Stereospecific deuterium substitution at the alpha-carbon position of dopamine and its effect on oxidative deamination catalyzed by MAO-A and MAO-B from different tissues.

Stereospecific replacement of deuterium in the alpha-carbon side chain position of dopamine (DA) was achieved by decarboxylation of L-3,4-dihydroxyphenylalanine (L-dopa) using hog kidney aromatic aminoacid decarboxylase. The S[alpha-2H1]DA enantiomer was obtained by decarboxylation of L-[alpha-2H1]dopa in H2O, while the R[alpha-2H1]DA enantiomer was obtained by decarboxylation of unsubstituted L-dopa in 2H2O. An inverse solvent isotope effect of L-dopa decarboxylation was observed in 2H2O. The deaminated aldehyde products of the four DA analogues, i.e. undeuterated DA, [alpha, alpha-2H2] DA, R[alpha-2H1]DA and S[alpha-2H1]DA, have been analyzed by the gas chromatography-mass spectrometry (GC-MS) method. It is evident that monoamine oxidase (MAO) catalyzes the stereochemical removal of only R-deuterium and that S-deuterium was maintained at the alpha-carbon atom of 3,4-dihydroxyphenylacetaldehyde. The steady-state kinetics of the oxidative deamination of undeuterated, [alpha, alpha-2H2], R[alpha-2H1], and S[alpha-2H1] dopamine were assessed by determination of the aldehyde products directly by high performance liquid chromatography (HPLC) using electrochemical detection (ECD). MAO-A from rat liver mitochondria (deprenyl-treated) and from human placenta, as well as MAO-B from rat liver (clorgyline-treated) and from human platelet were used in this study. The apparent isotope effects, i.e. (V/K)H/(V/K)D ratios of [alpha, alpha-2H2]DA and R[alpha-2H1]DA, were quite similar (2.34 and 3.13) with respect to both MAO-A and MAO-B. S[alpha-2H1]DA exhibited a slight secondary isotope effect. Formula: see text.