Enzymes in Body Fluids
Lecture outline
Dr. Essam H. Jiffri
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Upon completion of this chapter,
the following will be covered:
- Introduction
- Factors affecting serum enzyme
• Rate of entry of enzymes into blood
• Enzyme inhibitors
• Clearance of enzyme
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Specificity of serum enzyme
measurements
• Test results and clinical features
• Test pattern recognition
• Isoenzymes
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Major enzymes of diagnostic
interest
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Phosphatases
Transaminases
G-Glutamyl transferase
Amylase and Lipase
Cholinesterase
Creatine kinase
Lactate dehydrogenase
Serum enzymes in disease
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Myocardial infarction
Muscle disease
Liver disease
Bone disease
Enzymes in urine
Haematological disorders
Tissue enzymes
- Methods for the determination in serum
and urine
- Case histories
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Introduction
-Enzymes are protein catalysts which are found in
small amounts, mainly within cells such as
clotting factors.
-Most enzymes with diagnostic applications
function within the cells in which they are
synthesized and since they have a large
molecular mass, they do not cross cell
membranes readily.
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Introduction
-Normally only small quantities of intracellular
enzymes leak from cells into blood or other body
fluids.
-The amounts are too low for enzyme mass to be
measured, their activities can be monitored.
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Introduction
-Most clinical enzyme measurements using serum,
occasionally other fluids, such as urine and gut
secretions, are investigated.
-In general, increased rather than decreased
activities of enzymes are of diagnostic interest in
body.
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Factors Affecting Serum
Enzyme Activities
The activity of an enzyme in the circulation
depends on a balance between:
1- The rate of release from tissues (rate of entry
of enzyme into blood),
2- The presence of inhibitors, and
3- The rate of removal
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1.Rate of entry of enzyme
into blood
The main factors affecting the rate of entry of
enzyme into blood are:
• The rate of synthesis
• The mass of enzyme producing cells, and
• Cell damage
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Enzyme Synthesis
-The rate of enzyme synthesis is increased
particularly in conditions affecting the liver.
-Biliary obstruction causes increased synthesis of
enzyme located in the hepatobiliary tree.
-Some agents induce increased synthesis of
enzyme by hepatocytes, example
( phenobarbitone and phenyrtoin).
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Mass of Enzyme Producing cells
-Serum alkaline phosphatase originating from
bone reflects osteoblastic activity, this is
increased (leading to high serum alkaline
phosphatase activity in children who are actively
growing or where bone disease is present in
which increased osteoblastic activity occurs, e.g.
Paget’s disease).
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Mass of Enzyme Producing cells
-The placenta produces alkaline phosphatase,
causing increased levels in the third trimester.
-Metastatic carcinoma of the prostate produces
increased acid phosphatase levels.
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Cell Damage
-Increased amounts may leak from tissues that are
inflamed, necrotic, or metabolically abnormal,
leading to increased serum levels.
-Examples include raised transaminase levels in
hepatitis, creatine kinase (CK) following
myocardial infarction and lactate dehydrogenase
(LDH).
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2. Enzyme Inhibitors
-Organophosphate poisoning which irreversibly
inhibits cholinesterase.
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3. Clearance of Enzymes
-Serum enzyme activity is also affected by the rate of
removal of enzymes from the circulation, and
understanding of these mechanisms is incomplete.
-Possibilities include removal by reticuloendothelial
system.
-Renal excretion appears unimportant, except for
amylase which is small enough to be cleared by
the kidney.
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FACTORS AFFECTING SERUM
ENZYME ACTIVITY
Tissue damage
Rate of synthesis
Mass of enzyme
producing tissue
Rate of entry into blood
Inhibition
Serum enzyme activity
Rate of removal
Clearance
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Inactivation
Specificity of Serum Enzyme
Measurements
- Many enzymes which are used diagnostically
originate from more than one tissue which
potentially limits their specificity.
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Enzyme
Origin
Main applications
Acid phosphatase
Prostate, erythrocytes
Metastatic carcinoma of
prostate
Alanine aminotransferase (ALT,
SGPT)
Hepatocytes
Kidney
Hepatocelluar disease
Alkaline phosphatase (ALP)
Hepatobiliary tree
Bone
GI tract, placenta, kidney
Cholestatic disease
Bone disease
Lipase
Amylase
Pancrease
Pancrease, Salivary glands
Acute pancreatitis
Acute pancreatitis
Aspartate aminotransferase (AST,
SGOT)
Hepatocytes
Cardiac muscle
Skeletal muscle
Hepatocelluar disease
Myocardial infarction
Muscle disease
Cholinesterase
Liver
Organophosphorous
poisoning
Creatine kinase (CK)
Skeletal Muscle
Heart Muscle
Brain
Muscle disease
Myocardial infarction
y-Glutamyl transferase (GGT)
Liver
Pancrease
Kideny
Cholestasis
Alcohol abuse
Lactate dehydrogenase (LDH)
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Cardiac muscle
Skeletal muscle
Erythrocytes, liver
Myocardial infarction
Specificity of Serum Enzyme
Measurements
- Increased serum CK could be due to myocardial
infarction or skeletal muscle disease, and
increased LDH occurs through multiple causes.
- This would limit the usefulness of enzyme
measurements if their specificity was not
increased.
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Specificity of Serum Enzyme
Measurements
- Greater specificity is achieved in three ways:
• Interpreting investigations in the light of clinical
features (test results and clinical features).
• Test pattern recognition,
• Isoenzyme determination.
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Test Results and Clinical
Features
- Serum aspartate aminotransferase (AST) activity
may be raised due to myocardial infarction or
because of diseases affecting hepatocytes, such
as viral hepatitis.
- Occasionally, increased AST may originate from
the liver because of complications of myocardial
infarction, such as congestive cardiac failure.
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Test Pattern Recognition
- Investigations are rarely done in isolation and
recognition of test patterns may aid differential
diagnosis.
- Alkaline phosphatase is raised in cholestasis and
bone disease, in cholestasis, there are often
increases in bilirubin and transaminase levels, while
these do not occur in bone disease.
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Test Pattern Recognition
-If an isolated increase in alkaline phosphatase
occurs, the estimation of gamma-glutamyltranferase may be helpful, as high serum activities
of this enzyme occur in cholestasis while levels are
normal in bone disease.
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Isoenzymes
- Multiple forms of enzymes (isoenzymes) occur
which have similar catalytic activities but
different structures.
- Different isoenzymes are often organ-specific
and their determination may improve the
specificity of enzyme tests.
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Isoenzymes
- The heterogeneity of some isoenzymes is due to
different protein subunits which are coded for by
separate genes.
- Lactate dehydrogenase has four subunits of two
different types (H and M), five isoenzymes
occur, H4 originating from the heart and M4 from
the liver.
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Isoenzymes
- Creatine kinase has two subunits, M and B;
three isoenzymes occur, BB from brain, MM
from skeletal muscle and MB from the heart.
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Isoenzymes
- Isoenzymes may be differentiated because of
different physicochemical properties by
techniques such as:
• Electrophoresis
• Immunochemical properties (immunoassay) or
• Chemical properties (differential activity for some
substrates or susceptibility to inhibitors).
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