Describe the structure, secretion, function and regulation of insulin?
Insulin
Structure, Sythesis & Transport
- A peptide hormone made of 51 amino acids
- Formed of 2 chains (α and β) connected by disulphide bridges
- Synthesised as proinsulin with chains joined by C-peptide
- C-peptide excised by endopeptidases in the endoplasmic reticulum of β cells prior to release
- C-peptide is 31 amino acid sequence has no known biological function
- Can be used as marker of endogenous insulin secretion
- Stored in secretory granules and is secreted by the cells via exocytosis
- It enters the portal system with half cleared by first pass through the liver
- Circulates entirely in an unbound form
Function
- Key to glucose homeostasis - 'hypoglycaemic anabolic hormone':
- Liver:
- Promotes glycolysis and storage of glucose as glycogen (glycogenesis)
- Promotes conversion of glucose to triglycerides
- Muscle
- Promotes the uptake of glucose and its storage as glycogen
- Adipose tissue:
- Promotes uptake of glucose and its conversion to triglycerides for storage.
Mechanism of Action
- Binds to a specific receptor tyrosine kinase on the plasma membrane
- Increases its activity to of numerous numerous regulatory enzymes and promotes glucose transport in to cells by increasing membrane permeability
Regulation
Stimulated by:
- Raised plasma glucose
- Raised plasma fatty acid & ketone bodies
- Vagal nerve stimulation
- Amino acids
- Gut hormones (gastrin, secretin, CCK, and GIP)
Inhibited by:
- Sympathetic stimulation
- Somatostatin