diabetes mellitus - biochemistry
TRANSCRIPT
Diabetes mellitus is the 3rd leading cause of
death in many developed countries.
Diabetes is a major cause of blindness, renal
failure, amputation, heart attacks and stroke.
Diabetes mellitus is a characterized by
increased blood glucose level
(hyperglycemia) due to insufficient or
inefficient (incompetent) insulin.
Insulin is a polypeptide hormone produced
by the β-cells of islets of Langerhans of
pancreas.
It influences the metabolism of
carbohydrate, fat & protein.
It is an anabolic hormone, promotes the
synthesis of glycogen, triacylglycerols &
proteins.
Human insulin (mol. wt. 5,7341) contains 51
amino acids, arranged in 2 polypeptide chains.
A chain – 21 amino acids & B chain – 30 amino
acids.
Both are held together by 2 interchain disulfide
bridges, connecting A7 to B7 & A20 to B19.
There is an intrachain disulfide link in chain A
between the amino acids 6 & 11.
The gene for insulin synthesis is located on
chromosome 11.
The synthesis of insulin involves two
precursors, namely preproinsulin with 108
amino acids (mol. wt. 11,500) & proinsulin with
86 amino acids (mol. wt. 9,000).
They are sequentially degraded to form the
active hormone insulin & a connecting
peptide (C-peptide).
Insulin & C-peptide are produced in
equimolar concentration.
C-peptide is biologically inactive.
Its estimation is useful index for the
endogenous production of insulin.
In the β-cells, insulin (also proinsulin)
combines with zinc to form complexes.
In this complex form, insulin is stored in the
granules of the cytosol which is released in
response to various stimuli by exocytosis.
Factors stimulating insulin secretion:
Glucose & amino acids
Gastrointestinal hormones – secretin, gastrin,
pancreozymin increase the secretion.
Factors inhibiting insulin secretion:
Epinephrine from adrenal medulla is most
potent inhibitor of insulin secretion.
Insulin has a half-life of 4-5 minutes.
About 40-50 units of insulin is secreted daily
human pancreas.
The normal range of insulin: 20-30 μU/ml.
A protease enzyme – insulinase degrades
insulin.
Insulinase is mainly present in liver & kidney.
Effects on carbohydrate metabolism:
Insulin lowers blood glucose level by
promoting its utilization & storage & by
inhibiting its production.
Effect on glucose uptake by tissues:
Insulin is required for uptake of glucose by
muscle (skeletal, cardiac & smooth), adipose
tissue, leukocytes & mammary glands.
About 80% of glucose uptake in the body is
not dependent on insulin.
Effect on glucose utilization:
Insulin increases glycolysis in muscle & liver.
Insulin activates key enzymes of glycolysis –
glucokinase, PFK & pyruvate kinase.
Glycogen production is increased, due to
increased activity of glycogen synthase by
insulin.
Effect on glucose production:
Insulin decreases gluconeogenesis by
suppressing the enzymes pyruvate
carboxylase, phosphoenol pyruvate
carboxykinase & glucose 6- phosphatase.
Insulin also inhibits glycogenolysis by
inactivating the enzyme glycogen
phosphorylase.
Effects on lipid metabolism:
The net effect of insulin on lipid metabolism
is to reduce the release of fatty acids from
the stored fat & decrease the production of
ketone bodies.
Adipose tissue is the most sensitive to the
action of insulin.
Effect on lipogenesis:
Insulin favours the synthesis of
triacylglycerols from glucose by providing
more glycerol 3-phosphate & NADPH.
Insulin increases the activity of acetyl CoA
carboxylase, a key enzyme in fatty acid
synthesis.
Effect on lipolysis:
Insulin decreases the activity of hormone-
sensitive lipase & reduces the release of fatty
acids from stored fat.
The mobilization of fatty acids from liver is
also decreased by insulin.
Effect on ketogenesis:
Insulin reduces ketogenesis by decreasing
the activity of HMG CoA synthetase.
Effects on protein metabolism:
It stimulates the entry of amino acids into the
cells, increases protein synthesis & reduces
protein degradation.
Insulin promotes cell growth & replication.
This is mediated through certain factors such
as epidermal growth factor (EGF), platelet
derived growth factor & prostaglandins.
Insulin receptor mediated signal transduction:
Insulin receptor:
It is a tetramer consisting of 4 subunits – α2β2.
The subunits are in the glycosylated form.
They are held together by disulfide linkages.
