Transcript Insulin

The
endocrine portion of the
pancreas represents 1-2% of its
total weight and it is consisted of 12 million islets of Langerhans that
are collections of at least 4 types of
cells; A or α cells, B or β cells, D or
δ cells and F cells. These cells
secrete at least 4 types of hormones
glucagone, insulin, somatostatin,
and PP respectively.`
INSULIN
HISTORY:
In 1889 both Von Mering and Minkowski
demonstrated that pancreatectomy produced diabetes.
The link between the pancreatic islets and diabetes was
suggested by De Mayer in 1909 and by Sharpy –
Schaffer in 1917, however; it was Banting and Best who
proved this association in 1921. These investigators
used acid- ethanol to extract from the pancreatic tissue
an “ islet cell factor” that had potent hypoglycemic
activity. This factor was named insulin and within a year
both bovine and porcine insulin was widely used for
treatment of human diabetes.
Insulin was the first protein that:• proved to have hormonal bioactivity
• was crystallized
• was sequenced (1951)
• was
synthesized
biochemically
(1964)
• was synthesized by recombinant
DNA technology for commercial use.
CHEMICAL STRUCTURE:Insulin is a polypeptide consisting of two chains
(A and B) that are connected by interchain
disulfide bridges that connect A7 to B7 and A20
to B19. A third intrachain disulfide bridge
connects the residues 6 and 11 of the A chain.
The A and B chains contain 21 and 30 amino
acids respectively in most species.
Porcine and bovine insulin were the standard
therapy for diabetes before human insulin was
prepared by DNA technology. Porcine insulin
differs from human insulin in substituting alanin
instead of threonin at B30. However, bovine
insulin has this substitution in addition to 2 other
substitutions at A8 and A10.
Synthesis and Release
Insulin is first synthesized in the
pancreatic islet B cells as [preprohormone] with a molecular Wt. of
11500 D. This molecule is directed by the
hydrophobic 23 amino acid leader
sequence into the cisternae of the
endoplasmic reticulum and is then
removed to make the 9000 D proinsulin
molecule. Starting from the amino terminal
this molecule is [ B chain- connecting
peptide(C- peptide)- A chain].
The proinsulin molecule undergoes
a series of site specific peptide
cleavages that result in the
formation of the mature insulin and
C- peptide in eqimolar amounts.
Proinsulin molecule varies in its
length between 78- 86 a.a. due to
the variation in the C- peptide.
Proinsulin and insulin have the same solubility and
isoelectric point and both of them form hexamers with
zinc crystals which are present in the islets in high
concentrations. However, proinsulin has only 5% of
insulin bioactivity and a small amount of it is normally
secreted into the circulation together with insulin when
the release of the later is stimulated and in larger
amounts in pancreatic tumors. Proinsulin has a longer
half – life than insulin and it reacts with insulin anti-sera
in the RIA overestimating insulin bioactivity.
C- peptide molecule has no known biological activity
and with no distinct antigenic property and that’s why its
measurement can distinguish endogenous insulin from
that given exogenously as a therapy. The human insulin
gene was identified to be located on the short arm of
chromosome no. (11).
Regulation of Insulin Secretion
The human pancreas secrets 40- 50 units of insulin
daily. A number of mediators are implicated in insulin
release including:
1.Glucose:
High plasma glucose concentration is the most important
physiological regulator for insulin secretion. The
threshold conc. of fasting plasma glucose for insulin
release is 80- 100 mg/dl (while the max. response is at
FPG of 300- 500). Two theories can explain the role of
glucose ;
•Glucose receptors at the cell surface of B- cells.
•Rate of metabolites influx from different biological
processes including pentose- phosphate shunt, citric
acid cycle or glycolytic pathway.
2.Hormonal factors:
Alpha-adrenergic hormones (like ephedrine) inhibits
insulin release whereas β – adrenergic hormones
increase insulin release. Other hormones increase insulin
secretion including chronic exposure to GH, cortisol,
placental
lactogen,
estrogens
and
progestins
(pregnancy?).
3. Drugs:
Sulphonylurea compounds are used most frequently in
the therapy of type 2 diabetes. Drugs like tolbutamide
stimulate insulin secretion by a mechanism different from
glucose.
INSULIN METABOLISM
Insulin has no plasma carrier protein, thus
its plasma half- life is very short (3-5 min).
The major organs involved in insulin
metabolism are liver, kidneys, and placenta.
Two enzyme systems are involved in its
metabolism; the 1st includes an “insulin
specific protease” and the second one is “
hepatic glutathione- insulintranshydrogenase”
which reduces the disulfide bridges resulting
in insulin rapid degradation.
