Glutathione S-transferase A. A novel kinetic mechanism in which the major reaction pathway depends on substrate concentration.

Glutathione transferase A has been purified from rat liver. The enzyme catalyzes the conjugation of glutathione with compounds bearing an electrophilic site, especially those in which the electrophilic site is on, or cr to, an aromatic ring. The enzyme has a molecular weight of 45,000 and is composed of two similar subunits. Initial velocity, product inhibition, and binding studies indicate a biphasic kinetic mechanism in which the reaction pathway depends on the concentration of the substrates. At high concentrations of GSH, an ordered sequential pathway predominates in which GSH binds first. At low concentrations of GSH, a ping-pong pathway predominates in which the electrophilic substrate adds first. In accordance with a prediction of the general rate equation for the over-all mechanism, the breakpoint in the biphasic double reciprocal plot for GSH saturation was found to shift to lower GSH concentrations as the concentration of the electrophilic substrate was lowered. A numerical rate equation was developed which describes initial velocities over the entire range of substrate concentrations. An appendix presents a method for distinguishing among several formal kinetic mechanisms which yield nonlinear double reciprocal initial velocity plots as a result of multiple reaction pathways.