Reaction of dopa decarboxylase with L-aromatic amino acids under aerobic and anaerobic conditions

Analysis of the reaction of dopa decarboxylase (DDC) with L-dopa reveals th at loss of decarboxylase activity with time is observed at enzyme concentra tions approximately equal to the binding constant, K-d, of the enzyme for p yridoxal 5'-phosphate (PLP). Instead, at enzyme concentrations higher than K-d the course of product formation proceeds linearly until complete consum ption of the substrate. Evidence is provided that under both experimental c onditions no pyridoxamine 5'-phosphate (PMP) is formed during the reaction and that dissociation of coenzyme occurs at low enzyme concentration, leadi ng to the formation of a PLP-L-dopa Pictet-Spengler cyclic adduct. Taken to gether, these results indicate that decarboxylation-dependent transaminatio n does not accompany the decarboxylation of L-dopa proposed previously [O'L eary and Baughn (1977) J. Biol. Chem. 252, 7168-7173]. Nevertheless, when t he reaction of DDC with L-dopa is studied under anaerobic conditions at an enzyme concentration higher than K-d, we observe that (1) the enzyme is gra dually inactivated and inactivation is associated with PMP formation and (2 ) the initial velocity of decarboxylation is approximately half of that in the presence of O-2. Similar behaviour is observed by comparing the reactio n with L-5-hydroxytryptophan occurring in aerobiosis or in anaerobiosis. Th erefore the reaction of DDC with L-aromatic amino acids seems to be under O -2 control. In contrast, the reactivity of the enzyme with D-aromatic amino acids does not change in the presence or absence of O-2. These and other r esults, together with previous results on the effect exerted by O-2 on reac tion specificity of DDC towards aromatic amines [Bertoldi, Frigeri, Paci an d Borri Voltattorni (1999) J. Biol. Chem. 274, 5514-5521], suggest a produc tive effect of O-2 on an intermediate complex of the reaction of the enzyme with L-aromatic amino acids or aromatic amines.