Serum alkaline phosphatase is elevated in a patient and the source must be identified as bone or liver. Which property separates the two isoenzymes on heat-inactivation testing?

• A The bone isoenzyme is destroyed at room temperature within minutes
• B The liver isoenzyme is more heat labile and loses activity faster at 56 degrees Celsius
• C Both isoenzymes are equally heat stable and require electrophoresis only
• D The bone isoenzyme is more heat labile and loses activity faster at 56 degrees Celsius ✓

Explanation

Alkaline phosphatase isoenzymes differ in thermal stability. When serum is heated to 56 degrees Celsius, the bone isoenzyme is inactivated much more rapidly than the liver isoenzyme, so residual activity after heating indicates a hepatic source. Placental alkaline phosphatase is even more heat resistant and survives heating to about 65 degrees Celsius. This simple test remains a classic examination point on isoenzyme characterisation.

Reference: Harper's Illustrated Biochemistry, 31st ed.

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