Stereoselectivity in the microsomal conversion of n-nitrosodimethylamine to formaldehyde

The possibility that N-nitrosodimethylamine (NDMA) might be metabolized preferentially at either the syn (relative to the nitroso oxygen) or the anti methyl group has been examined by comparing the rates of formaldehyde production when unlabeled NDMA, its fully deuteriated analogue (NDMA-d₆), and (Z)- or (E)-N-nitrosomethyl(methyl-d₃)arnine (NDMA-d₃) were incubated in turn at concentrations of 0–2.4 mM with acetone-induced rat liver microsomes. The Km values for the conversion of (Z)- and (E)-NDMA-d₃ to formaldehyde were identical to each other within experimental error (32 ± 2 and 35 ± 1 µM, respectively) but different from those for NDMA (24 ± 6 µM) and NDMA-dg (116 ± 3 µM); similar V_max values were observed for the four isotopic variants [7.5–8.1 nmol/(mg of protein-min)]. The observed similarity of kinetic parameters for (Z)- and (E)-NDMA-d₃ suggested that the isotopic composition of the methyl group is an energetically more important determinant of its rate of oxidation at the NDMA demethylase active site than is its orientation relative to the nitroso oxygen atom. The absence of syn vs anti stereospecificity was confirmed via product isolation studies, in which the formaldehyde generated from each of the four isotopomers was trapped as the dimedone adduct and assayed for deuterium content by mass spectrometry; again, a strong preference for metabolism at CH₃ vs CD₃ regardless of stereochemistry was observed, though the data on CH₂O generation suggested that there may be a slight net excess of anti attack. The results indicate that the microsomal enzymes employed display little regioselectivity in metabolizing the syn vs anti methyl groups of NDMA.