Inhibition of aromatic L-amino acid decarboxylase by coenzyme-amino acid adducts

The coenzyme-amino acid adducts N-(5'-phosphopyridoxyl)-L-3,4-dihydroxyphenylalanine and N-(5'-phosphopyridoxyl)-L-m-NH sub(2)-tyrosine inhibit hog kidney aromatic L-amino acid decarboxylase (Dopa decarboxylase). Kinetic studies on the nature of the inhibition caused by these adducts appeared to distinguish two distinct decarboxylase activities in purified enzyme preparations. It is proposed that Dopa decarboxylase has either a second active site which has a low affinity for pyridoxal phosphate or a site(s) which when occupied by pyridoxal phosphate leads to an increase in the activity of the enzyme.