INSULIN

INSULIN

Insulin is a polypeptide hormone produced by the beta-cells of the Islets of Langerhans of the pancreas of all vertebrates. It is secreted directly into the bloodstream where it regulates carbohydrate metabolism, influences the synthesis of protein and of RNA, and the formation and storge of neutral lipids. It was the first protein for which chemical structure and precise molecular weight were determined.

Insulin consists of two chains of amino acids, the A and B chains, connected by two disulphide bridges. Insulin produced by different species contains the same basic structure but has different sequences of amino acids in the chain. Porcine insulin differs from human insulin in only one amino acid in the β chain whereas bovine insulin varies from human insulin in the same amino acid in the B chain and in two acids in the A chain.

Commercially insulins are obtained by extraction of bovine or porcine or these mixed pancreases and purified by recrystallization only. An extract which has been recrystallized only once can be separated into 3 components or fractions called as a, b, and c fractions, The 'a' fraction contains high molecular weight substances and is present in very impure preparations. This fraction is purified by repeated recrystallization. The 'b' fraction consists mainly of proinsulin and insulin dimers. In 'c' fraction, insulin, insulin esters, arginine insulin and desamidoinsulin are present. Other pancreatic peptides such as glucagon, pancreatic polypeptide, somatostatin, and vasoactive intestinal peptide are also present in products.

Proinsulin is removed by gel filtration and ion-exchange chromatagraphic techniques. Prior to the discovery of recombinant DNA technology, the major source of insulin was the pancreases of slaughtered animals.

Human insulin, sometimes called semisynthetic human insulin, is now produced by the enzymatic modification of insulin obtained from the porcine pancreas. Human insulin is also produced by the chemical combination of A and B chains which have been obtained from bacteria genetically modified by recombination of DNA technology, or from proinsulin obtained from bacteria or from a precursor obtained from a yeast genetically. Human insulin obtained by recombinant DNA technology is called as biosynthetic human insulin.

Insulin is a protein containing the elements carbon, hydrogen, oxygen, nitrogen and sulphur. The minimum molecular weight is 6000. All crystalline insulin preparations contain small amounts of heavy metals, such as zinc, nickel, cobalt or cadmium as , components of the molecule. The amino acid composition and physical structure of mammalian insulin varies only slightly from species to the next, except for the rat and the guinea pig which have large structural differences. Insulin is hydrolyzed by an enzyme, insulinase, which is prepared from hog pancreas

Insulin occurs as an amorphous powder or as crystals, usually obtained as flat rhombohedra and containing 0.4 per cent zinc. It is readily soluble in dilute acids and alkalies. It exists as the dimer in mild acid and as the monomer in 30 per cent acetic acid. Practically, it is insoluble in pH 4.5 to 7.0. Its isoelectric point is from 5.30 to 5.35.

Insulin is inactivated when the phenolic, hydroxyl. carboxyl. imidazole and amide groups are blocked by chemical reactions such as acetylation, esterification; or the disulphide bridges are broken by reduction, oxidation or treatment with cysteine. Insulin forms complexes with metals like zinc, nickel, cobalt and cadmium.

insulin plays a role in :

1.     transport of glucose through cell membrane;

2.      protein synthesis in muscles, adipose tissue and liver; 3. 4 5

3.     fatty acid synthesis, increased in liver and adipose tissue;

4.     glycogen synthesis in muscle and adipose tissue

5.      glucose uptake, enhanced in muscle, adipose tissue and liver

6.     oxidation phosphorylation, i.e., formation of "energy rich' phosphate bonds;

7.     oxidations in tricarboxylic acid cycle,

8.      formation of glucose-6-phosphate from glucose and ATP + hexokinase.

The role of insulin in the reversible transport of glucose through cell membranes is important. This transport is stereoselective in character and L-glucose is not transported. Insulin does not effect the transport of phosphorylated glucose through the cell membrane.

A deficiency of insulin yields hyperglycemia, followed by glucosuria, disturbance of electrolyte and water balance, increased fat metabolism and increased amino acid metabolism with increased nitrogen excretion. This results in decrease in blood pressure, collapse of the peripheral blood vessels, failure of kidney function and shock.

Slightly esterified insulin is active and not very strongly antigenic. Unmodified insulin has a rapid onset of activity and a short duration of action.

Proinsulin (Porcine) is a single polypeptide which is composed of the A and B chains of insulin joined together by a polypeptide containing 33 amino acid units and 3 disulphide bridges.

Insulin Injection (Brand names : Actrapid; Endopancrine, Humulin S; Insular; Insulyll; Iszilin; Regular iletin; Velosulin) is a steriled, acidified or neutral solution of insulin. It has a potency of not less than 95 per cent. The potency is 40, 80. 100 or 500 U.S.P insulin units in each ml. The injection containing not more than 100 U.S.P. units/ml is colourless, that containing 500 units may be straw-coloured; substantially free from turbidity and from insoluble matter. Onset of action occurs within l hour after injection, reaching its maximum in 2 to 3 hours.

Insulin zinc suspension is a sterile suspension, in a buffered water medium, of insulin modified by the addition of zine chloride in a manner such that the solid phase of the suspension consists of a mixture of crystals and amorphous material in a ratio of approximately 7:3. It is almost colourless suspension of a mixture of characteristic crystals and many particles have no uniform shape.

Uses :

 Insulin is used to treat diabetes.The most frequent complication of insulin therapy is hypoglycaemia resulting hunger, pallor, sweating, palpitations, anxiety and tremulousness. Other symptoms include headache, visual disturbances, slurred speech, paraesthesia of the mouth, alterations in behaviour, and impaired mental ability .

.