Nonequivalence of the Two Subunits of Horse Erythrocyte Glutathione Transferase in Their Reaction with Sulfhydryl Reagents

Glutathione transferase (EC 2.5.1.18) from horse erythrocytes has been purified and some molecular and kinetic properties have been investigated. It appears to be a dimeric protein composed of subunits of about 23 kDa, indistinguishable either in sodium dodecyl sulfate or in urea electrophoresis. Amino acid composition, substrate specificities, sensitivity to inhibitors, CD spectra, and immunological studies provide evidence that the horse enzyme is related to the pi class transferases. This enzyme has only two reactive thiol groups/dimer whose integrity appears to be essential for the activity. A peculiar feature of these protein thiol groups is that they react nonidentically with a number of thiol blocking reagents, i.e. iodacetamide, bromopyruvate, N-ethylmaleimide, and 1-chloro-2,4-dinitrobenzene. Also many disulfides react with one thiol group 5- to 10-fold more rapidly than with the other. The two mixed disulfides so formed also have different rates of reactivation by dithiothreitol. All the structural and kinetic data reported in this paper indicate a nonsymmetrical association of two identical subunits, or alternatively heterodimeric structure with subunits of very similar charge and size.