Dopa decarboxylase exhibits low pH half‐transaminase and high pH oxidative deaminase activities toward serotonin (5‐hydroxytryptamine)

Abstract

Dopa decarboxylase (DDC) catalyzes not only the decarboxylation of l-aromatic amino acids but also side reactions including half-transamination of d-aromatic amino acids and oxidative deamination of aromatic amines. The latter reaction produces, in equivalent amounts, an aromatic aldehyde or ketone (depending on the nature of the substrate), and ammonia, accompanied by O₂ consumption in a 1 : 2 molar ratio with respect to the products. The kinetic mechanism and the pH dependence of the kinetic parameters have been determined in order to obtain information on the chemical mechanism for this reaction toward 5-hydroxytryptamine (5-HT). The initial velocity studies indicate that 5-HT and O₂ bind to the enzyme sequentially, and that d-Dopa is a competitive inhibitor versus 5-HT and a noncompetitive inhibitor versus O₂. The results are consistent with a mechanism in which 5-HT binds to DDC before O₂. The pH dependency of log V for the oxidative deaminase reaction shows that the enzyme possesses a single ionizing group with a pK value of ∼7.8 that must be unprotonated for catalysis. In addition to an ionizing residue with a pK value of 7.9 similar to that found in the V profile, the (V/K)_5-HT profile exhibits a pK value of 9.8, identical to that of free substrate. This pK was therefore tentatively assigned to the α-amino group of 5-HT. No titrable ionizing residue was detected in the (V/K)_O2 profile, in the pH range examined. Surprisingly, at pH values lower than 7, where oxidative deamination does not occur to a significant extent, a half-transamination of 5-HT takes place. The rate constant of pyridoxamine 5′-phosphate formation increases below a single pK of ∼6.7. This value mirrors the spectrophotometric pK_spec of the shift 420–384 nm of the external aldimine between DDC and 5-HT. Nevertheless, the analysis of the reaction of DDC with 5-HT under anaerobic conditions indicates that only half-transamination occurs with a pH-independent rate constant over the pH range 6–8.5. A model accounting for these data is proposed that provides alternative pathways leading to oxidative deamination or half-transamination.