Insulin (from lat. insula – island) – a hormone of a peptide nature, is formed in the beta cells of the islets of Langerhans of the pancreas. It has a multifaceted effect on metabolism in almost all tissues. The main action of insulin is to lower the concentration of glucose in the blood.
Insulin increases the permeability of plasma membranes for glucose, activates key glycolysis enzymes, stimulates the formation of glycogen from glucose in the liver and muscles, and enhances the synthesis of fats and proteins. In addition, insulin inhibits the activity of enzymes that break down glycogen and fats. That is, in addition to the anabolic effect, insulin also has an anti-catabolic effect.
Disruption of insulin secretion due to the destruction of beta cells – absolute insulin deficiency – is a key link in the pathogenesis of type 1 diabetes mellitus. Disruption of insulin action on tissues – relative insulin deficiency – plays an important role in the development of type 2 diabetes mellitus.
One way or another, insulin affects all types of metabolism throughout the body. However, first of all, the effect of insulin concerns precisely the metabolism of carbohydrates. The main effect of insulin on carbohydrate metabolism is associated with increased glucose transport across cell membranes. Activation of the insulin receptor triggers an intracellular mechanism that directly affects the flow of glucose into the cell by regulating the amount and function of membrane proteins that carry glucose into the cell.
The transport of glucose in two types of tissues depends to the greatest extent on insulin: muscle tissue (myocytes) and adipose tissue (adipocytes) – this is the so-called. insulin-dependent tissues. Composing together almost 2/3 of the entire cellular mass of the human body, they perform such important functions in the body as movement, respiration, blood circulation, etc., and store the energy released from food.
Like other hormones, insulin acts through a receptor protein.
The insulin receptor is a complex integral protein of the cell membrane, built of 2 subunits (a and b), each of which is formed by two polypeptide chains.
Insulin with high specificity binds and is recognized by the a-subunit of the receptor, which changes its conformation upon attachment of the hormone. This leads to the appearance of tyrosine kinase activity in the b subunit, which triggers a branched chain of reactions to activate enzymes, which begins with receptor autophosphorylation.
The whole complex of biochemical consequences of the interaction of insulin and the receptor is not yet completely clear, however, it is known that at an intermediate stage, the formation of secondary mediators occurs: diacylglycerols and inositol triphosphate, one of the effects of which is the activation of the enzyme, protein kinase C, with the phosphorylating (and activating) action of which on enzymes and changes in intracellular metabolism are associated.
An increase in the flow of glucose into the cell is associated with the activating effect of insulin mediators on the incorporation into the cell membrane of cytoplasmic vesicles containing the glucose transporter GLUT 4.
Insulin has a complex and multifaceted effect on metabolism and energy. Many of the effects of insulin are realized through its ability to act on the activity of a number of enzymes.,
Insulin is the main hormone that lowers blood glucose (glucose levels are also reduced by androgens, which are secreted by the reticular adrenal cortex), this is realized through:
- increased absorption of glucose and other substances by cells;
- activation of key glycolysis enzymes;
- an increase in the intensity of glycogen synthesis – insulin accelerates the storage of glucose by liver and muscle cells by polymerizing it into glycogen;
- a decrease in the intensity of gluconeogenesis – the formation of glucose in the liver from various substances decreases
Below is a list of insulins: