Deuterium isotope effect on metabolism of N-nitrosodimethylamine in vivo in rat

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Abstract
The maximal rates of metabolic oxidation of N-nitrosodimethylamine (NDMA) and N-nitrosodimethylamine-d6 (NDMA-d6) in viva (VH and VD, respectively) have been measured by following 14CO2 exhalation in rats after intraperitoneal injection of the two 14C-labelled carcinogens at high doses (20 or 40 mg/kg). Complete deuteration of NDMA reduced only slightly the maximal rate of metabolism when the two substrates were administered separately (VH/VD ∼1.2). However, much larger(∼4-fold) deuterium isotope effects were observed when mixtures of NDMA with NDMA-d6 were injected. These results are tentatively interpreted as evidence that C-H bond cleavage is not a rate-limiting feature of overall metabolism, but that the complex between NDMA and the principal enzyme(s) metabolizing it in vivo freely equilibrates with unbound substrate. Single, large, intrapentoneal doses of NDMA and NDMA-d6 produced a similar alkylation of rat liver DNA and also of kidney DNA. However, a small oral dose (54 μg/kg) of NDMA-d6 produced 1/3 less alkylation of liver DNA and 3 times as much alkylalion of kidney DNA as did an equimolar dose of NDMA. The reduction in alkylation of liver DNA correlates well with, and possibly explains, the decreased ability of NDMA-d6 to induce liver tumors in rats. The associated increase in the alkylation of kidney DNA suggests that this change is due to a decrease in the amount of nitrosamine removed from the portal blood on the first pass through the liver.