Reaction specificity of native and nicked 3,4-dihydroxyphenylalanine decarboxylase

3,4-Dihydroxyphenylalanine (Dopa) decarboxylase is a stereospecific pyridox al 5'-phosphate (PLP)-dependent cu-decarboxylase that converts L-aromatic a mino acids into their corresponding amines, We now report that reaction of the enzyme with D-5-hydroxytryptophan or D-Dopa results in a time-dependent inactivation and conversion of the PLP coenzyme to pyridoxamine 5'-phospha te and PLP-D-amino acid Pictet-Spengler adducts, which have been identified by high performance liquid chromatography. We also show that the reaction specificity of Dopa decarboxylase toward aromatic amines depends on the exp erimental conditions. Whereas oxidative deamination occurs under aerobic co nditions (Bertoldi, M., Moore, P. S., Maras, B., Dominici, P., and Borri Vo ltattorni, C, (1996) J. Biol. Chem. 271, 23954-23959; Bertoldi, M,, Dominic i, P., Moore, P, S,, Maras, B., and Borri Voltattorni, C. (1998) Biochemist ry 37, 6552-6561), half-transamination and Pictet-Spengler reactions take p lace under anaerobic conditions. Moreover, we examined the reaction specifi city of nicked Dopa decarboxylase, obtained by selective tryptic cleavage o f the native enzyme between Lys(334) and His(335). Although this enzymatic species does not exhibit either decarboxylase or oxidative deamination acti vities, it retains a large percentage of the native transaminase activity t oward D-aromatic amino acids and displays a slow transaminase activity towa rd aromatic amines, These transamination reactions occur concomitantly with the formation of cyclic coenzyme-substrate adducts. Together with addition al data, we thus suggest that native Dopa decarboxylase can exist as an equ ilibrium among "open," "half-open," and "closed" forms.