Observations on the heat-stability of alkaline phosphatase isoenzymes in serum

Serum specimens which apparently contain the same isoenzyme of alkaline phosphatase nevertheless show wide variations in the stabilities of their phosphatase activities on heating at 56°. A number of factors which may influence the stability of bone and liver phosphatases in serum have been studied and their contributions to the observed variability of serum phosphatase heat-stabilities have been assessed. Differences between specimens in protein or urea content do not appear to be significant in this respect, but differences in pH may cause variability in enzyme heat-stability. When differences between serum samples in characteristics such as pH have been taken into account, however, the range of variation which still remains suggests that a particular phosphatase isoenzyme entering the circulation in different individuals and in various diseases may not have constant stability properties.