Simple inhibition studies for distinction between homodimeric and heterodimeric isoenzymes of glutathione transferase

Simple inhibition studies in which fractional velocity is measured as a function of inhibitor concentration were used to distinguish heterodimeric from homodimeric isoenzymes of glutathione transferase. Homodimeric isoenzymes from rat, mouse, and human tissues were shown to give graphs of fractional velocity versus the logarithm of inhibitor concentration that were characterized by a sigmoid curve shape and a maximal slope of -0.58 at 50% inhibition, characteristic for linear inhibition of monomeric or non-cooperative oligomeric enzymes. In contrast, inhibition curves for heterodimeric isoenzymes from rat liver displayed significant deviations from these characteristics. The basis for the identification of heterodimers was the finding that the kinetic properties of subunits were additive such that the inhibition curve of a heterodimeric isoenzyme could be predicted from those of the corresponding homodimers. The method should be valuable in the differentiation between the multiple forms of glutathione transferase in tissues not previously characterized. It is suggested that the method should be applicable for discrimination also in other isoenzyme families consisting of oligomeric structures of identical and nonidentical subunits.