Species-specific inhibition of homologous enzymes by modification of nonconserved amino acids residues. The cysteine residues of triosephosphate isomerase.

The possibility of using non-conserved amino acid residues to produce selective inhibition of homologous enzymes from different species has been further explored with triosephosphate isomerase. S-phenyl-p-toluenethiosulfonate (MePhSO2-SPh), which produces phenyl disulfides with accessible Cys residues, inhibits the activity of rabbit triosephosphate isomerase. The inhibition is due to derivatization of one of the five Cys residues of rabbit triosephosphate isomerase. The effect of MePhSO2-SPh on triosephosphate isomerase from Saccharomyces cerevisiae, Escherichia coli, chicken and Schizosaccharomyces pombe was also determined. MePhSO2-SPh did not affect the activity of triosephosphate isomerase from S. cerevisiae and E. coli but it inhibited triosephosphate isomerase from chicken and S. pombe. From an analysis of the Cys content of the various triosephosphate isomerases, it was evident that amongst the ones studied only those that have a Cys in position 217 (or in an equivalent position) were sensitive to MePhSO2-SPh. Methyl metanethiosulfonate (MeSO2-SMe), which produces methyl disulfides, had no effect on triosephosphate isomerases that lack Cys217 (S. cerevisiae and E. coli). In triosephosphate isomerases that have Cys217, MeSO2-SMe inhibited by 40-50% the activity of that from S. pombe, 20-25% that from rabbit but had no effect on the chicken enzyme. In the three latter triosephosphate isomerases, MeSO2-SMe protected against the strong inhibiting action of MePhSO2-SPh. The latter observations suggest that MeSO2-SMe and MePhSO2-SPh derivatize the same Cys and that significant inhibition of activity requires perturbation by the relatively large phenyl group. The intrinsic fluorescence of rabbit triosephosphate isomerase that had been derivatized to a phenyl disulfide was almost identical to that of the native enzyme. Thus, modification of Cys217 did not produce gross structural alterations, albeit it brought about important kinetic alterations, i.e. a nearly fivefold increase in the K(m) for glyceraldehyde 3-phosphate and a 65% decrease in Vmax. The effect of derivatizating Cys217 differs markedly from that produced by derivatization of Cys14 (another non-conserved cysteine). The differences may be explained from their position in the three-dimensional structure of the enzyme.