α – subunit (mol. wt. 135,000) is extracellular &
it contains insulin binding site.
β – subunit (mw. 95,000) is a transmembrane
protein which is activated by insulin.
The cytoplasmic domain of β – subunit has
tyrosine kinase activity.
The insulin receptor is synthesized as a single
polypeptide & cleaved to α & β subunits which
are then assembled.
Insulin receptor has a half-life of 6-12 hours.
About 20,000 receptors/cell in mammals.
Signal transduction:
Insulin binds to the receptor, a conformational
change is induced in the α-subunits of insulin
receptor.
This results in the generation of signals which
are transduced to β-subunits.
The net effect is that insulin binding activates
tyrosine kinase activity of intracellular β-
subunit of insulin receptor.
This causes the autophosphorylation of
tyrosine residues on β-subunit.
Receptor tyrosine kinase also phosphorylates
insulin receptor substrate (IRS).
The phosphorylated IRS, in turn, promotes
activation of other protein kinases &
phosphatases, finally leading to biological
action.
Insulin-mediated glucose transport:
The binding of insulin to insulin receptor signals the
translocation of vesicles containing glucose
transporters from intracellular pool to the plasma
membrane.
The vesicles fuse with the membrane recruiting the
glucose transporters.
The glucose transporters are responsible for the
insulin-mediated uptake of glucose by the cells.
As the insulin level falls, the glucose
transporters move away from the membrane
to the intracellular pool for storage & recycle.
Insulin mediated enzyme synthesis:
Insulin promotes the synthesis of enzymes
such as glucokinase, PFK & pyruvate kinase.
This is brought about by increased
transcription & translation.
Glucagon, secreted by α-cells of the pancreas.
It is a polypeptide hormone composed of 29
amino acids (mol. wt. 3,500) in a single chain.
It is synthesized as proglucagon, on sequential
degradation releases active glucagon.
Its amino acid sequence is the same in all
mammalian species & half-life. i.e. about 5
minutes.
The secretion of glucagon is stimulated by
low blood glucose concentration, amino
acids derived from dietary protein & low
levels of epinephrine.
Increased blood glucose level markedly
inhibits glucagon secretion.
Glucagon enhances the blood glucose level
(hyperglycemic).
Primarily, glucagon acts on liver to cause
increased synthesis of glucose & enhanced
degradation of glycogen.
Effects on lipid metabolism:
Glucagon promotes fatty acid oxidation
resulting in energy production & ketone
body synthesis.
Effects on protein metabolism:
Glucagon increases the amino acid uptake
by liver & promotes gluconeogenesis.
The maintenance of glucose level in blood
within narrow limits is a very finely &
efficiently regulated system.
It is essential to have continuous supply of
glucose to the brain.
Following a meal, glucose is absorbed from
the intestine and enters the blood.
The rise in blood glucose level stimulates the
secretion of insulin.
The uptake of glucose by most extrahepatic
tissues, except brain is dependent on insulin.
Insulin helps in the storage of glucose as
glycogen or its conversion to fat.
Normally, 2 to 2½ hours after a meal, blood
glucose level falls to near fasting levels.
It may go down further; but this is prevented
by processes that contribute glucose to the
blood.
For another 3 hours, hepatic glycogenolysis
will take care of the blood glucose level.
Thereafter gluconeogenesis will take charge
of the situation.
Liver is the major organ that supplies the
glucose for maintaining blood glucose level.
Glucagon, epinephrine, glucocorticoids,
growth hormone, ACTH & thyroxine will keep
the blood glucose level from falling.
They are referred to as antiinsulin hormones
or hyperglycemic hormones.
Random blood sugar
Fasting blood sugar
Post-prandial blood sugar
Hyperglycemia
Hypogycemia
Glucose is estimated by GOD/POD or
hexokinase method.
Insulin:
It is produced in response to hyperglycemia.
Some amino acids, free fatty acids, ketone
bodies, drugs such as tolbutamide also cause
the secretion of insulin.
It is hypoglycemic hormone that lowers in
blood glucose level.
Glucagon:
Hypoglycemia stimulates its production.
It increases blood glucose concentration.
It enhances gluconeogenesis & glycogenolysis.
Epinephrine:
It is secreted by adrenal medulla.
It acts on muscle & liver to bring about
glycogenolysis by increasing phosphorylase
activity.
Thyroxine:
It is a hormone of thyroid gland.
It elevates blood glucose level by stimulating
hepatic glycogenolysis & gluconeogenesis.
