Pancreatic hormones – Biosynthesis, regulation and secretion of insulin, and glucagon

1. Dr. ACHLA JAIN
GUEST LECTURER
SOS IN LIFE SCIENCES
PT. RAVISHANKAR SHUKLA UNIVERSITY, RAIPUR

2.  Pancreas contains exocrine & endocrine cells.
 Roughly 99% of the cells of the pancreas are
arranged in clusters called acini. The acini produce
digestive enzymes, which flow into the GIT.
 Endocrine cells are called Islets of Langerhans, 1-
2% of pancreatic tissue.

4. There are 4-types of
endocrine cells
1. α-cells: secret
glucagon (25%)
2. β-Cells: secret
insulin (65%)
3. δ-Cells: secret
somatostatin (8%)
4. F-Cells: secret
pancreatic
polypeptides (2%)

5. INSULIN
 The word "insulin" is derived from Latin, insula, meaning
island (islet). In 1869, Langerhans identified the alpha and
beta cells in islets of pancreas. In 1889, von Mering and
Minkowski produced experimental diabetes by
pancreatectomy.
 In 1922, Banting and Best extracted insulin from pancreas.
Insulin was the first hormone to be isolated in a pure form.
For this work Banting was awarded Nobel prize in 1923.
 In 1927, Abel crystallized the insulin. In 1954, Sanger
studied the amino acid sequence of insulin; it was the first
protein in which complete amino acid sequencing was done.
For this work Sanger got Nobel prize in 1958. Insulin is the
first protein produced by recombinant DNA technology
(1982).

6. Insulin is a protein hormone with 2 polypeptide chains. The
A chain has 21 amino acids and B chain has 30 amino acids.
These two chains are joined together by two interchain
disulphide bonds, between A7 to B7 and A20 to B19. There
is also an intrachain disulphide link in A chain between 6th
and 11th amino acids .Species variation is restricted to
amino acids 8,9 and 10 of A chain and C terminal of B
chain.

7. Primary structure of human insulin

8. i. Insulin is a protein synthesized and secreted by the beta cells
of the islets of Langerhans of the pancreas.
ii. The insulin is synthesized as a larger precursor polypeptide
chain, the pre-pro-insulin. It has 109 amino acids. It is
rapidly converted to pro-insulin in the endoplasmic reticulum
by removal of leader sequence of 23 amino acid residues.
iii. The proinsulin with 86 amino acids is transported to Golgi
apparatus where it is cleaved by a protease. Thus C-peptide or
connecting peptide with 33 amino acids is removed. (The
number of amino acids in C peptide may vary according to
species). Insulin with 51 amino acids is thus formed .

9. Insulin biosynthesis

10. The insulin is packed into granules. The molecules take
shape of a hexamer with 2 zinc ions and one calcium ion.
Approximately 50 units of insulin is secreted per day.
Normal insulin level in blood is 5-15 microunits/ml.
Proinsulin contributes 5 to 10% of the total insulin
measured in plasma.
Proinsulin has about one-third biological activity as that of
insulin. Insulin and C-peptide are synthesized and secreted
in equimolar quantities. Therefore, measurement of C-
peptide is an index of rate of secretion of insulin. Mutations
causing changes in amino acid sequence at the cleavage
points can lead to familial pro-insulinemia.

11. 1. Glucose
Glucose is the major stimulant of insulin secretion. As blood
glucose level increases, the insulin secretion also
correspondingly increases. Glucose induces a biphasic
response to insulin secretion. A discharge of insulin from the
beta cell storage pool occurs during the initial rapid phase of
insulin release within first 2 minutes. The second phase of
insulin release lasting for 5-10 minutes is of smaller magnitude
and is due to discharge of newly synthesized hormone. The
beta cells have GluT 2 receptors, through which glucose is
absorbed. Glucose is oxidised, so that more ATP is produced.
ATP stimulates an ATP Binding Cassette protein (ABC
protein) which is referred to as Sulfonyl Urea Receptor
(SUR).

