Clinical Enzymology
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Enzyme activity in plasma/serum in normal healthy individuals (with steady cell turnover) represents what?
balance between its rate of liberation into the extracellular (E.C.) space and its rate of clearance/uptake from E.C. space (+ auto degradation of enzyme).
• General cases in which enzymes in plasma are elevated:
• Cell proliferation: There is an increase in cell turnover. • Non-proliferative increase in the rate of cell turnover • Cell damage
Enzymes are mainly influenced by:
pH (aminio acid sequence does not change with pH but protein’s shape may change) Temperature (directly proportional with rate of rxn; very high > 37C =denaturation)
Characteristics of enzymes
• very efficient • all are proteins (charge and conformation may be changed; (-) = slower) specific to substrates (due to receptors on membranes) • partly specific to tissues • Assay by measure of rate of specific reaction catalyzed by that enzyme
This model shows the concentration of substrate when the reaction velocity is equal to one half of the maximal velocity for the reaction
Michaelis menten model *reaction slows down and plateaus after vmax/2
measure of how well a substrate complexes with a given enzyme
binding affinity
ATP’s role in elevated enzyme activities* *??
maintenance of cell permeability (for retaining cytosolic enzyme within cells) *In the absence of added ATP to medium containing cells, the rate of escape of intracellular enzyme is strictly related to the rate of depletion of intracellular ATP.
Factors which may affect elevated enzymes:
Anoxia Hypoxia Drugs/Poison Lipid peroxidation
Factors which may affect elevated enzymes: How does anoxia affect elevated enzymes?
It may allow escape of intracellular enzyme from cells even in the absence of organ damage such as in sever cardiac/respiratory disease
Factors which may affect elevated enzymes: When does hypoxia happen?
during intense exercise or etc. (may allow transient escape of enzymes from muscle cells)
Factors which may affect elevated enzymes: examples of drugs which inhibit energy-yielding processes
chlorpromazine and promethazine (exert direct effect on cell membrane to accelerate enzyme release)
Poison (?) which causes increase in serum enzyme activities
bacterial toxins
Factors which may affect elevated enzymes: Role of electrolytes and osmotically active molecules
maintain cell integrity
This factor which affects elevated enzymes cases serum enzy elevations to be non-specific
drugs/poison
Factors which may affect elevated enzymes: Example of indices related to cell necrosis when patient is with drugs(??)
Mitochondrial enzymes (only liberated during cell necrosis e.g. Glu-DH, AST (exist in mitochondrial form, distinct from cytosolic form)
This affects release of membrane-associated enzymes
Lipid peroxidation
[Lipid Peroxidation: Hepatic (microsomal) enzyme induction] These are elevated in epileptic patients taking anti-convulsants
ALP, GGT, 5-nucleotdase
[Lipid Peroxidation: Hepatic (microsomal) enzyme induction] When do bile acids promote liberation of enzymes from damaged hepatic cell?
in biliary obstruction
[Lipid Peroxidation: Hepatic (microsomal) enzyme induction] The enzyme rate of clearance and elmination is measured through half lives. what are the half-lives of AST, ALT and CK
ALT - 6.3 days AST - 2.0 days CK - 1.4 days
Different mechanisms of enzyme clearance
• Amylase and lipase - Excreted in urine (detection of urinary amylase is essential in acute pancreatitis) • ALP excreted partly in the bile. • Reticuloendothelial system clears some enzymes. • Proteolytic degradation. • Non-biologic decay (same as where serum specimens are not kept refrigerated). • Conjugation of enzyme with Abs (enzyme loses its biological potency) e.g. CK/LDH
Deals with the enzymatic reaction from the different parts or tissues of the body as it measures enzyme activity for the diagnosis and treatment of diseases (i.e.: calcium cytotoxic injury)
Diagnostic enzymology
unit for enzyme
IU: μmol of substrate transformed per minute under standard conditions. • New unit: Katal: amount of activity that convert one mole of substrate/second (1 Katal = 6x107 IU) - (amount of enzymes that catalyzes the conversion of one micromole of substrate to product per minute)
Why are there low concenctrations of enzymes in blood?
enzymes are normally intracellular (endoenzyme) *detection only through tissue destruction (release from intracellular to extra)
examples of extracellular enzymes
amylase and lipase in digestive tract