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

Abstract

Stereospecific replacement of deuterium in the α-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[ α- 2H 1]DA enantiomer was obtained by decarboxylation of l-[α- 2H 1]dopa in H 2O, while the R[ α- 2H 1]DA enantiomer was obtained by decarboxylation of unsubstituted l-dopa in 2H 2O. An inverse solvent isotope effect of l-dopa decarboxylation was observed in 2H 2O. The deaminated aldehyde products of the four DA analogues, i.e. undeuterated DA, [α,α- 2H 2] DA, R[ α- 2H 1]DA and S[ α- 2H 1]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 α-carbon atom of 3,4-dihydroxyphenylacetaldehyde. The steady-state kinetics of the oxidative deamination of undeuterated, [α,α- 2H 2], R[ α- 2H 1], and S[ α- 2H 1] 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 [α,α- 2H 2]DA and R[ α- 2H 1]DA, were quite similar (2.34 and 3.13) with respect to both MAO-A and MAO-B. S[ α- 2H 1]DA exhibited a slight secondary isotope effect.