Insulin is a member of a family of structurally-related regulatory proteins; other proteins in this group include the insulin-like growth factors and relaxin. Insulin is produced by the β-cells of the pancreatic islets and is initially synthesized as a 12 kDa pre-prohormone, which undergoes intracellular processing to a 9 kDa, 86-amino acid prohormone and subsequent packaging in storage granules. Within these granules, disulfide bonds are formed between the A and B chains of the insulin molecule and the C-peptide region is cleaved, resulting in the 51-amino acid, 6 kDa mature insulin molecule. Upon stimulation, the islet cells release equimolar amounts of insulin and C-peptide, and small amounts of proinsulin and other intermediates (<5% of normal total insulin secretion)1.
Insulin is the most important hormone of the fed-state, and is the only physiologic hormone which significantly lowers blood glucose levels. In response to a number of substrates and other stimuli, including glucose and amino acids, insulin is secreted into the hepatic portal circulation1,2. Fifty-percent of the insulin is removed on first-pass through the liver, the remainder enters the general circulation and is carried to other target tissues. Insulin then binds to specific cell-surface receptors3 and, through incompletely defined mechanisms, facilitates substrate uptake and intracellular utilization, resulting in net increases in intracellular lipid, protein and glycogen1-4. In addition to its role in peripheral metabolism, insulin may influence central regulation of energy balance5. Insulin is rapidly cleared both by liver uptake, tissue utilization and renal clearance (T1/2 of about 4 mins), and circulating insulin levels are very low during fasting. In contrast, C-peptide of insulin does not undergo significant liver or extra-renal metabolism and, therefore, has a much longer circulating half-life (~30 min).1
Basal- and glucose-stimulated circulating insulin concentrations are relatively stable during infancy and childhood, and increase during puberty due to decreased insulin sensitivity [6]. Insulin concentrations tend to be higher in obese individuals, particularly those with an increased proportion of visceral (abdominal) fat7. Glucose counter-regulatory hormones, such as glucagon, glucocorticoids, growth hormone and epinephrine, decrease insulin sensitivity and action; insulin levels may increase during exogenous administration of these substances. 1,2
Measurement of circulating insulin concentrations may be useful in the clinical evaluation of several conditions. Elevated serum insulin levels in the presence of low glucose concentrations may be indicative of pathologic hyperinsulinism, e.g. nesidioblastosis and islet-cell tumor [8]. Elevated serum fasting insulin levels with normal or elevated glucose concentrations, and exaggerated insulin and glucose response to exogenous glucose administration are characteristic of the insulin-resistant forms of glucose intolerance and diabetes mellitus and other insulin resistant conditions.7, 9, 10 High circulating insulin concentrations may be involved in the pathogenesis of hypertension and cardiovascular disease10,11. Conversely, low insulin concentrations in the presence of hyperglycemia suggests insulin-deficiency, e.g. insulin-dependent or Type I diabetes mellitus.
Although the C-peptide of insulin is biologically inactive, it has a longer circulating half-life than insulin and undergoes relatively minimal hepatic metabolism. In addition, C-peptide of insulin assays may be analytically more sensitive than insulin assays. Because of these factors, measurement of C-peptide of insulin may be useful in evaluating insulin secretion in a variety of clinical conditions.12-14
References:
1. Gerich JE: Hormonal control of homeostasis. IN Galloway JA, Potvin JH, Shuman CR (eds): Diabetes Mellitus, ninth edition. Eli Lilly Co, Indianapolis, 1988, pp 46-63.
2. Rasmussen H, Zawalick KC, Ganesan S, Calle R, Zawalich WS: Physiology and pathophysiology of insulin secretion. Diab Care 13:655-666, 1990.
3. Gammeltoft S: Insulin receptors: binding kinetics and structure-function relationship of insulin. Physiol Rev 64:1321-1378, 1984.
4. Rosen OM: After insulin binds. Science 237:1452-1458, 1987.
5. Schwartz MW, Figlewicz D, Baskin DG, Woods SC, Porte D Jr: Insulin in the brain: A hormonal regulator of energy balance. Endocrin Rev 13:387- 414, 1992.
6. Amiel SA, Caprio S, Sherwin RS, Plewe G, Haymond MW, Tamborlane WV: Insulin resistance of puberty: a defect restricted to peripheral glucose metabolism. J Clin Endocrinol Metab 72:277-282, 1991.
7. Björntorp P: Metabolic implications of body fat distribution. Diab Care 14:1132 - 1143, 1991.
8. Haymond MW: Hypoglycemia in infants and children. Endocrinol Metab Clin North Amer 18:211-2552, 1989.
9. O'Rahilly S, Moller DE: Mutant insulin receptors in syndromes of insulin resistance. Clin Endocrinol 36:121-132, 1992.
10. Reaven GM: Insulin resistance, hyperinsulinemia, hypertriglyceridemia, and hypertension. Parallels between human disease and rodent models. Diab care 14:195-202, 1991.
11. Fontbonne AM, Eschwege EM: Insulin and cardiovascular disease. Paris prospective study. Diab Care 14:461-469, 1991.
12. Horwitz DL, Kuzaya H, Rubenstein AH: Circulating serum C-peptide: a brief review of diagnostic implications. New Engl J Med 295:207-209, 1976.
13. Bommen M, Stanhope R, Kurtz AB, Brook CGD: Plasma C peptide in hyperinsulinaemic hypoglycaemia. Arch Dis Child 59:1096-1098, 1984.
14. Field JB: Hypoglycemia. Definition, clinical presentations, classification, and laboratory tests. Endocrinol Metab Clin North Am 18:27-43, 1989.
15. HHS Publication, 5th ed., 2007. Biosafety in Microbiological and Biomedical Laboratories. Available http://www.cdc.gov/biosafety/publications/bmbl5/BMBL5
16. DHHS (NIOSH) Publication No. 78€“127, August 1976. Current Intelligence Bulletin 13 - Explosive Azide Hazard. Available http:// www.cdc.gov/niosh.
17. Approved Guideline €“ Procedures for the Handling and Processing of Blood Specimens, H18-A3. 2004. Clinical and Laboratory Standards Institute.
18. Kricka L. Interferences in immunoassays €“ still a threat. Clin Chem 2000; 46: 1037€“1038.
