Insulin

Insuli is a pancreatic hormone produced by islets of Langerhans of beta cells in the pancreas. It is synthesized as preproinsulin, which is processed into proinsulin, a 74 amino acids polypeptide. Proinsulin is modified in the endoplasmic reticulum of beta cells. It is cleaved at two places producing active insulin hormone composed of two polypeptide chains (chain A= 21 amino acids and chain B=30 amino acids) joined together by disulfide bonds and an intervening biologically inactive called C peptide. Insulin and C peptides are stored in secretory granules in beta cells. After a meal, blood glucose levels rise, stimulating the pancreas to secret insulin. Insulin secretion by glucose stimulation is a complex process. Glucose enters beta cells through glucose transporters (GLUT) and is metabolized, ultimately signaling to secret insulin from beta cells into the blood circulation. Insulin secretion is also enhanced by certain amino acids, fatty acids, keto acids, and other hormones secreted by the GI tract. Once insulin is secreted, it binds with insulin receptors in insulin-sensitive tissues like muscle and adipose tissues. It allows the blood glucose to enter the cells for further metabolism. The action of insulin lowers blood glucose levels, suppressing the synthesis of insulin hormone. The balance of insulin secretion is very critical for the metabolic process. As decreased insulin secretion is responsible for hyperglycemia (diabetes), increased insulin secretion (hypoglycemia) is also life-threatening. [Md. Zakir Hossain Howlader]