Question

Insulin is made of …………
A. Alpha chain of 21 amino acids and second beta chain of 30 amino acids.
B. Alpha chain of 31 amino acids and second beta chain of 20 amino acids.
C. Disulfide linkages.
D. Both A and C.

Answer 1

Hint: Pancreas secrete two hormones insulin and glucagon, which are essential for normal regulation of glucose, lipid and protein metabolism. Pancreas also secrete other hormones, such as amylin, somatostatin and pancreatic polypeptide. Insulin is an anabolic hormone mainly concerned with the lowering of blood sugar level.

Complete Answer:
- In 1922, insulin was isolated by Banting and Best. It has a profound effect on carbohydrate metabolism and is associated with blood sugar. Its deficiency also affects fat metabolism and protein metabolism leads to wasting of tissues and many cellular functional disorders.
- Insulin is a small protein. In humans, it has a molecular weight of 5808. Structure of insulin comprises two amino acid chains connected by disulphide linkages. Insulin will lose its functional activity when two amino acid chains are split apart.
- Insulin is synthesized in beta cells.

Steps of insulin synthesis:
1. First preproinsulin is formed in ribosomes.
2. In the endoplasmic reticulum, preproinsulin is cleaved to form proinsulin consisting of peptide chains Alpha (A), Beta (B) and Connecting peptide (C).
3. Proinsulin is further cleaved in Golgi apparatus to form insulin, composed of alpha and beta chains connected by disulfide linkages and C chain peptide.

- Proinsulin and C peptide have no insulin activity. Insulin and C peptide is packaged in secretory vesicles and secreted in equimolar amounts.
- Alpha chain of insulin contains 21 amino acids and its beta chain has 30 amino acids. Alpha and beta chains are connected together by disulfide linkages.
- When insulin is secreted into blood, it circulates in an unbound form. It has a plasma half-life of 6 minutes, so it is cleared within 10 to 15 minutes.

Functions of insulin are the following:
- It accelerates the transport or uptake of glucose from blood into cells, especially skeletal muscles.
- It stimulates the cellular oxidation of glucose.
- It promotes glycogenesis, lipogenesis and the storage of glycogen in the liver.
- It inhibits glycogenolysis and gluconeogenesis.
- It promotes protein synthesis, inhibits the catabolism of proteins and fats, and promotes fat storage.
- Deficiency of insulin causes diabetes mellitus. It is characterised by hypercalcaemia, glycosuria, ketonemia, ketonuria, hypercholesterolemia. Polydipsia, polyuria and polyphagia.
Overproduction of insulin accelerates glycolysis and glycogenesis.

So, here the correct answer is, (D) Both A and C.

Additional information:
- Pancreas is a heterocrine gland with enzyme secreting exocrine and hormone secreting endocrine portions.
- Endocrine part is less than 2 percent of pancreatic tissue.
- Overproduction of insulin causes abnormal fall in blood sugar level, often resulting in a dangerous condition called insulin shock.

Note: The pancreas is composed of two types of tissues, one is acini, which secrete digestive juice into duodenum. Second is islets of Langerhans, which secrete insulin and glucagon directly into blood. Human pancreas has 1 to 2 million islets of Langerhans. Each islet is only about 0.3 mm in diameter and contains 3 major types of cells alpha, beta and delta cells.