Deuterium isotope effect on the metabolism of N-nitrosodimethylamine and related compounds by cytochrome P4502E1

Abstract

1. Deuteration of N-nitrosodimethylamine (NDMA) decreases its carcinogenicity, and produces an isotope effect on its metabolism in vivo. Consistent with these results are the observations that deuteration caused a 5-fold increase in the apparent K_m, but not the V_max the demethylation and denitrozation of NDMA in acetone-induced rat liver microsomes. These microsomes are a good source of cytochrome P4502E1.

2. For demethylation of Z-[²H₃]NDMA and E-[²H₃)NDMA, the K_m values were indistinguishable, and were between the values for those of NDMA and [²H₆]NDMA. Almost all the formaldehyde formed was derived from the non-deuterated methyl group. indicating a lack of stereoselectivity in the demethylation of NDMA.

3. NDMA and [²H₆]NDMA displayed apparent K_i values of 59 and 441 μM, respectively, for N-nitrosodiethylamine deethylase, showing an apparent isotope effect of 0.13, and displayed an isotope effect of 0.21 in the K_i values for p-nitrophenol hydroxylase.

4. With acetone and deuterated acetone as inhibitors for p-nitrophenol hydroxylase, the isotope effect on the K_i was 0.11. Similar deuterium isotope effects were also observed with acetone and dimethylformamide as competitive inhibitors for NDMA demethylase.

5. In the microsomal oxidation of ethanol, a deuterium isotope effect of about five was observed in the V_max/K_m when carbon-1 was deuterated, but was not observed in the V_max.

6. Results illustrate a unique deuterium isotope effect on the K_m values of reactions catalysed by P4502E1.