Glucocorticoids:
Glucocorticoids increases gluconeogenesis.
The glucose utilization by extrahepatic tissues is
inhibited by glucocorticoids.
The overall effect of glucocorticoids is to elevate
blood glucose concentration.
GH & ACTH also increases blood glucose.
A fall in plasma glucose less than 50 mg/dl is
called as hypoglycemia.
Hypoglycemia is life-threatening.
The manifestations include headache,
anxiety, confusion, sweating, slurred speech,
seizures & coma, and, if not corrected, death.
Post-prandial hypoglycemia:
This is also called reactive hypoglycemia & is
observed in subjects with an elevated insulin
secretion following a meal.
This causes transient hypoglycemia & is
associated with mild symptoms.
The patient is advised to eat frequently rather
than the 3 usual meals.
Fasting hypoglycemia:
Fasting hypoglycemia is not very common.
It is observed in patients with pancreatic β-
cell tumor & hepatocellular damage.
Hypoglycemia due to alcohol intake:
Alcohol consumption causes hypoglycemia
This is due to the accumulation of NADH,
which diverts pyruvate & oxaloacetate to
form lactate & malate.
Finally gluconeogenesis is reduced due to
alcohol consumption.
Hypoglycemia due to insulin overdose:
The most common complication of insulin
therapy in diabetic patients is hypoglycemia.
This is particularly observed in patients who
are on intensive treatment.
Diabetes mellitus (DM) is a metabolic disease
due to absolute or relative insulin deficiency.
DM is a common clinical condition.
It is a major cause for morbidity & mortality.
Mainly two types.
Type 1 diabetes mellitus (T1DM).
Type 2 diabetes mellitus (T2DM).
Also known as IDDM or (less frequently)
juvenile onset diabetes, mainly occurs in
childhood (between 12 -15 years age).
IDDM accounts for about 10 to 20% of the
known diabetics.
Characterized by almost total deficiency of
insulin due to destruction of β-cells.
The β-cell destruction may be caused by drugs,
viruses or autoimmunity.
Due to certain genetic variation, the β-cells are
destroyed by immune mediated injury.
Symptoms of diabetes appear when 80-90% of
the - β cells have been destroyed.
The pancreas ultimately fails to secrete insulin
The patients of IDDM require insulin therapy.
Also called as non-insulin dependent diabetes
mellitus (NIDDM).
Accounting for 80 to 90% of diabetic population.
NIDDM occurs in adults (above 35 years) & is
less severe than IDDM.
The causative factors of NIDDM include genetic
& environmental.
NIDDM commonly occurs in obese individuals.
Gestational diabetes mellitus (GDM):
This term is used when carbohydrate intolerance is
noticed, for the first time, during a pregnancy.
A known diabetic patient, who becomes pregnant, is
not included in this category.
Glucose challenge test (GCT) is done between 22 & 24
weeks of pregnancy by giving an oral glucose load
of 50 g of glucose regardless of the time.
If the 2-hour post-glucose value is >140 mg/dl, the test
is positive.
Impaired glucose tolerance (IGT):
Also called as Impaired Glucose Regulation (IGR).
Plasma glucose values are above the normal level, but
below the diabetic levels.
In IGT, the FBS value is 110 & 126 mg/dl & PPBS value is
between 140 & 200 mg/dl.
Requires careful follow-up because IGT progresses to
frank diabetes at the rate of 2% patients per year.
Impaired fasting glycemia (IFG):
In this condition, fasting plasma glucose is
above normal (between 110 & 126 mg/dl); but
the 2 hour post-glucose value is within
normal limits (less than 140 mg/dl).
These persons need no immediate treatment;
but are to be kept under constant check up.
Secondary to other known causes:
Endocrinopathies (Cushing's disease,
thyrotoxicosis, acromegaly)
Drug induced (steroids, beta blockers, etc.)
Pancreatic diseases (chronic pancreatitis,
fibrocalculus pancreatitis, hemochromatosis,
cystic fibrosis).
The diagnosis of diabetes can be made on
the basis of individual's response to the oral
glucose load, commonly referred to as oral
glucose tolerance test (OGTT).
Preparation of the subject:
Carbohydrate-rich diet for at least 3 days
prior to the test.
All drugs known to influence carbohydrate
metabolism should be discontinued (2 days).
The subject should avoid strenuous exercise
on the previous day of the test.
Person should be in an overnight fasting
state.
During the course of GTT, the person should
be comfortably seated & should refrain from
smoking & exercise.
