Sulphur Metabolism: Functions and Distribution | Organisms | Biology

In this article we will discuss about:- 1. Brief Life History of Sulphur 2. Distribution of Sulphur 3. Source 4. Functions 5. Excretion.

Brief Life History of Sulphur:

Sulphur metabolism may be divided into two parts:

i. Exogenous, and

ii. Endogenous.

i. Exogenous:

The inorganic sulphates of food cannot be utilised for building tissue proteins.

They are excreted as such:

The sulphur-containing amino acids of food serve two broad functions:

First, they are utilised by the body to build up tissue proteins and for the synthesis of various sulphur compounds mentioned above.

Secondly, these amino acids break down and the sulphur liberated from them is mostly oxidised in the liver into sulphuric acid. This acid is partly excreted as inorganic sulphates and partly as the ethereal sulphates in the urine.

ii. Endogenous:

Sulphur derived from the breakdown of the endogenous proteins undergoes a different fate. It escapes further oxidation and is excreted as neutral sulphur compound which is a true index of endogenous protein metabolism.

Distribution of Sulphur:

i. It is found as keratin in the hard appendages of the body, i.e., hair, nails, hoofs, horns, feathers, etc.

ii. In the cartilage.

iii. In various glycoproteins, such as mucin.

iv. In nervous tissue as sulpholipids.

v. In traces in almost all cells in the form of glutathione.

vi. The enzyme rennin, lipase, phosphatase, etc., contain sulphur.

vii. The hormone insulin is a sulphur compound.

viii. Heparin contains sulphur.

ix. β-mercapto ethylamine, a constituent of coenzyme A is derived from sulphur-containing amino acids.

Source of Sulphur:

The various forms in which sulphur is taken are as follows:

i. Organic:

a. Sulphur-Containing Amino Acids:

Sulphur-containing amino acids Viz., cysteine, methionine, etc.

b. Sulphur-Containing Proteins:

Sulphur-containing proteins, viz., mucin (mucoitin-sulphuric acid), cartilage proteins (chondroitin-sulphuric acid) etc.

c. Sulpho Lipidsor Sulphatids:

Sulpho lipids or sulphatids present in the nervous tissue.

d. Thiamine and Biotin:

Thiamine and biotin (members of vitamin B complex).

ii. Inorganic:

a. Sulphates of sodium, potassium, magnesium, etc.

Functions of Sulphur:

Functions of sulphur should be regarded to be the same as the functions of sulphur-containing compounds.

From the distribution and forms in which it remains in the body, functions of sulphur may be summarised as follows:

i. It is related to the maintenance of hardness with certain amount of elasticity in various tissues of the body, for instance, hair, hoofs, horns, cartilages, nails, etc.

ii. It is a constituent of essential amino acid methionine.

iii. Relation with bile salt. Taurocholic acid of bile is derived from taurine, which is synthesised from sulphur-containing amino acid cystine.

iv. It is related to O₂ carriage and the oxidative processes of the body.

For instance:

a. Glutathione is a sulphur-containing tripeptide.

b. It takes an important part in tissue oxidation.

c. It is an ingredient of the hormone insulin which is concerned with the metabolism of carbohydrate mainly, and partly protein and fat.

d. As a constituent of thiamine and biotin, it takes part in tissue oxidation.

v. As the active radicle of some enzymes. The enzymes lipase and phosphatase, etc. contain ‘SH’ (sulphydryl) radicle, upon which their characteristic action depends.

vi. Relation with milk clotting. The milk-coagulating enzyme ‘rennin’ contains sulphur.

vii. Relation with blood clotting. Heparin, which is believed to prevent intravascular clotting normally, contains sulphur.

viii. Detoxication. The sulphur-containing amino acid cystine is used in the detoxication of toxic substances, such as bromobenzene, which is excreted as mercapturic acid. Exogenous sulphur is oxidised into sulphuric acid. This sulphuric acid is partly used to detoxicate substances like indoxyl and forms ethereal sulphate. A part of it forms inorganic sulphate by neutralising alkalies and thus helping to maintain blood reaction constant.

Excretion of Sulphur:

Sulphur is excreted in the following ways:

i. In the Saliva:

As thiocyanates and inorganic sulphates (traces). The former is specially found in smoker’s saliva. It gives a deep brown colour with ferric chloride.

ii. In the Stool:

As mucin, H₂S gas and altered bile salts.

iii. In the Urine:

The total sulphur of urine is equivalent to about 3-5 gm of H₂SO₄ per day.

It is present in three forms:

i. Inorganic sulphates 80%,

ii. Ethereal sulphates 5%, and

iii. Neutral sulphur (organic sulphide sulphur), 15-20%.

i. Inorganic Sulphates:

Such as those of sodium, potassium, magnesium, calcium, ammonium, etc. It con­stitutes about 85-90% of total urinary sulphur. It is an index of exogenous protein metabolism. The excretion of sulphates and urea run closely parallel, showing that both of them are of exogenous origin.

ii. Ethereal Sulphates:

It constitutes about 5% of total urinary sulphur.

It is formed in the following ways:

a. Tyrosine or phenylalanine undergoes bacterial putrefaction in the intestine and is converted into phenol and cresol. These are absorbed, carried to liver where they are conjugated with sulphuric acid and form one group of ethereal sulphates.

Recently it has been shown that in the process of esterification active sulphate is first formed by the combination of ATP and sulphate. The active sul­phate (adenosine-3′-phosphate-5’phosphosulphate) in the presence of an enzyme sulphate transferase transfers its sulphate group to phenols, alcohols, amines, etc. They are excreted by the kidneys as ethereal sulphates.

b. Tryptophan also undergoes bacterial putrefaction and is converted into indole and skatole. These are absorbed and are toxic substances. They are detoxicated in the liver. During detoxication they are at first oxidised into indoxyl and skatoxyl sulphuric acids and then they are conjugated with sulphuric acid to form indoxyl and skatoxyl sulphates.

They are excreted in urine as K salts and they form another class of ethereal sulphates. Indican, which is chemically potassium indoxyl sulphate, is an example of ethereal sulphate. The presence of ethereal sulphate, in the urine indicates bacterial putrefaction of proteins in the intestine. The common clinical condition is constipation.

iii. Neutral Sulphur:

It constitutes about 4-6% of total urinary, sulphur.

a. Chemistry and Forms:

Here, the sulphur remains in unoxidised form. The examples are sulphur-containing amino acids (viz., cystine), urochrome, thiosulphates, mercaptans, thiocyanates and traces of taurocholic acid, oxyproteic acid, etc. Amount in blood 2.2-4.5 mgm per 100 ml of whole blood and 17-3-5 mgm per 100 ml of serum. This proves that one quarter remains in the corpuscle and three quarters in the plasma.

b. Significance:

It increases in that rare metabolic disease, in which cystine is excreted in a large amount in urine (cystinuria). It also rises in cases of melanotic sarcomas when an abnormal sulphur-containing pigment appears in the urine.