The sulphatase of ox liver. XXI: Kinetic studies of the substrate-induced inactivation of sulphatase A

Abstract

The theoretical basis is given for methods of determining the apparent velocity constant, k ∗ , for the substrate-induced inactivation of sulphatase A (aryl-sulphate sulphohydrolase, EC 3.1.6.1) and the initial velocity, v o , of the catalytic reaction. The expression is of the same form as the empirical relationships previously used but the significance of the various terms is clearly established. The method has been applied to the characterisation of the inactivation occurring during the hydrolysis of a number of substrates and it has been shown that k ∗ varies with s o in a hyperbolic relationship described by k, a velocity constant at infinite substrate concentraions and by K, a constant analogous to the Michaelis constant. Although K varies considerably for different susbtrates, and is consistently less than the corresponding K m , k is almost constant at 0.23 min −1. It is therefore suggested that the inactivation of the enzyme does not proceed through an enzyme · substrate complex but through the enzyme × SO 2− 4 complex produced during the catalytic reaction. The effects of several variables on these parameters are described.