Q5 LONG ANSWER 10M ★ High-yield

Describe the diagnostic importance of serum enzymes. Give the enzyme profile in myocardial infarction and liver disease. Add a note on isoenzymes.

Biochemistry · 1st Professional — Pre-Clinical

Principle

Diagnostic enzymology = measurement of enzyme activity in plasma to detect and monitor tissue damage.Enzymes appear in plasma when cells are damaged, when their turnover increases, or when their excretion is obstructed.

Group	Meaning	Example
Plasma-specific (functional)	Act in plasma; synthesised by the liver	Clotting factors, lipoprotein lipase, pseudocholinesterase
Non-plasma-specific	Have no function in plasma; leak from tissue	All the diagnostic enzymes
Secretory	Normally secreted into a duct	Amylase, lipase, alkaline phosphatase

Cardiac Markers

Marker	Rises	Peaks	Returns to normal	Comment
Myoglobin	1–3 h	6–9 h	24 h	Earliest but non-specific
CK-MB	4–6 h	24 h	48–72 h	Best for detecting RE-infarction, as it normalises quickly
Troponin I and T	4–6 h	24–48 h	7–10 days	Most sensitive and specific; the gold standard
Ast	12 h	24–48 h	3–5 days	Non-specific
LDH	12–24 h	48–72 h	8–14 days	Useful in late presentation

Troponin has replaced all others for diagnosis; the others retain value only for timing

Liver Enzymes

Enzyme	Chief significance
ALT (SGPT)	More specific for liver; markedly raised in viral hepatitis (> 10×)
Ast (SGOT)	Liver, heart, muscle. ast:ALT > 2 suggests alcoholic liver disease
Alkaline phosphatase	Markedly raised in obstructive jaundice and bone disease (Paget, rickets, metastases)
GGT	Most sensitive index of alcohol intake; rises with ALP in cholestasis, confirming a hepatic rather than bony source
5′-nucleotidase	Confirms that a raised ALP is of hepatobiliary origin

Other Diagnostic Enzymes

Enzyme	Raised in
Amylase and lipase	Acute pancreatitis; lipase is more specific and stays up longer
Creatine kinase (CK-MM)	Duchenne muscular dystrophy (very high), rhabdomyolysis, myositis
Acid phosphatase	Carcinoma prostate (now superseded by PSA)
Pseudocholinesterase	Decreased in liver disease and organophosphate poisoning
Aldolase	Muscle disease
ACE	Sarcoidosis

Isoenzymes and Their Diagnostic Value

Isoenzymes = physically distinct forms of an enzyme catalysing the same reaction, but differing in structure, kinetics and electrophoretic mobility.

Lactate dehydrogenase — 5 isoenzymes

A tetramer of H (heart) and M (muscle) subunits
LDH-1 (H4) — heart and RBC
LDH-2 (H3M) — reticuloendothelial system; normally the highest in serum
LDH-3 — lung; LDH-4 — kidney, placenta
LDH-5 (M4) — liver and skeletal muscle
"Flipped pattern" LDH-1 > LDH-2 indicates myocardial infarction

Creatine kinase — 3 isoenzymes

CK-MM — skeletal muscle; CK-MB — heart; CK-BB — brain

Factors to Consider Before Interpreting a Raised Enzyme

Tissue specificity — how confined is the enzyme to one organ?
Timing — when did the injury occur relative to the sample?
Magnitude — a rise of 10× means something different from 1.5×
Half-life — determines how long the abnormality persists
Route of clearance — renal or hepatic failure may raise levels without tissue damage
Physiological causes — ALP rises in growing children, in pregnancy, and after a fatty meal (intestinal isoenzyme)
Artefacts — haemolysis of the sample falsely raises LDH, ast and potassium

Applied Aspects

Enzymes are used therapeutically too — streptokinase and urokinase as thrombolytics, asparaginase in leukaemia, pancreatin in pancreatic insufficiency
Enzymes as laboratory reagents — glucose oxidase for blood glucose, urease for urea, and horseradish peroxidase in ELISA
Timing matters — a normal troponin within 4 hours of chest pain does not exclude infarction; the test must be repeated
The pattern of enzymes is more informative than any single value — ALT with ALP, or ast with ALT, localises the disease far better than either alone
Serial measurement beats a single reading — a falling trend indicates recovery, a rising one continuing damage
Normal enzymes do not exclude disease — in established cirrhosis the transaminases may be normal because little viable liver remains

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