Evidence of a critical histidine residue in soluble aspartic aminotransferase.

Photooxidation of extramitochondrial cy-aspartate aminotransferase in the presence of methylene blue or Rose bengal leads to a loss of enzymatic activity which follows fist order kinetics. Amino acid analysis shows that histidine is the only amino acid residue significantly affected by photooxidation. Of the 8 hi&dine residues present in the enzyme monomer, 2 are oxidized rapidly at a rate identical with that of the activity loss, while the other 6 are destroyed much more slowly. The pH dependence of the rate of the photo-induced lnactivation of the enzyme corresponds to that expected for the photooxidation of imidazole groups. The behavior of the enzyme in Sephadex G-200 is identical before and after extensive photooxidation, while the starch gel electrophoretic pattern changes after photooxidation. It is concluded that the loss of enzyme activity caused by photooxidation is related to the destruction of 1 hi&dine residue.