12. Simultaneously potassium channels are closed and calcium
channels are opened .Increased intracellular calcium causes
the insulin secretion.
Mechanism of insulin secretion

13. 2. Gastrointestinal hormones: Insulin secretion is enhanced
by secretin, pancreozymin and gastrin. After taking food,
these hormones are increased.
3. Proteins and amino acids: Leucine and arginine are
stimulants.
4.Parasympathetic and beta-adrenergic stimulation.
5. Glucagon and growth hormone.
6. Drug, Tolbutamide.
7.Incretin hormones: Glucose dependent insulinotropic
polypeptide (GIP, 41 amino acids) and Glucagon likepeptide
1 (GLP-1, 31 amino acids) are involved in the release of
insulin following nutrient entry into stomach.

14. 1. Insulin Receptors
Insulin acts by binding to a plasma membrane receptor on the
target cells. In obesity, the number of receptors are decreased
and target tissue becomes less sensitive to insulin (diabetes
mellitus Type 2). Insulin receptor is a glycoprotein with 4
subunits; 2 alpha and 2 beta subunits. The alpha units (135
kD) are located on the extracellular side, to which insulin
binds. The beta subunits (95 kD) traverse the membrane and
are exposed on the cytoplasmic side. Beta subunit has
tyrosine kinase activity.

15. Insulin receptor

16. Insulin binds to the alpha subunit. This binding
activates the tyrosine kinase activity of the beta
subunit, leading to autophosphorylation of the beta
subunit .This event, in turn, phosphorylates insulin
receptor substrates (IRS). The message is later
transmitted into a series of serine/threonine kinases,
such as IRS → Ras → Raf → MAPK, etc. which
causes cellular responses.

17. 3.Gene Transcription (new enzyme synthesis) Insulin acts
at the transcriptional level to regulate synthesis of more
than 100 proteins.
3-A. Insulin induces the following enzymes:
i. Glucokinase
ii. Pyruvate kinase
iii. Phospho fructo kinase
iv. Acetyl CoA carboxylase
3-B.Insulin represses the following enzymes:
i. Glucose-6-phosphatase
ii. Phospho enol pyruvate carboxy kinase
iii. Fructose-1,6-bisphosphatase

18. 4. Activation of Enzymes
Insulin activates the existing molecules of enzymes by
covalent modification (phosphorylation or
dephosphorylation). There are more than 50 enzymes
activated by this mechanism. Insulin activates protein
phosphatase I (PPI) which dephosphorylates enzyme
proteins.

20. 5. DNA Synthesis
Through the IRS-1 pathway, (Insulin receptor substrate
1 (IRS-1)) insulin increases DNA synthesis, cell
growth and anabolism. In all the above mentioned
pathways, intracellular mediators have been implicated
in insulin action. These are Ca++ and cyclic AMP.
Insulin activates phospho diesterase and thereby
decreases cAMP. So, reactions dependent on cAMP are
inhibited, e.g. glycogen phosphorylase
6. Glucose Uptake
Insulin increases the recruitment of GluT4 in
cells.

21. Insulin plays a central role in regulation of the metabolism of
carbohydrates, lipids and proteins.
1. Uptake of Glucose by Tissues
Insulin facilitates the membrane transport of glucose.
Facilitated diffusion of glucose in muscle is enhanced by
insulin. In diabetes mellitus, the transporter, GIuT4 is
reduced. However, glucose uptake in liver (by GluT2) is
independent of insulin.

23. 2. Utilization of Glucose
i. Glycolysis is stimulated by insulin. The activity and
amount of key glycolytic enzymes (glucokinase,
phosphofructokinase and pyruvate kinase) are increased.
ii. Glycogen synthase enzyme is activated, and so insulin
favors glucose storage as glycogen.
iii. Insulin favors synthesis of fatty acid from glucose and
so glucose utilization is increased.

