Insulin is a protein hormone which is secreted by the islet cells of the pancreas. Chemically speaking it is composed of two polypeptide chains that are connected by disulfide bond or S-S bond. It has a molecular weight of 5808. Although insulin is known for its clinical effect in diabetes mellitus in which it regulates the blood level of the sugar glucose it is also known for its effect on fat and protein metabolism.
Insulin function by stimulating the entry of glucose into the muscle cells in which it is used for energy production through the glycolytic pathway. In addition insulin facilitates the uptake of glucose by the liver cells. Insulin functions by to help in storing glucose in liver cells in the form of the biopolymer glycogen.
Glucose is released into the circulation upon demand by the body tissues or during hypoglycemic situations such as in strenuous exercise in which case the sympathetic nervous system hormone epinephrine stimulates the formation of glucose by degradation of glycogen in the liver.
Lack of insulin as occurs in diabetes mellitus induce a hyperglycemic state. In this case of inability of the glucose molecules to enter the cell due to the lack of the hormone insulin, the body starts to use fatty acids instead as a source of energy. The hormone insulin also has effect on fat metabolism although not as apparent to the eye as that of the effect on glucose metabolism.
Lack of insulin can for example cause atherosclerosis that can cause or precipitate heart attack or cerebral stroke due to the formation of plugs in the blood of affected individuals. Insulin stimulates fatty acids synthesis in the same way like it does for glucose metabolism.
Deficiency of the hormone insulin induces the utilization of fat as a source of energy instead of glucose. In addition in the absence of the hormone insulin cholesterol and phospholipids synthesis are enhanced. Oxidation of fatty acids which occurs during insulin deficiency leads to the formation of acetyl coenzyme A which is then used to form ketone bodies such as acetoacetate compound.
Insulin also has effect on proteins and amino acids metabolism. Insulin stimulates the uptake of amino acids by the cells in the same way it does with glucose in the liver cells in which case they are used to synthesize new proteins. Amino acids are also used for the process gluconeogenesis or the synthesis of glucose from amino acids. It is also used for the generation of energy by the deamination process of these amino acids.
In the case of diabetes mellitus in which case insulin secretion is diminished or abolished completely the proteins resources in the body are converted to amino acids which can be used in different metabolic pathways such as gluconeogenesis to form glucose during starvation or the production of energy by their deamination process.
Lack of insulin in the blood stimulates the degradation of proteins. Thus forming amino acids in excess. Also insulin is required for the synthesis of proteins through its action of stimulating the liver cells to uptake more and more amino acids. Insulin is similar to growth hormone in this respect of proteins build up.
In the case of insulin deficiency a hyperglycemic state induces the secretion of insulin from the islet cells of the pancreas. Specific amino acids in the body can also stimulate the secretion of insulin like the effect of glucose deficiency on the secretion of glucose.