Studies on alkaline phosphatase. Catalytic activity of human-placental alkaline-phosphatase variants.

Abstract

The three common homozygous variants of human placental alkaline phosphatase, types F1, S1 and I1 were partly purified and the concentration of active sites determined using a stopped‐flow procedure. A comparison of the variant steady‐state activities indicated that any differences in catalytic‐centre activity are marginal, both at alkaline pH (with 4‐nitrophenyl phosphate) where phosphorylation is rate limiting and at acid pH (with 4‐methylumbelliferyl phosphate) where dephosphorylation is rate limiting.Though the I1 variant is significantly more heat labile, it is suggested that differences in activity on starch gel and in placental tissue are due to a lower rate of synthesis of the I1 subunit, approximately one half that of the F1 and S1 subunits.It is proposed that where kinetic identity between closely‐related phosphatases is observed, especially with regard to inhibition by l‐phenylalanine, then the catalytic activities are probably identical.