Studies on Alkaline Phosphatase

Abstract

The three common homozygous variants of human placental alkaline phosphatase, types F 1 , S 1 and I 1 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 I 1 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 I 1 subunit, approximately one half that of the F 1 and S 1 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.