title: Computational evidence for α-nitrosamino radical as initial metabolite for both the P450 dealkylation and denitrosation of carcinogenic nitrosamines.
abstract: The mutagenic and carcinogenic potency of α-CH(n)-nitrosamines such as N-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 (•CH(2)N(CH(3))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 (ΔG(‡)) 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(n)-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.
authors.affiliation: UFZ Department of Ecological Chemistry, Helmholtz Centre for Environmental Research, Leipzig, Germany.
authors.forename: Li
authors.initials: L
authors.lastname: Ji
authors.forename: Gerrit
authors.initials: G
authors.lastname: Schüürmann
mesh_terms: D055162:Biocatalysis; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry / Q000378:metabolism*; D004128:Dimethylnitrosamine / Q000737:chemistry / Q000378:metabolism; D005609:Free Radicals / Q000737:chemistry / Q000378:metabolism*; D006900:Hydroxylation; D007700:Kinetics; D009602:Nitrosamines / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D005609:Free Radicals; D009602:Nitrosamines; D003577:Cytochrome P-450 Enzyme System; D004128:Dimethylnitrosamine