Objectives – Learning outcome Flashcards
Objections
The meaning of diagnostic enzymes
● Characteristics of enzymes
● Some enzymes used for disease diagnosis
● Sources and uses of some diagnostic
plasma enzymes
● What causes decrease or increase in plasma
enzymes
Characteristics of enzymes
Enzymes are proteins that act as biological
catalysts, altering the rate, and providing a means
of regulating metabolic reactions
● most easily measured in terms of biological
activity rather than their mass
● fairly constant in healthy individuals, some
increase after muscular exercise (CK) or after a
meal (intestinal isoenzymes of ALP)
Characteristics of enzymes cont’d
Most diagnostic enzymes have half-life of 10
hours to >5 days
● enzyme activity is usually measured -
concentrations can also be measured
● unit of measurement of enzyme is International
Unit (IU): defined as ‘that amount of enzyme
which under given assay conditions will catalyse
the conversion of 1
mol of substance per
minute’.
Causes of enzyme increase in plasma:
severe damage to cell – ischaemia or toxic
substances mostly affecting cytoplasmic enzymes
(e.g. ALT)
● increased rate of cell turnover – occurs during
periods of active growth (physiological e.g. ALP),
tissue repair or in association with several forms of
malignant disease (cancer)
Causes of enzyme increase in plasma cont’d
increased concentration of enzymes within
cells – enzymes whose synthesis is induced by
disease or drugs e.g.GT synthesis is induced by
ethanol
duct obstruction – occurs in enzymes that are
normally present in exocrine secretions. This is
regurgitated into the blood if the normal route of
outflow is obstructed – e.g. ALP
Causes of decreased levels of plasma
enzymes
reduced cell population (severe liver failure)
● genetic deficiencies (e.g.
hypophosphatasemia)
● reduced activity (organophosphate poisoning
of pseudocholinesterase)
Factors affecting the numerical values of
enzyme results
nature and concentration of substrate
● reaction temperature
● pH (H+ concentration)
● type of buffer
● nature and concentration of co-factors etc
Questions to consider before requesting to do
plasma enzyme tests
which organ/tissue is damaged? - test specificity
● what is the extent of damage? - test sensitivity
● Is the plasma enzyme activity changing during
the course of the disease?
Choose a biomarker ye clerkship
Isoenzymes
These are proteins that possess similar
catalytic activity but show genetically
determined differences in their structure and
certain other properties such as
electrophoretic mobility, stability to heat etc.
● Isoenzymes aid in localizing the tissue of
origin in an increased plasma enzyme
activity.
Common diagnostic plasma enzymes
- Amylase:
Source: pancreas, salivary gland, lungs,
fallopian tubes, ovary and prostrate.
Half life: ~ 12 h
Use: diagnosis of acute pancreatitis,
alcoholism, biliary tract disease,
hyperlipidaemia, truma etc
Common diagnostic plasma enzymes cont’d
- Creatine Kinase
- has 2 subunits of 2 polypeptides designated M or B.
These subunits combine to form 3 isoenzymes MM,
MB and BB.
Source: MM skeletal and cardiac muscle MB cardiac muscle
BB brain (present in CSF)
Use: diagnosis of cardiac diseases:- myocardial
infarction (MI), myocarditis, skeletal muscle
damage etc
Common diagnostic plasma enzymes cont’d
- Aspartate aminotransferase (AST)
Source: heart, liver, skeletal muscle,
pancreas, kidney,
erythrocytes.
Use: diagnosis of myocardial
disease, liver disease, skeletal
muscle disease etc
Common diagnostic plasma enzymes cont’d
- Alanine aminotransferase (ALT)
Source as above but liver
dominant.
Use diagnosis of hepatocellular
damage, more liver specific than
AST. Raised in cardiac failure,
muscle disease etc
Common diagnostic plasma enzymes cont’d
- Lactate dehydrogenase (LD)
- LD is a tetramer with 2 subunit H (heart) and M
(muscle) & 5 isoenzymes - LD1, LD2, LD3, LD4,
LD5
LD1 & LD2 - heart
LD5 - liver
Source: widespread cytosolic enzyme,
Major: heart, skeletal muscle, liver, kidney, erythrocytes etc.
Use: diagnosis of myocardial disease, liver disease, skeletal muscle disease etc
Common diagnostic plasma enzymes cont’d
- Alkaline phosphate (ALP)
- has 4-5 known but poorly characterised
isoenzymes
Source: bone (osteoblasts), liver, intestine, kidney, placenta.
Use: diagnosis of liver disease (cholestasis), bone disease (osteoblastic
activity) e.g.
Paget’s disease, hyperparathyroidism, malignancy, bone tumours, liver tumours
etc
