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The chemical structure, target-cells, mechanism of action, physiologic effects, regulation of secretion of insulin

Insulin is formed in B-cells of pancreas.

The chemical structure: It is unfull value protein consistsing of 51 amino acids (except of Met and Try). Amino acids of insulin are arranged to 2 polypeptide chains (A and B). A-chain consists of 21 amino acids; B-chain does 30 amino acids. These chains are linked by 2 disulfidic bonds. Third disuldidic bond occurs in A-chain. Monomers of Insulin are combined with zinc forming polymer consisting of 2-8 molecules of insulin. Insulin is broken up in liver under action of insulinase.

Mechainism of action: 3-rd one

Receptor of insulin consists of 4 subunits (2 alfa and 2 beta). Subunits are bound by disulfidic bridges. B-chain of insulin joins with alfa-subunits of receptor. Beta-subunits of receptor are catalytic part of receptor (tyrosine-kinase).

Target-cells: all cells of the body except nervous system, lens, erythrocytes. Coupled insulin has receptors in adipose tissue only.

Physiologic action: insulin is anabolic hormone that is stimulates synthesis of glycogen, proteins and lipids. The cleavage of proteins, lipids and glycogen is decreased.

I. The influence on Carbohydrate Metabolism: 1) increase the permeability of membranes for glucose; 2) increase glycogenogenesis; 3) increase of glycolysis; 4) decrease glycogenolysis

II. The influence on lipid metabolism: 1) increase of synthesis of lipids (incl FFA, Cholseterol); 2) increase of liponeogenesis from glucose and its intermediaries; 3) increase oxidation of glycerol; 4) decrease beta-oxidation of FFA, ketogenesis

III. The influence on protein metabolism: 1) increase protein synthesis; 2) decrease protein cleavage

IV. Stimulation of cells division

Regulation of secretion: low level of glucose in blood plasma decreases insulin formation. high level of glucose increases insulin formation

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