Glucose tolerance test should be conducted
preferably in the morning (ideal 9 to 11 AM).
A fasting blood sample is drawn and urine
collected.
The subject is given 75 g glucose orally,
dissolved in about 300 ml of water, to be
drunk in about 5 minutes.
Blood & urine samples are collected at 30
minute intervals for at least 2 hours.
All blood samples are subjected to glucose
estimation while urine samples are
qualitatively tested for glucose.
The fasting plasma glucose level is 75-110 mg/dl
in normal persons.
On oral glucose load, concentration increases
& peak value (140 mg/dl) is reached in less
than an hour which returns to normal by 2
hours.
Glucose is not detected in any of the urine
samples
In individuals with impaired glucose
tolerance, the fasting (110-126 mg/dl) as well as
2 hour (140-200 mg/dl) plasma glucose levels
are elevated.
These subjects slowly develop frank diabetes.
Dietary restriction & exercise are advocated
for the treatment of impaired glucose
tolerance.
Condition Plasma glucose concentration as mmol/l (mg/dl)
Normal IGT Diabetes
Fasting <6.1
(<110)<7.0
(<126)>7.0
(>126)
2 hours after glucose
<7.8(<140)
<11.1(<200)
>11.1(>200)
For conducting GTT in children, oral glucose is
given on the basis of weight (1.5 to 1.75 g/kg).
In case of pregnant women, 100 g oral
glucose is recommended.
Mini GTT carried out in some laboratories,
fasting and 2 hrs. sample (instead of 1/2 hr.
intervals) of blood & urine are collected.
To evaluate the glucose handling of the body
under physiological conditions, fasting blood
sample is drawn, the subject is allowed to
take heavy breakfast, blood samples are
collected at 1 hour & 2 hrs (post-prandial-
meaning after food).
Urine samples are also collected.
This type of test is commonly employed in
established diabetic patients for monitoring
the control.
For individuals with suspected malabsorption,
intravenous GTT is carried out.
Corticosteroid stressed GTT is employed to detect
latent diabetes.
Glycosuria:
The commonest cause of glucose excretion in urine
(glycosuria) is diabetes mellitus.
Glycosuria is the first line screening test for diabetes.
Normally, glucose does not appear in urine until the
plasma glucose concentration exceeds renal
threshold (180 mg/dl).
Renal glycosuria:
Renal glycosuria is a benign condition due to
a reduced renal threshold for glucose.
It is unrelated to diabetes & should not be
mistaken as diabetes.
Further, it is not accompanied by the classical
symptoms of diabetes.
Alimentary glycosuria:
In certain individuals, blood glucose level rises
rapidly after meals resulting in its spill over
into urine.
This condition is referred to as alimentary
glycosuria.
It is observed in some normal people & in
patients of hepatic diseases, hyperthyroidism
& peptic ulcer.
Hyperglycemia:
Elevation of blood glucose concentration is the
hallmark of uncontrolled diabetes.
Hyperglycemia is primarily due to reduced
glucose uptake by tissues & its increased
production via gluconeogenesis &
glycogenolys.
Glucose is excreted into urine (glycosuria).
Ketoacidosis:
Increased mobilization of fatty acids results
in overproduction of ketone bodies which
often leads to ketoacidosis.
Hypertriglyceridemia:
Conversion of fatty acids to TAGs & secretion
of VLDL & chylomicrons is higher in diabetics.
Plasma levels of VLDL, chylomicrans, TAGs &
cholesterol are increased.
Glycosuria – glucose excretion in urine.
Due to osmotic effect, more water
accompanies the glucose (polyuria).
To compensate for this loss of water, thirst
center is activated & more water is taken
(polydypsia).
To compensate the loss of glucose & protein,
patient will take more food (polyphagia).
Diabetic keto acidosis (DKA):
DKA more common in T1DM.
Normally the blood level of ketone bodies is
<1 mg/dl & only traces are excreted in urine.
Increased synthesis causes the accumulation
of ketone bodies in blood.
It causes ketonemia, ketonuria & smell of
acetone in breath.
Together constitute ketosis.
Detected by Rothera's test.
Supportive evidence may be derived from
estimation of serum electrolytes, acid–base
parameters & glucose estimation.
The urine of a patient with diabetic keto
acidosis will give positive Benedict's test as
well as Rothera's test.
But in starvation ketosis, Benedict's test is
negative, but Rothera's test will be positive.
