Metabolism of the β-oxidized intermediates of N-nitrosodi-n-propylamine: N-nitroso-β-hydroxypropylpropylamine and N-nitroso-β-oxopropylpropylamine

The rat liver carcinogen N-nitrosodi-n-propylamine (NDPA) is metabolized to a propylating and methylating species in vivo. Metabolism to a methylating species is believed to require an initial hydroxylation by cytochrome P450s (P450s) to N-nitroso-β-hydroxypropylpropylamine (NHPPA), which is oxidized to N-nitroso-β-oxopropylpropylamine (NOPPA), followed by a P450-mediated depropylation to β-oxopropyldiazotate, which non-enzymatically breaks down to the methylating agent. Purified rat liver P450 2B1 and rabbit liver 2E1 in the reconstituted system and liver microsomes from phenobarbital (PB) and pyridine (Pyr) treated rats readily metabolized NOPPA to a methylating species as determined by the in vitro formation of 7-methylguanine (m⁷Gua) in DNA. Exposure of cells derived from the human liver epithelium transfected with human 2E1 (T5-2E1) to NOPPA resulted in the formation of m⁷Gua DNA adducts and a dose dependent toxicity. In vitro incubation of NHPPA with microsomes from PB, Pyr and non-treated (NT) rats and a human microsomal sample also resulted in m⁷Gua formation. P450s 2B1 and 2E1 oxidized NHPPA to NOPPA, forming 16.5 ± 3.1 and 20.0 ± 4.4 pmol NOPPA/pmol P450 in 1 h, respectively. Rat liver cytosol, in the presence of NAD⁺, oxidized NHPPA to NOPPA at a rate of 13.7 ± 3.0 pmol/min/mg protein while microsomes from NT rats catalyzed this reaction at 95.6 ± 16.5 pmol/min/mg protein. Cells derived from hamster lung tissue (V79 control) and T5-neo cells oxidized NHPPA to NOPPA. This oxidation was about 15 fold higher in T5-2E1 or V79 cells transfected with human 2E1 or rat 2B1, respectively. The results are consistent with the putative sequential oxidation pathway and suggest that, at the concentrations tested, oxidation of NHPPA to NOPPA may be predominantly mediated by cytochrome P450s. In addition, it appears that rabbit, rat and human P450 2E1 can catalyze both oxidations.