Which statement does not accurately describe properties associated with lactate dehydrogenase?

Which statement does not accurately describe properties associated with lactate dehydrogenase?

• A. Optimum pH for the catalysis of lactate to pyruvate is 7.4-7.8.
• B. LD is increased in a hemolyzed serum specimen.
• C. LD catalyzes the oxidation of lactate to pyruvate with mediation of nicotinamide-adenine dinucleotide.
• D. LD-4 and LD-5 are labile in the cold.

Answer: (A)

The key idea is that LDH works best in alkaline conditions. The forward reaction, converting lactate to pyruvate with NAD+, reaches its optimum activity at a fairly alkaline pH, typically around pH 9.0 to 9.5 in common enzyme assays. A pH around 7.4–7.8 is closer to physiological pH and does not represent the enzyme’s peak activity for this direction, so describing it as the optimum is not accurate. The other statements reflect true properties: lactate dehydrogenase activity increases in hemolyzed serum because red blood cells release LDH when they rupture; the enzyme catalyzes lactate to pyruvate with NAD+ as the cofactor; and the LD-4 and LD-5 isoenzymes are more labile when stored cold, meaning they can degrade more quickly at low temperatures.

The key idea is that LDH works best in alkaline conditions. The forward reaction, converting lactate to pyruvate with NAD+, reaches its optimum activity at a fairly alkaline pH, typically around pH 9.0 to 9.5 in common enzyme assays. A pH around 7.4–7.8 is closer to physiological pH and does not represent the enzyme’s peak activity for this direction, so describing it as the optimum is not accurate.

The other statements reflect true properties: lactate dehydrogenase activity increases in hemolyzed serum because red blood cells release LDH when they rupture; the enzyme catalyzes lactate to pyruvate with NAD+ as the cofactor; and the LD-4 and LD-5 isoenzymes are more labile when stored cold, meaning they can degrade more quickly at low temperatures.