Insulin

Insulin (Latin insula, "island", as it is produced in the Islets of Langerhans in the pancreas) is a polypeptide hormone that regulates carbohydrate metabolism. Apart from being the primary effector in carbohydrate homeostasis, it also has a substantial effect on small vessel muscle tone, controls storage and release of fat (triglycerides) and cellular uptake of both amino acids and some electrolytes. In this last sense, it has anabolic properties. Its concentration (more or less, prsence or absence) has extremely widespread effects throughout the body.

Structure and production

Insulin is synthesized in humans and other mammals within the beta cells (B-cells) of the islets of Langerhans in the pancreas. One to three million islets of Langerhans (pancreatic islets) form the endocrine part of the pancreas, which is primarily an exocrine gland. The endocrine part accounts for only 2% of the total mass of the pancreas. Within the islets of Langerhans, beta cells constitute 60–80% of all the cells.

Related Topics:
Beta cell - Islets of Langerhans - Endocrine - Pancreas - Exocrine - Gland

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Insulin is synthesized from the proinsulin precursor molecule by the action of proteolytic enzymes known as prohormone convertases (PC1 and PC2). Active insulin has 51 amino acids and is one of the smallest proteins known. Beef insulin differs from human insulin in three amino acid residues, and pork insulin in one residue. Fish insulin is also close enough to human insulin to be effective in humans. In humans, insulin has a molecular weight of 5734. Insulin is structured as 2 polypeptide chains linked by 2 sulfur bridges (see figure shown above). Chain A consists of 21, and chain B of 30 amino acids. Insulin is produced as a prohormone molecule – proinsulin – that is later transformed by proteolytic action into the active hormone.

Related Topics:
Amino acid - Molecular weight - Sulfur bridges - Prohormone - Proinsulin - Proteolytic

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The remaining part of the proinsulin molecule is called C-peptide. This polypeptide is released into the blood in equal amounts to the insulin protein. Since exogenous insulins contain no C-peptide component, serum levels of C peptide are good indicators of endogenous insulin production. C-peptide has recently been discovered to have itself biological activity; the activity is apparently confined to an effect on the muscular layer of the arteries.

Related Topics:
C-peptide - Arteries

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