ISOTOPE EFFECTS IN THE HYDROXYLATION OF DOPAMINE BY DOPAMINE-ß-HYDROXYLASE (DBH)

α, α′ -Dideutero dopamine (α-DA) and β, β′ -dideuterodopamine (β-DA) were synthesized with positional specificities shown to be greater than 90 percent by gas chromatography-mass spectrometry (GC/MS) in the chemical ionization mode. Kinetic comparisons with homogeneous bovine adrenal DBH with GC/MS indicate a pseudo-primary isotope effect of 2.5 for the β position and 1.7 for the α position. This indicates that the rate-limiting step in the conversion of dopamine to norepinephrine is the βC-H cleavage. The secondary effect suggests that the rate of hydroxylation is modified by the geometry of the adjacent C-H bond. Trihydroxyindole determinations gave similar results for studies with α-DA but an unusually high isotope effect (8.3) was observed with β-DA. This is due to an additional primary isotope effect in conversion to the trihydroxyindole.