Insulin is a peptide hormone with a molecular weight of approximately 6000 daltons. It is secreted by the B‑cells of the pancreas and passes into circulation via the portal vein and the liver. Insulin is generally released in pulses, with the parallel glucose cycle normally about 2 minutes ahead of the insulin cycle.
The insulin molecule consists of two polypeptide chains, the α‑chain with 21 and the β‑chain with 30 amino acids. Biosynthesis of the hormone takes place in the β‑cells of the islets of Langerhans in the form of single‑chain preproinsulin, which is immediately cleaved to give proinsulin. Specific proteases cleave proinsulin to insulin and C‑peptide which pass into the bloodstream simultaneously. About half of the insulin, but virtually none of the C‑peptide, is retained in the liver. Circulating insulin has a half‑life of 3‑5 minutes and is preferentially degraded in the liver, whereas inactivation or excretion of proinsulin and C‑peptide mainly takes place in the kidneys. The amino acid sequence of insulin has remained surprisingly constant during evolution, with the result that prior to the development of genetically engineered human insulin it was possible to successfully use porcine or bovine insulin in the therapy of diabetes mellitus.
The action of insulin is mediated by specific receptors and primarily consists of facilitation of the uptake of sugar by the cells of the liver, fatty tissue and musculature; this is the basis of its hypoglycemic action.
Serum insulin determinations are mainly performed on patients with symptoms of hypoglycemia. They are used to ascertain the glucose/insulin quotients and for clarification of questions concerning insulin secretion, e.g. in the tolbutamide test and glucagon test or in the evaluation of oral glucose tolerance tests or hunger provocation tests.
Although the adequacy of pancreatic insulin synthesis is frequently assessed via the determination of C‑peptide, it is still generally necessary to determine insulin. For example, therapeutic administration of insulins of non - human origin can lead to the formation of anti‑insulin antibodies. In this case, measurement of the concentration of serum insulin shows the quantity of free - and hence biologically active - hormone, whereas the determination of C‑peptide provides a measure of the patient's total endogenous insulin secretion.
A disorder in insulin metabolism leads to massive influencing of a number of metabolic processes. A too low concentration of free, biologically active insulin can lead to the development of diabetes mellitus. Possible causes of this include destruction of the β‑cells (type I diabetes), reduced activity of the insulin or reduced pancreatic synthesis (type II), circulating antibodies to insulin, delayed release of insulin or the absence (or inadequacy) of insulin receptors.
On the other hand, autonomous, non - regulated insulin secretion is generally the cause of hypoglycemia. This condition is brought about by inhibition of gluconeogenesis, e.g. as a result of severe hepatic or renal failure, islet cell adenoma, or carcinoma. Hypoglycemia can, however, also be facilitated intentionally or unintentionally (factitious hypoglycemia). In 3 % of persons with reduced glucose tolerance, the metabolic state deteriorates towards diabetes mellitus over a period of time. Reduced glucose tolerance during pregnancy always requires treatment. The clearly elevated risk of mortality for the fetus necessitates intensive monitoring.
SI units Conversion Calculator. Convert Insulin level to pmol/L, µIU/mL, mIU/L. Clinical laboratory units online conversion from conventional or traditional units to Si units. Table of conversion factors for Insulin unit conversion to pmol/L, µIU/mL, mIU/L.