Studies on sulphatases. 22. The anomalous kinetics of arylsulphatase A of human tissues: interpretation of the anomalies

Abstract

The anomalous time course of the reaction between human arylsulfatase A and dipotassium 2-hydroxy-5-nitrophenyl sulfate is markedly affected by the products of the reaction, i.e. 4-nitrocatechol and SO42- ions. Both these products, when present in small amounts, are without effect on the initial stages of the enzyme reaction but exert an activating effect during the later stages of the reaction. The effect of SO42- ions can be abolished by addition of a slight excess of Ba2+ ions and subsequently restored by the further addition of a slight excess of SO42- ions. PO43-, P2O74- and F- ions are similar to SO42- ions with regard to their effect on the time course of the enzyme reaction. When present in higher concentrations, these ions inhibit enzyme activity. A modified form of the enzyme can be separated from the incubation mixture at the end of the initial period of high enzyme activity. The modified form of enzyme can be reconverted into the original form by incubation with sodium acetate solution at pH 8A possible explanation of the anomalous kinetics of arylsulfatase A is offered in which it is suggested that the observed effects result from the slow exposure of a 2d substrate-binding site as the enzymic reaction proceeds. This 2d site can bind substrate to give a form of enzyme which is virtually inactive or it can bind SO42-, PO43- and P2O74- ions and 4-nitrocatechol to give a form of enzyme which is still active. The final rate of reaction is governed by the relative affinity of the new site for substrate or reaction products.