MLT GlobeChemistry: Enzymes & cardiac markers – page 6
132 Chemistry MCQs on Enzymes & cardiac markers with answers and explanations.
What is the ESC-recommended NT-proBNP rule-out cut-off for acute heart failure in a breathless patient in the emergency department?
An NT-proBNP below 300 pg/mL makes acute heart failure unlikely. Higher, age-related values (450, 900, 1800 pg/mL) are used to rule in. The non-acute rule-out value is lower, 125 pg/mL.
A non-drinker on long-term phenytoin has GGT 150 U/L with normal ALT, ALP and bilirubin. The most likely explanation is:
Phenytoin and other enzyme-inducing drugs increase hepatic GGT synthesis without liver damage, so an isolated GGT rise is common. Biliary obstruction would also raise ALP.
Amylase measured on EDTA plasma gives a falsely low result because amylase requires:
Amylase is a calcium-containing metalloenzyme (also activated by chloride). EDTA and citrate chelate calcium and inhibit it, so serum or heparin plasma should be used.
A farmer has miosis, heavy salivation and bradycardia after pesticide spraying. Which result supports organophosphate poisoning?
Organophosphates irreversibly inhibit acetylcholinesterase (and serum butyrylcholinesterase), so acetylcholine accumulates. Red cell AChE best reflects inhibition at nerve synapses.
Serum tartrate-resistant acid phosphatase isoform 5b (TRAP 5b) is a marker of:
TRAP 5b is secreted by osteoclasts and reflects bone resorption. Prostatic acid phosphatase is inhibited by tartrate. Bone ALP is a marker of osteoblast (formation) activity.
LD-1, the fastest-migrating lactate dehydrogenase isoenzyme, is composed of:
LD is a tetramer of H (heart) and M (muscle) subunits. LD-1 is H4, found in heart, red cells and kidney; LD-5 is M4, found in liver and skeletal muscle.
In the lactate-to-pyruvate (forward) LD method, enzyme activity is measured as:
Lactate + NAD+ → pyruvate + NADH; NADH absorbs at 340 nm, so absorbance rises. The reverse (pyruvate-to-lactate) method measures a fall in NADH absorbance.
Under the revised Atlanta classification, which enzyme result counts as one criterion for acute pancreatitis?
Two of three are needed: typical abdominal pain, lipase or amylase at least 3 × URL, and typical imaging findings. Smaller rises occur in many non-pancreatic conditions.
A woman with fatigue, weight gain and cold intolerance has CK 900 U/L with normal troponin. She has no recent exercise, injury or injections. Which test is most useful next?
Hypothyroidism causes a myopathy with raised CK, often with these symptoms. TSH is needed before investigating for primary muscle disease. Normal troponin makes a cardiac cause unlikely.
A patient arrives 1 hour after chest pain began. The first high-sensitivity troponin is within the reference range. What is the correct next step?
Troponin may not yet be raised soon after symptom onset. Serial measurements are needed to detect a rise and/or fall that confirms myocardial injury.
Enzyme A has a Km of 0.1 mmol/L and enzyme B has a Km of 5 mmol/L for the same substrate. Which statement is correct?
Km is the substrate concentration at half-maximal velocity. A lower Km means half-maximal rate is reached at a lower substrate level, indicating higher affinity.
A 5-year-old boy has progressive leg weakness and difficulty climbing stairs. Serum CK is 15,000 U/L. Which condition is most likely?
Duchenne muscular dystrophy causes very high serum CK (often more than 10 times normal) from early childhood because of ongoing skeletal muscle damage.
On a Lineweaver–Burk plot, lines with and without an inhibitor meet on the y-axis. The inhibitor is:
Same y-intercept means unchanged Vmax, while a steeper slope shows raised apparent Km, which is competitive inhibition. Non-competitive lines meet on the x-axis; uncompetitive lines are parallel.
Why do IFCC reference methods add pyridoxal-5′-phosphate to ALT and AST assays?
Transaminases need PLP; in patients with low B6 some enzyme circulates as inactive apoenzyme, so adding PLP avoids underestimating activity. Substrates are the amino acids and α-ketoglutarate.
In metabolic control analysis, which parameter shows how much pathway flux changes when one enzyme's concentration changes?
The flux control coefficient links fractional change in flux to fractional change in enzyme amount. Elasticity describes an enzyme's sensitivity to a metabolite.
In metabolic control analysis, which term describes how sensitive one enzyme's rate is to the concentration of a metabolite?
The elasticity coefficient is a local property of one enzyme. The flux control coefficient is a system property describing the whole pathway.
Making sphingosine from serine and palmitoyl-CoA needs several cofactors. Which one is NOT used?
PLP-dependent serine palmitoyltransferase starts the pathway. NADPH reduces 3-ketosphinganine, and an FAD-linked desaturase adds the double bond at the ceramide stage.
On a Lineweaver–Burk plot, a noncompetitive inhibitor changes which value?
A noncompetitive inhibitor binds outside the active site and lowers Vmax, raising 1/Vmax (y-intercept) while Km stays the same. An unchanged y-intercept is the pattern for competitive inhibition.
Total CK is 5000 U/L and CK-MB is 40 U/L. What is the relative index, and the likely source?
Relative index = CK-MB ÷ total CK × 100 = 40 ÷ 5000 × 100 = 0.8%. Values below about 2.5–3% point to a skeletal muscle source despite a raised CK-MB.
Diadenosine pentaphosphate and AMP are added to CK reagents to:
Adenylate kinase from red cells or platelets also forms ATP and would falsely raise CK; these inhibitors block it. Sulfhydryl protection is the role of N-acetylcysteine.