Insulin

Insulin is produced by the beta cells of the islets of Langerhans in the pancreas. It consists of an A-chain of 21 amino acids and a B-chain of 30 aminoacids. Pro-insulin, the precursor of insulin, is a single polypeptide chain 1.5 times the size of insulin. It is cleaved into insulin and C-peptide which are therefore secreted in equimolar amounts. C-peptide levels reflect endogenous insulin secretion and are widely used for research purposes. About 3-5% of pro-insulin escapes the cleavage process and is secreted intact.

There are various types of insulin preparation used in the treatment of diabetes mellitus:

• very rapidly-acting insulin e.g. insulin lispro or insulin aspart
• short-acting e.g. human actrapid
• intermediate-acting insulin e.g. human insulatard
• long-acting-acting insulin
• mixtures containing specific proportions of short and intermediate-acting insulins

Commercial insulin is generally recombinant human insulin; although animal insulins are still available.

Notes:

• long-acting human insulin analogue are an alternative to intermediate-acting insulin