Selective modification of tyrosine and cysteine residues in aspartate aminotransferase from pig heart cytosol

Abstract

In the region of the active site of aspartate amino-transferase two amino acid residues — one Tyr and one Cys — are accessible to selective modification by appropriate reagents. Modification of each of the two residues singly results in certain changes of the enzyme's physico-chemical properties, but does not abolish its ability to catalyse the transamination reaction. Complete inactivation, associated with irreversible amination of the protein-bound pyridoxal-P to pyridoxamine-P, is observed only on modification of both residues.