O-18 KINETIC ISOTOPE EFFECTS IN THE DOPAMINE BETA-MONOOXYGENASE REACTION - EVIDENCE FOR A NEW CHEMICAL MECHANISM IN NONHEME METALLOMONOOXYGENASES

Previous studies of dopamine beta-monooxygenase (DbetaM) have implicat ed the formation of a substrate-derived benzylic radical via a hydroge n atom abstraction mechanism [Miller & Klinman (1985) Biochemistry 24, 2114]. We now address the nature of the oxygen species catalyzing C-H bond cleavage through the measurement of oxygen-18 isotope effects as a function of substrate structure. Using deuterium isotope effects, t ogether with experimental O-18 isotope effects with protonated and deu terated substrates, it has been possible to calculate intrinsic O-18 i sotope effects. Since the DbetaM mechanism includes many steps which m ay involve changes in bond order at dioxygen, e.g., the reversible bin ding of O2 to the active-site copper and its reductive activation to a copper-hydroperoxide species, the intrinsic O-18 isotope effect is ex pected to be the product of two terms: (1) an overall equilibrium O-18 isotope effect on steps leading from O2 binding to the formation of t he intermediate which catalyzes C-H bond cleavage and (2) a kinetic O- 18 isotope effect on the C-H bond cleavage step. Thus, the magnitude o f a single O-18 isotope effect measurement cannot reveal the nature of the bonding at oxygen during substrate activation. In the present stu dy we have measured the change in O-18 isotope effect as a function of substrate structure and reactivity, finding values of 18(V/K) which d ecrease from 1.0281 +/- 0.001 to 1.0216 +/- 0.0003 as the rate of the C-H bond cleavage step decreases from 680 to 2 s-1. As described, this trend in O-18 isotope effect with reactivity can only be explained if the O-O bond of dioxygen undergoes cleavage prior to substrate activa tion. A new chemical mechanism is proposed for DbetaM which may serve as a general paradigm for non-heme, metallomonooxygenases.