Acylation of aspartate aminotransferase.

1. Acetylation of aspartate aminotransferase from pig heart inhibits completely the enzymic activity when the coenzyme is in the amino form (pyridoxamine phosphate) or when the coenzyme has been removed, but not when the coenzyme is in the aldehyde form (pyridoxal phosphate). 2. The group the acylation of which is responsible for the inhibition has been identified with the in-amino group of a lysine residue at the coenzyme-binding site. Moreover, in the pyridoxamine-enzyme the amino group of the coenzyme is also acetylated. 3. The reactivity of the coenzyme-binding lysine residue is greatly different in the pyridoxamine-enzyme and in the apoenzyme, suggesting the possibility of an interaction of its in-amino group with pyridoxamine or with other groups on the protein.