Computational Evidence for α-Nitrosamino Radical as Initial Metabolite for Both the P450 Dealkylation and Denitrosation of Carcinogenic Nitrosamines

The mutagenic and carcinogenic potency of α-CH_<i>n</i>-nitrosamines such as <i>N</i>-nitrosodimethylamine (NDMA) is caused by their P450-catalyzed α-hydroxylation and subsequent dealkylation, yielding alkyl diazonium ions (R−NN⁺) as potent electrophiles. Alternatively, P450s may also catalyze their denitrosation as metabolic detoxification. DFT calculations at the UB3LYP/LANL2DZ(Fe)/6-31G+**(H,C,N,O,S)//LANL2DZ(Fe)/6-31G(H,C,N,O,S) level of theory show that H-abstraction from the α-C of NDMA as initial metabolic step yields an α-nitrosamino radical (•CH₂N(CH₃)NO) as common first intermediate for both the oxidative dealkylation (toxification) and denitrosation (detoxification) pathways. In particular, the calculated kinetic isotope effect for the P450-mediated dealkylation of NDMA is in good agreement with experimental information. The results show further that the initial α-hydroxylation of NDMA may proceed in two spin states. Besides a stepwise high-spin (HS, quartet) route with a separate rebound barrier, there is a concerted low-spin (LS, doublet) pathway. Interestingly, the resultant two-state reactivity appears to discriminate between metabolic toxification and detoxification: Evaluation of calculated free energy barriers of the H-abstraction (Δ<i>G</i>^‡) through the Eyring equation suggests that the dealkylation:denitrosation product ratio is governed by the LS:HS ratio of the overall metabolic process. Moreover, inclusion of three further α-CH_<i>n</i>-nitrosamines in the computational analysis demonstrates that the initial H-abstraction barrier is proportional to the C–H bond dissociation enthalpy (BDE) of the substrates, which enables the estimation of spin-averaged reaction barriers through ground-state BDE calculations. The discussion includes also reductive denitrosation pathways that according to current computational evidence appear to be unlikely for aliphatic nitrosamines