Inherited occurrence of a heat stable alkaline phosphatase in the absence of malignant disease.

We describe a family with an inherited persistent elevation of serum alkaline phosphatase activity in the absence of malignant disease, observed for at least 15 yr. Isoenzyme studies revealed that this increased activity was due to an enzyme which showed similarities to serum placental alkaline phosphatase from pregnant women having the following properties: high heat stability; reactivity to anti-placental alkaline phosphatase antiserum; lack of inhibition by L-homoarginine; moderate inhibition by EDTA; and lack of interaction with wheat germ lectin. The enzyme was less sensitive than placental alkaline phosphatase to inhibition by L-phenylalanine, L-tryptophan, L-leucine, L-leucyl-glycyl-glycine and L-phenylalanyl-glycyl-glycine. The enzyme also differed from the placental alkaline phosphatase in its electrophoretic mobility, isoelectric heterogeneity and apparent molecular mass. We conclude that the enzyme is an inherited heat stable alkaline phosphatase variant which might correspond to a rare phenotype of placental alkaline phosphatase.