If less than 20% activity remains after heating serum to 56 degrees Celsius for 10 minutes, which ALP is most likely present?

The scenario describes a heat stability test for different alkaline phosphatase (ALP) isoenzymes present in serum. When serum is heated to 56 degrees Celsius for 10 minutes, certain isoenzymes exhibit different levels of heat stability.

Bone alkaline phosphatase (ALP) is known to be thermolabile, meaning it is sensitive to heat and loses activity when exposed to elevated temperatures. If the activity measured after heating is less than 20%, this significant reduction indicates that the ALP present is primarily of bone origin, as liver ALP and other isoforms tend to exhibit more thermal stability and would retain more activity following such a treatment.

In contrast, liver ALP is more heat-stable than bone ALP, often retaining more than 20% activity after the same heating. Placental and germ cell ALPs also show varying levels of heat stability, but bone ALP is the most relevant isoform associated with a marked decrease in activity under these conditions.

Therefore, based on the criteria given in the question regarding the remaining activity after heat treatment, the isoenzyme most likely present in the serum is indeed bone alkaline phosphatase.