4-Aminobutryrate aminotransferase,the reaction of lysine residues connected with enzymatic activity

4-Aminobutyrate aminotransferase from pig brain is inactivated by incubation with o-phthalaldehyde at pH 7.4. The reaction of 6 lysil residues per dimer brings about 90% loss of aminotransferase activity. The substrate 2-oxoglutarate at concentrations higher than the KM 0.1 mM affords complete protection against inactivation. Several lines of experimental evidence indicate that o-phthalaldehyde reacts with lysyl residues other than those involved in the binding of Pyridoxal-5-P.It is postulated that the carboxyl groups of 2-oxoglutarate interacts with positively charged lysyl residues located at the catalytic site.