24. 3. Hypoglycemic Effect
i. Insulin lowers the blood glucose level by promoting
utilization and storage.
ii. Gluconeogenesis is inhibited by insulin by
repressing the key enzymes, pyruvate carboxylase (PC)
phosphoenol pyruvate carboxykinase (PEPCK) and
glucose-6- phosphatase.
iii.Insulin inhibits glycogenolysis by favoring the
inactivation of glycogen phosphorylase and inhibiting
glucose-6-phosphatase. The net effect of all these three
mechanisms, blood glucose level is lowered.

25. 4. Lipogenesis
i. Lipogenesis is favored by providing more acetyl CoA
by pyruvate dehydrogenase reaction.
ii. Insulin increases the activity of acetyl CoA
carboxylase and provides glycerol for esterification
of fatty acids to TAG .
iii. Insulin also provides NADPH by increasing the
GPD activity of the HMP shunt pathway.

26. 5. Anti-lipolytic Effect
i. Insulin inhibits lipolysis in adipose tissue due to
inhibition of hormone sensitive lipase.
ii. The increased level of FFA in plasma in diabetes is
due to the loss of this inhibitory effect on lipolysis.
6. Anti-ketogenic Effect
i. Insulin depresses HMG CoA synthase and so
ketogenesis is decreased.

27. ii. In presence of insulin, acetyl CoA is completely utilized in
the citric acid cycle, because oxaloacetate generated from
glucose is available in plenty. Insulin also favors fatty acid
synthesis from acetyl CoA.
iii. All these factors reduce the availability of acetyl CoA, so
that production of ketone bodies reduced.
7. Other General Effects
i. Protein synthesis is promoted and degradation is retarded.
ii. It is an anabolic hormone.
iii. Insulin stimulates replication of cells. Insulin is an essential
growth factor for all mammalian cells.

28. 1. Glucagon
2. Epinephrine or Adrenaline
3. Glucocorticoids
4. ACTH
5. Growth hormone
6. Thyroxine
All these are anti-insulin hormones.
Glucagon
It is a polypeptide hormone with 29 amino acids. It is
secreted by the alpha cells of pancreas. Enteroglucagon is
a peptide hormone secreted by duodenal mucosa, having
same immunological and physiological properties of
glucagon. Glucagon is synthesized as a longer
proglucagon precursor.

29. Fig. Combined action of insulin and glucagon will keep the blood
glucose level within normal limits.
High blood glucose stimulates insulin secretion (yellow pathway). Low
blood glucose causes glucagon secretion (blue pathway)

30. Glucagon has a half-life in plasma at about 5 minutes.
It is inactivated in the liver. The major regulator of secretion of
glucagon is glucose. An increase in blood glucose level
inhibits secretion of glucagon.
Physiological Actions of Glucagon
1.Glucagon is the most potent hyperglycemic hormone. It is
anti-insulin in nature. Therefore, the net effect is decided by
the insulin-glucagon ratio.
2. Glucagon is mainly glycogenolytic. The active form of
glycogen phosphorylase is formed under the influence of
glucagon. Liver is the primary target for the
glycogenolytic effect of glucagon.

31. 3. It depresses glycogen synthesis.
4. Gluconeogenesis is favored by glucagon by inducing
enzymes like PEPCK (Phosphoenolpyruvate carboxykinase),
glucose-6- phosphatase and fructose-1,6-bisphosphatase.
5. Glucagon increases plasma free fatty acid level. In
adipose tissue glucagon favors beta-oxidation, as it
activates carnitine acyl transferase. The mitochondrial acetyl
CoA level increases
6. Ketogenesis is favored.

32.  Glucagon combines with a membrane bound receptor.
 This activates the GDP-bound G-protein, by converting it into
GTP form. The alpha subunit of the G protein dissociates from the
beta and gamma subunits.
 The alpha subunit binds to GTP. The GTP-G protein will in turn
activate adenylate cyclase to convert ATP to cAMP.
 Cyclic AMP is the second messenger, which combines with the
regulatory subunit of the protein kinase so that the catalytic
subunit is free to act.
 The active protein kinase will phosphorylate enzyme proteins and
alter their activity; covalent modification and activation of
glycogen phosphorylase, inactivation of glycogen synthase, etc.