Analysis of the inhibition of N-nitroso-dimethylamine activation in the liver by N-nitro-dimethylamine using a new non-linear statistical method

N-nitro-dimethylamine (NTDMA) is carcinogenic to rats: it induces nasal cav ity tumours, It can be demethylated to N-nitromethylamine and formaldehyde and reduced to N-nitroso-dimethylamine (NDMA): a potent liver carcinogen an d also of the nasal cavity if activation in the liver is blocked. To explai n the mechanism of NTDMA carcinogenicity we compared its demethylation with that of NDMA in liver microsomes from female and male rats, untreated, fas ted or treated with ethanol to induce cytochrome P450 2E1 (CYP2E1), Kinetic parameters were analysed by nonlinear statistical methods, which yielded u nbiased parameter estimates for the calculated K-m and V-max values. K-m fo r both compounds was very similar in females (24-47 mu M) whereas V-max for NTDMA was consistently higher than for NDMA as substrate: 1.07-4.70 nmol f ormaldehyde/mg microsomal protein x min and 0.52-2.76 nmol, respectively. I n liver microsomes from induced male rats NTDMA was found to be a much more effective inhibitor of NDMA activation (K-EI 39.6-73.6 mu M) than NDMA of NTDMA demethylation (K-EI 224-286 mu M). Nasal microsomes can demethylate b oth NDMA and NTDMA but the kinetics are vastly different. NTDMA is demethyl ated at a linear rate and similar to 10-fold more effectively than NDMA, Th e mechanism of carcinogenicity of ingested NTDMA, we propose, is a partial reduction to NDMA in the liver and inhibition of NDMA activation in the liv er by residual NTDMA, which enables NDMA to reach the nasal mucosa where it is activated to DNA-alkylating species and the observed tumours are formed .