INSULIN ACTIONS
Insulin has got a major role in the metabolism of CHO, protein, and
lipid metabolism. These include:
High Blood Glucose
Increase
Signals release
Glucagon
Insulin
Decrease
Signals release
Low Blood Glucose
1. Effects on the cell membrane transport:
Insulin enhances the entry of glucose from the
extracellular to the intracellular compartments
using [carrier mediated facilitated diffusion] in
almost all body tissues mainly adipose and muscle
tissues when entry of glucose is followed by
glucose phosphorylation and further metabolism.
In the liver tissue insulin does not enhance
facilitated diffusion, but it acts on enhancing
phosphorylation of free glucose through inducing
the activity of the enzyme ‘ hepatic glucokinase’
and in this way it enhances the glucose entry by
simple diffusion (based on the conc. gradient of
free glucose).
2. Effects on glucose utilization:
Insulin enhances the intracellular utilization of glucose
where about 50% of glucose is converted to energy
through the “ glycolytic pathway” , while the other 40%
is converted to fat and about 10% is converted to
glycogen. It stimulates several key enzymes in the
glycolytic
pathway
including
glucokinase,
phosphofructokinase, and pyruvate kinase. Insulin
stimulates the conversion of glucose to glucose- 6phosphate by glucokinase and hexokinase II enzymes
mainly in the liver and the muscle tissues, which is
then converted to glucose-1- phosphate that will be
incorporated in the structure of glycogen through the
action of ‘glycogen synthase’ enzyme which is under
direct stimulation by insulin.
3. Effects on glucose production:
Insulin inhibits the production of glucose from non- CHO
sources where it decreases the amount of the key
enzyme [PEPCK]; an effect that is usually opposed by
glucagone, glucocorticosteroids, angiotesin II, and others.
4. Effects on Lipid Metabolism:
Insulin is well –known to have a potent lipogenic
activity, meanwhile, it is a potent inhibitor of lipolysis in
the liver and the adipose tissue. It inhibits [hormonesensitive lipase] enzyme activity. It thus decreases the
amount of circulating free fatty acids and hence it affects
the glucose metabolism as well as the free fatty acids
inhibit glycolysis at many steps and stimulates
gluconeogenesis.
In patients with insulin deficiency, the
hormone- sensitive lipase activity gets
increased resulting in increased concentration
of circulating free fatty acids together with
increased glucagon release (which opposes
most actions of insulin). The increased free
fatty acids will be converted to acetyl Co A
which enters the citric acid cycle and converted
to CO2 and water. However, in this case the
capacity of this cycle is usually exceeded
resulting in the conversion of the excess acetyl
Co A into ketone bodies (acetone, acetoacetic
acid, and β- hydroxybutyric acid) in high
concentration resulting in ketoacidosis.
5. Effects on Protein Metabolism:
Insulin is known to have an anabolic
role in protein metabolism where it
enhances the entry of neutral amino
acids into the muscle cells and thus
protein synthesis and it however
prevents protein degradation.
Glucagon
Glucagon
It is a single- chain polypeptide hormone (M
Wt ~ 3500 D) synthesizes mainly in the A cells
of the pancreatic islets. It consists of 29 amino
acids and it is also synthesized from a
proglucagon precursor molecule of ~ 9000 D
first. It shares some biological and biophysical
properties with (enteroglucagon) which is
released from the duodenal mucosal cells.
It also flows freely in the plasma without
being associated to any carrier protein and thus
its plasma half – life is only about 5 min. It is
inactivated in the liver by removing the 1st 2
amino acids from the amino terminal end.
Secretion of glucagon is inhibited by glucose,
an action that emphasizes the opposing
metabolic roles of glucagon and insulin.
However, many other substances including
amino acids, fatty acids and ketones, GIT
hormones
and
neurotransmitters
affect
glucagons secretion.
Glucagon Actions
In general, the actions of glucagon oppose
those of insulin. As insulin promotes energy
storage
by
stimulating
glycogenesis,
lipogenesis, and protein synthesis, glucagons
causes rapid mobilization of potential energy
sources
into
glucose
by
stimulating
glycogenolysis and into fatty acids by
stimulating lipolysis. It is also the most potent
gluconeogenic hormone.
The liver is the primary site of action
for glucagon where it binds to specific
cell surface receptors and its action is
mediated through c- AMP mechanism. In
the liver, it inhibits ‘glycogen synthetase’
enzyme and thus decreases glycogen
synthesis, and meanwhile it promotes
glycogenolysis. This action on glycogen
metabolism is tissue specific i.e. it does
not affect muscle glycogen.
It promotes the synthesis of more [PEPCK]
enzyme and thus the conversion of more
amino acids to glucose through the process
of gluconeogenesis and this is the opposite
action to insulin which decreases the gene
transcription of PEPCK. In the adipose tissue
it stimulates the[ hormone- sensitive lipase]
enzyme resulting in the increased rate of
lipolysis, this enzyme induction is through
increasing adipose cell c- AMP level.