterior pituitary controls urine output and indirectly controls chloride balance.

Functions:

i. Chlorine provides for about 2/3rds of the anions of plasma and is the chief anion of the body. Consequently, it is a main factor for regulating body reaction.

ii. Chlorides, such as NaCl, KCl, etc., are important agents in the regulation of osmotic pressure in the body.

iii. HCl of the gastric juice is ultimately derived from the blood chlorides.

iv. Chlorine ions are essential for the action of ptyalin and pancreatic amylase.

v. Chloride shift is an important phenomenon by which chlorine ion shifts alternately from plasma to cell and helps CO₂ carriage and regulation of blood reaction.

7. Iodine:

Sources:

(a) In traces as iodides in water, food, common salts, etc.

(b) Sea-water—0.2 mgm per litre. Sea weeds and spongy shells are very rich in iodine.

Daily Requirement:

0.05 mgm. In children more.

Absorption:

Readily absorbed from the small intestine.

Distribution:

Total 25 mgm in the body, half of it remains in the thyroid.

Iodine remains in the blood as:

i. Inorganic Iodide:

Inorganic iodide in the plasma and corpuscles and varies from 0.5 µ gm to per 100 ml.

ii. Bound Iodine:

Thyroxine and tri-iodothyronine in the plasma in combination with α-globulin. It is called protein – bound iodine or PBI. The normal value of PBI varies from 5.0 µ gm to 8.0 µ gm per 100 ml.

Iodine Metabolism:

It is controlled by thyroid. Deficiency leads to simple goitre. Iodine is excreted chiefly in the urine.

Functions:

Iodine is an essential constituent of thyroid hormones and therefore essential for normal life.

8. Bromine:

Sources:

In foodstuff, specially in the animal tissues.

Distribution:

Distributed throughout the tissues in traces, and remains mixed with chlorides. In blood its concentration is higher than iodine.

Absorption and Excretion:

Readily absorbed from the small intestine.

Functions:

Bromine ions (like chlorine ions) activate ptyalin and pancreatic amylase.

9. Fluorine:

Sources:

Traces in food and water. Drinking small water should not contain more than 1—2 parts per million.

Distribution:

Distributed throughout the body in traces. Enamel of teeth is particularly rich in it.

Functions:

In bones it remains as CaF, but neither essential nor constant. Very small amount possibly helps in development of tooth, but excessive intake causes mottling of teeth. Fluoride is a poison for some enzymes of glycolytic pathways, not clear if it is an essential element.

10. Zinc:

Sources:

Traces in ordinary articles of diet.

A vital enzyme, carbonic anhydrase, contains zinc. It is also essential for the activity of some enzymes and hence essential for the life of the organism. Other enzymes, viz., several dehydrogenases (alcohol, glutamic, certain pyridine nucleotide) and pancreatic carboxypeptidase contain zinc.

Daily Requirement:

About 12 mgm.

Distribution:

Traces in all tissues.

Excretion:

Chiefly in urine.

Functions:

i. Although active zinc free insulin preparations are available, yet zinc is an integral and essential element of insulin molecule.

ii. Insulin crystallises readily as a zinc compound and zinc prolongs insulin action.

iii. Possibly helps storage of insulin in the pancreas; because pancreas is rich in Zn.

Effects of Deficiency:

Reduced growth, alopaecia, skin lesions, etc., are observed.

11. Aluminium:

This is another trace element distributed widely in the body. Its function in the body is not properly known. Use of aluminium utensils does not produce any aluminium poisoning.

12. Cobalt:

Traces are essential for cattle life. Being a component of vitamin B₁₂ (4.5%) it is also essential for human haemopoiesis. Only minute amounts in daily diet are sufficient. [Vide Vitamin B₁₂.]

Administration of excess cobalt causes degeneration of α-cells of islets of Langerhans and abnormal in­crease in number of circulating R.B.C. (cobalt polycythemia) possibly due to stimulation of erythropoietin, a hormone which accelerates erythropoiesis.

13. Selenium:

Excess selenium has some toxic effects in rats. This element is a constituent of Factor 3 which prevents liver necrosis, muscular dystrophy, white muscle disease, etc., in animals.