Which CK isoenzymes are described as heat sensitive at 60°C?

• A. CK-1 and CK-2
• B. CK-MM
• C. CK-BB
• D. CK-MV

Answer: (A)

Heat sensitivity here means how readily an enzyme loses activity when exposed to high temperature. Among CK isoenzymes, one form is notably more unstable to heat than the others. The two isoforms described as heat sensitive at 60°C are the ones labeled CK-1 and CK-2. In older naming schemes, these correspond to the two heat-labile CK forms, which lose activity more quickly with heating than the muscle form. The muscle CK isoform is relatively heat-stable, so it’s not described as heat sensitive at this temperature. While CK-BB is known to be more labile, the reference here groups the heat-sensitive forms as CK-1 and CK-2, making them the best answer.