Diabetes Mellitus:
The combination of hyperglycemia,
glucosuria, ketonuria & ketonemia is called
diabetic ketoacidosis (DKA).
Untreated diabetes mellitus is the most
common cause for ketosis.
Deficiency of insulin causes accelerated
lipolysis & more fatty acids are released into
circulation.
Oxidation of these fatty acids increases the
acetyl CoA pool.
Enhanced gluconeogenesis restricts the
oxidation of acetyl CoA by TCA cycle, since
availability of oxaloacetate is less.
In starvation, dietary supply of glucose is
decreased.
Available oxaloacetate is channelled to
gluconeogenesis.
The increased rate of lipolysis provides
excess acetyl CoA which is channeled to
ketone bodies.
The high glucagon favors ketogenesis.
Hyperemesis (vomiting) in early pregnancy may
also lead to starvation-like condition & may lead to
ketosis.
In both diabetes mellitus & starvation, the
oxaloacetate is channelled to gluconeogenesis.
Acetyl CoA cannot be fully oxidized in TCA cycle.
This excess acetyl CoA is channelled into ketogenic
pathway.
Metabolic acidosis:
Acetoacetate & β-hydroxy butyrate are
accumulated, causes metabolic acidosis.
There will be increased anion gap.
Reduced buffers:
The plasma bicarbonate is used up for
buffering of these acids.
Kussmaul's respiration:
Patients will have typical acidotic breathing due to
compensatory hyperventilation.
Smell of acetone in patient's breath.
Osmotic diuresis induced by ketonuria may lead to
dehydration.
Sodium loss:
The ketone bodies are excreted in urine as their
sodium salt, leading to loss of cations from the body.
High potassium:
Due to lowered uptake of potassium by cells
in the absence of insulin.
Dehydration:
Sodium loss further aggravates dehydration.
Coma:
Hypokalemia, dehydration & acidosis
contribute to the lethal effect of ketosis.
Parenteral administration of insulin & glucose.
Intravenous bicarbonate to correct acidosis.
Correction of water imbalance by normal
saline.
Correction of electrolyte imbalance.
Hyperglycemia is directly or indirectly
associated with several complications.
These include
Atherosclerosis
Retinopathy
Nephropathy
Neuropathy.
Dietary management:
A diabetic patient is advised to consume low
calories (i.e. low carbohydrate & fat), high
protein & fiber rich diet.
Diet control & exercise will help to a large
extent obese NIDDM patients.
Hypoglycemic drugs:
The oral hypoglycemic drugs are broadly of
two categories-sulfonylureas & biguanides.
Sulfonylurea such as acetohexamide,
tolbutamide & gibenclamide are frequently
used.
They promote the secretion of endogenous
insulin & help in reducing blood glucose level.
Management with insulin:
Two types of insulin preparations are
commercially available – short acting & long
acting.
The short acting insulins are unmodified &
their action lasts for about 6 hours.
The long acting insulins are modified ones &
act for several hours, which depends on the
type of preparation.
Glycated hemoglobin:
Refers to the glucose derived products of normal adult
hemoglobin (HbA).
Glycation is a post-translational, non-enzymatic
addition of sugar residue to amino acids of proteins.
Among the glycated hemoglobins, the most abundant
form is HbA1c.
HbA1c is produced by the condensation of glucose
with N-terminal valine of each β-chain of HbA.
The rate of synthesis of HbA1c is directly related to
the exposure of RBC to glucose.
The concentration of HbA1c serves as an indication of
the blood glucose concentration over a period.
HbA1c concentration is about 3-5%.
In diabetic patients, HbA1c is elevated (15%).
HbA1c reflects the mean blood glucose level over 2
months period prior to its measurement.
Other proteins in the blood are glycated.
Glycated serum proteins (fructosamine) can
also be measured in diabetics.
Albumin is the most abundant plasma
protein, glycated albumin largely contributes
to plasma fructosamine measurements.
Albumin has shorter half-life than Hb.
Glycated albumin represents glucose status
over 3 weeks prior to its determination.
Microalbuminuria is defined as the excretion of 30-
300 mg of albumin in urine per day.
Microalbuminuria represents an intermediary stage
between normal albumin excretion (2.5-30 mg/d) &
macroalbuminuria (>300 mg/d).
The small increase in albumin excretion predicts
impairment in renal function in diabetic patients.
It indicates reversible renal damage.
Textbook of Biochemistry – U Satyanarayana
Textbook of Biochemistry – DM Vasudevan