Syncatalytic sulfhydryl group modification in mitochondrial aspartate aminotransferase from chicken and pig heart

One sulfhydryl group of the mitochondrial isoenzyme of aspartate aminotransferase from both chicken and pig heart exhibits syncatalytic reactivity changes similar to those found previously in the cytosolic isoenzyme from pig heart (Birchmeier, W., Wilson, K.J., and Christen, P. (1973) J. Biol. Chem. $\text{248}$, 1751–1759). The reactivity of the only titratable sulfhydryl group toward 5,5′-dithiobis-(2-nitrobenzoate) is at a minimum in the free pyridoxal and pyridoxamine form of the enzyme and is increased by approximately one order of magnitude when covalent enzyme-substrate intermediates are formed. The modification of the sulfhydryl group does not affect enzymatic activity. This finding supports the earlier conclusion that the syncatalytic reactivity changes are not due to a direct participation of this group in the active site but rather to conformational adaptations of the enzyme-coenzyme-substrate compound occurring in the catalytic mechanism of aspartate aminotransferases.