Dopa decarboxylase exhibits low pH half-transaminase and high pH oxidativedeaminase activities toward serotonin (5-hydroxytryptamine)

Dopa decarboxylase (DDC) catalyzes not only the decarboxylation of L-aromat ic amino acids but also side reactions including half-transamination of D-a romatic amino acids and oxidative deamination of aromatic amines. The latte r reaction produces, in equivalent amounts, an aromatic aldehyde or ketone (depending on the nature of the substrate), and ammonia, accompanied by O-2 consumption in a 1 :2 molar ratio with respect to the products. The kineti c mechanism and the pH dependence of the kinetic parameters have been deter mined in order to obtain information on the chemical mechanism for this rea ction toward 5-hydroxytryptamine CS-HT). The initial velocity studies indic ate that 5-HT and O-2 bind to the enzyme sequentially, and that D-Dopa is a competitive inhibitor versus 5 HT and a noncompetitive inhibitor versus O- 2. The results are consistent with a mechanism in which 5-HT binds to DDC b efore O-2. 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 similar to7.8 that must be unprotonated for catalysis. In addition to an io nizing residue with a pK value of 7.9 similar to that found in the V profil e, 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 ar-amino group of 5-HT. No titrable ionizing residue was detected in the (V/K)O-2 p rofile, 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' ph osphate formation increases below a single pK of similar to6.7. This value mirrors the spectrophotometric pK(spec) of the shift 420-384 nm of the exte rnal aldimine between DDC and 5-HT, Nevertheless, the analysis of the react ion of DDC with 5-HT under anaerobic conditions indicates that only half-tr ansamination occurs with a pH-independent rate constant over the pH range 6 -8.5. A model accounting for these data is proposed that provides alternati ve pathways leading to oxidative deamination or half-transamination.