Insulin

 Insulin 

Insulin was discovered in 1921 by banting and best who demonstrated the hypoglycaemic  action of an extract of pancreas prepared after degeneration of the exocrine part due to ligation of pancreatic duct .

It was first obtained in pure crystalline form in 1926 and the chemical structure was fully worked out in 1956 by sanger.

Insulin is a two chain polypeptide  having 51 amino acids and MW about 6000.

The A-chain has 21 while B-chain has 30 amino acids. There are minor differences between human, pork and beef insulins: 

Species	A - chain		B-chain
	8th AA	10th AA	30th AA
Human	THR	ILEU	THR
PORK	THR	ILEU	ALA
BEEF	ALA	VAL	ALA

 

THUS pork insulin is more homologous to human insulin than is beef insulin. The A and B chains are held together by two disulfide bonds.

Insulin is synthesized in the  β cells of pancreatic islets as a single chain peptide pre pro insulin (110AA) from which 24 AAs are first removed to produce Pro insulin the connecting or C peptide (35AA) is split off by proteolysis in galgi apparatus both insulin and C peptide are stored in granules within the cell. 

The C peptide is secreted in the blood along with insulin.

ASSAY Insulin is bio assayed by measuring blood sugar depression in rabbits (1 U reduces blood glucose of a fasting rabbit of 45mg/dl) or by its potency to induce hypoglycemic convulsions in mice. 

1 mg of the international standard of insulin = 28 units. 

Within the availability of pure preparations , it can now be assayed chemically also.

Plasma insulin can be measured by radio-immunoassay or enzyme immunoassay.

Regulation of insulin secretion :-

Under basal condition 1U insulin is secreted per hour by human pancreas.  Much larger quantity is secreted after every meal. 

Secretion of insulin from β cells is regulated by chemical, hormonal and neural mechanisms.

Chemical : 

The β cells have a glucose sensing mechanism dependent on entry of glucose into β cells and its phosphorylation by glucokinase.

Glucose entry and activation of the glucoceptor  indirectly inhibits the ATP sensitive K+ channel resulting in partial depolarization of the β cells.

Hormonal  :-

A number of hormones , e.g. growth hormone, corticosteroids, thyroxin  modify insulin in response to glucose.

More important are the intra-islet paracrine interactions between the hormones produced by different types of islet cells.

1. Somatostatin inhibits release of both insulin and glucagon.

2. Insulin inhibits glucagon secretion.

Neural : 

The islets are richly supplied by sympathetic and vagal nerves. 

1. Adrenergic α2 receptor activation decreases insulin release by inhibiting β cell adenylyl cyclase.

2. Adrenergic β2 stimulation increases insulin release  by stimulating β cell  adenylyl cyclase.

3. Cholinergic – muscarinic activation by Ach or vagal  stimulation causes insulin secretion through IP3/DAG- inserted intracellular ca2+ in the β cell.

Actions of insulin : 

The overall effects of insulin are to favor  storage of fuel. The actions of insulin and the results of its deficiency can be summarized as : 

1. Insulin facilitates glucose transport across cell membrane; skeletal muscle and fat are highly sensitive. The availability of glucose intracellularly is the limitating  factor for its utilization in these and some other tissues.

2. The first step in intracellular utilization of glucose is its phosphorylation to form glucose -6- phosphate.

3. Insulin inhibits gluconeogenesis in liver by gene mediated decreased synthesis of phospheonol pyruvate carboxykinase.

4. Insulin inhibits lipolysis in adipose tissue and favours  triglyceride synthesis.

5. Insulin enhances transcription of vascular endothelial lipoprotein lipase and thus increases clearance of VLDL and chylomicrons.

6. Insulin facilitates AA entry and their synthesis into proteins, as well as inhibits protein break down in muscle and most other cells.

Most of the above metabolic actions of insulin are exerted within seconds or minutes and are called the rapid actions..