Studies on the subunit composition of rat liver glutathione S-transferases

Native glutathione S-transferases are composed of subunits with apparent molecular weights of 25,000, 23,500, or 22,000 which form either homo- or heterodimers. Glutathione S-transferases A, C, and X which contain two subunits with molecular weights of 23,500 yielded similar but nonidentical proteolytic fragmentation patterns. Fragments unique to the subunits of the homodimers A and X were present in decreased intensities in the patterns of form C. Two-dimensional electrophoresis under denaturing conditions showed single nonoverlapping spots for transferases A and X, while form C yielded two spots corresponding in position to those obtained from forms A and X. Renaturation of dissociated glutathione S-transferase C yielded enzymatically active transferases A, C, and X. These results indicate that form C is a heterodimer composed of one subunit from the homodimeric transferases A and X. This was substantiated by NH2-terminal sequence analysis showing extensive NH2-terminal homology amongst all three forms. However, in the positions where forms A and X yielded different residues, both amino acids were detected in the sequence of form C, indicating that the two subunits of Mr = 23,500 are the products of two different genes. NH2-terminal sequence analysis of the heterodimeric glutathione S-transferase B which is composed of subunits with molecular weights of 22,000 and 25,000 revealed a single unique sequence which bore no resemblance to the sequences of either forms A or X. Despite the identical NH2-terminal sequences, proteolytic fragmentation of the separated subunits showed markedly different fragmentation patterns. This indicates that two different mRNAs code for these two subunits.