Definition
Insulin is a peptide hormone produced by the β‑cells of the pancreatic islets of Langerhans. It plays a central role in the regulation of carbohydrate, lipid, and protein metabolism by facilitating cellular uptake of glucose and influencing anabolic processes.
Chemical Structure
Human insulin consists of 51 amino acids arranged in two polypeptide chains: an A‑chain of 21 residues and a B‑chain of 30 residues. The chains are linked by two inter‑chain disulfide bonds (A7‑B7 and A20‑B19) and an intra‑chain disulfide bond within the A‑chain (A6‑A11). The mature hormone is derived from a larger precursor, preproinsulin, through enzymatic cleavage and folding in the endoplasmic reticulum.
Biosynthesis and Secretion
Insulin is synthesized as preproinsulin in the rough endoplasmic reticulum of β‑cells. The signal peptide is removed to form proinsulin, which folds and forms the characteristic disulfide bonds. Proinsulin is transported to the Golgi apparatus, packaged into secretory granules, and cleaved by prohormone convertases to yield mature insulin and C‑peptide. Secretion is primarily triggered by elevated blood glucose concentrations; other stimuli include certain amino acids, fatty acids, gastrointestinal hormones (e.g., incretins), and autonomic nervous system signals.
Physiological Role
- Glucose Homeostasis: Insulin promotes glucose uptake in insulin‑responsive tissues such as skeletal muscle, adipose tissue, and the liver by stimulating the translocation of glucose transporter type 4 (GLUT4) to the plasma membrane.
- Glycogen Synthesis: In the liver and muscle, insulin activates glycogen synthase, enhancing glycogen storage.
- Lipogenesis: Insulin stimulates fatty acid synthesis and inhibits lipolysis in adipocytes.
- Protein Metabolism: It promotes amino acid uptake and protein synthesis while reducing proteolysis.
- Other Effects: Insulin influences electrolyte balance (e.g., potassium uptake), cell growth, and gene expression.
Regulation
Insulin secretion follows a biphasic pattern: an initial rapid release of pre‑stored granules, followed by a sustained phase dependent on continued glucose metabolism and calcium influx. Counter‑regulatory hormones such as glucagon, epinephrine, cortisol, and growth hormone antagonize insulin’s actions, raising blood glucose when necessary.
Clinical Significance
- Diabetes Mellitus: Deficiency of insulin production (type 1 diabetes) or impaired insulin action (type 2 diabetes) leads to chronic hyperglycemia. Exogenous insulin therapy is the cornerstone of management for type 1 diabetes and is employed in many cases of advanced type 2 diabetes.
- Insulin Analogs: Modified insulin molecules (e.g., rapid‑acting lispro, aspart; long‑acting glargine, detemir) have altered pharmacokinetic profiles to better mimic physiological secretion patterns.
- Hypoglycemia: Excessive insulin or inappropriate dosing can cause dangerously low blood glucose levels, requiring prompt treatment with glucose administration.
Historical Discovery
Insulin was first isolated in 1921 by Frederick Banting, Charles Best, James Collip, and John Macleod at the University of Toronto. Their work enabled the first successful clinical use of insulin to treat diabetes in 1922, for which Banting and Macleod received the Nobel Prize in Physiology or Medicine in 1923.
Pharmacology
Therapeutic insulin is administered subcutaneously, intravenously, or via inhalation. Its onset, peak, and duration of action depend on formulation (e.g., regular, NPH, analogs). Insulin dosing is individualized based on carbohydrate intake, blood glucose monitoring, and patient-specific factors such as renal function.
References
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