What is Insulin? Human Insulin

Definition of Insulin: Insulin derived from the Latin word ''insula'' means an island. As insulin is secreted from the β cells of islets of Langerhans of the pancreas, so it is called insulin.




Circulating insulin binds with β-globulin. The bound form is biologically inactive.

Chemistry of insulin

1) it's a little protein with a relative molecular mass in humans of 5808. It contains two chains A and B. a sequence has 21 amino acids and B chain has 30 amino acids. Two chains are linked by a disulfide bond when this bond breaks insulin becomes inactive.

 2) Pro-insulin, an extended single-chain molecule, is processed within Golgi body and packed into granules, where it's hydrolyzed into insulin and a residual connecting segment called C-peptide by removal of 4 amino acids.

3) Insulin and C-peptide are secreted in equimolar amounts in response to all insulin secretagogue. Pro-insulin may have some mild hypoglycaemic action. C-peptide has no known physiologic function.

4) Granules within a β-cells store the insulin in the forms of crystals consisting of two atoms of zinc and six molecules of insulin. 

Source of insulin

  • Human insulin
  • Bovine insulin
  • Porcine insulin

Human insulin

It is made by:
  • Recombinant DNA technology
  • Enzymatic modification of porcine insulin
  • Bacterial (crb) insulin
Human insulin is secreted from β-cells of pancreatic islets. The daily secretion of insulin is about 50 units.

Properties of human insulin


  • Less antigenic
  • Given subcutaneously
  • Absorbs more rapidly from the site of injection
  • Shorter duration of action
  • The dose must be adjusted as the duration of action is short
  • Less incidence of resistance
  • Most expensive
  • Most hypoglycaemic attack

Formulation of insulin



There are two types of insulin.
  1. Insulin prepared in a clear solution: these insulin are short-acting and are the only insulin to be used in emergencies such as diabetic ketoacidosis or for surgical operations.
  2. Insulin premixed with retarding agents: these insulin are intermediate or long-acting.

Storage of insulin


Stored in β-cells of the pancreas in the form of a crystalline complex of six molecules of insulin and two atoms of zinc.
 

Pharmacokinetics of insulin


R/O administration: Subcutaneous, I/M, I/V.
                                Orally insulin in digested because it is a protein in nature.
Absorption             : Slow from the subcutaneous site.
Metabolism            : Liver 60%, Kidney 40%.
                              : Destruction is by breaking the disulfide banned by insulinase.
Plasma half-life      : 3-9 minutes.
Excreation              : Urine.

Actions of insulin


Insulin is a powerful anabolic hormone.

1. It decreases blood glucose by:
  • Increase glucose uptake (liver)
  • Increase glycogen synthesis.
  • Decrease gluconeogenesis.
  • Decrease glycogenolysis.
2. Increase the transport of amino acid to muscle cells.
3. Decrease the breakdown of fat.