After heating at 56°C for 10 minutes, what are the expected residual activities for bone and liver ALP?

• A. Bone: <90%; Liver: <50%
• B. Bone: <50%; Liver: <90%
• C. Bone: <70%; Liver: <30%
• D. Bone: <100%; Liver: <60%

Answer: (A)

Different ALP isoenzymes have distinct heat stability, which allows a heat inactivation test to differentiate their contributions. Bone alkaline phosphatase is relatively heat-stable, while liver alkaline phosphatase is more heat-labile.

When a serum sample is heated to 56°C for 10 minutes, much of the liver ALP activity is inactivated, leaving less than half of its original activity. The bone ALP, being more resistant to heat, retains much of its activity, though not perfectly unchanged. In practice, this pattern yields a residual activity for bone that is reduced somewhat but typically remains below about 90% of the original, whereas liver activity drops to below about 50%.

Thus the expected residuals after this heat treatment are bone under 90% of the original activity and liver under 50% of the original activity. This reflects the greater heat stability of bone ALP compared with liver ALP.