Chemical modifications of histidine residues in cytoplasmic asparate aminotransferase from beef kidney

Summary Holo and apoenzyme of aspartate aminotransferase from beef kidney are 80% inactivated by photoxidation in the presence of 2 × 10^-6m tetraiodofluroescein with the modification of two histidine residues per enzyme protomer. At a higher concentration (1 × 10^-5m) a tyrosine residue is also modified. The keto substrates, ketoglutarate and oxalacetate, protect the enzyme from photoxidation.