Metabolic-intermediate complex formation with cytochrome P450: Theoretical studies in elucidating the reaction pathway for the generation of reactive nitroso intermediate

• Published in: Journal of computational chemistry 2012-08, Vol.33 (21), p.1740-1747
• Main Authors: Taxak, Nikhil, Desai, Prashant V., Patel, Bhargav, Mohutsky, Michael, Klimkowski, Valentine J., Gombar, Vijay, Bharatam, Prasad V.
• Format: Article
• Language: English
• Subjects: Amines - chemistry; Amines - metabolism; Biotransformation; Chemical reactions; Cytochrome P-450 Enzyme System - metabolism; density functional theory; mechanism-based inhibition of cytochrome P450; metabolic-intermediate complex with cytochrome P450; Metabolites; Nitroso Compounds - chemistry; Nitroso Compounds - metabolism; nitroso intermediate; Oxidation; Oxidation-Reduction; Proteins; Protons; Quantum physics; Quantum Theory; reaction mechanism
• ISSN: 0192-8651; 1096-987X
• DOI: 10.1002/jcc.23008

Mechanism‐based inhibition (MBI) of cytochrome P450 (CYP) can lead to drug–drug interactions and often to toxicity. Some aliphatic and aromatic amines can undergo biotransformation reactions to form reactive metabolites such as nitrosoalkanes, leading to MBI of CYPs. It has been proposed that the nitrosoalkanes coordinate with the heme iron, forming metabolic‐intermediate complex (MIC), resulting in the quasi‐irreversible inhibition of CYPs. Limited mechanistic details regarding the formation of reactive nitroso intermediate and its coordination with heme‐iron have been reported. A quantum chemical analysis was performed to elucidate potential reaction pathways for the generation of nitroso intermediate and the formation of MIC. Elucidation of the energy profile along the reaction path, identification of three‐dimensional structures of reactive intermediates and transition states, as well as charge and spin density analyses, were performed using the density functional B3LYP method. The study was performed using Cpd I [iron (IV‐oxo] heme porphine with SH− as the axial ligand) to represent the catalytic domain of CYP, simulating the biotransformation process. Three pathways: (i) N‐oxidation followed by proton shuttle, (ii) N‐oxidation followed by 1,2‐H shift, and (iii) H‐ion followed by rebound mechanism, were studied. It was observed that the proton shuttle pathway was more favorable over the whole reaction leading to reactive nitroso intermediate. This study revealed that the MIC formation from a primary amine is a favorable exothermic process, involving eight different steps and preferably takes place on the doublet spin surface of Cpd I. The rate‐determining step was identified to be the first N‐oxidation of primary amine. © 2012 Wiley Periodicals, Inc. Aliphatic and aromatic amines can undergo biotransformation reactions to form reactive metabolites such as nitrosoalkanes, leading to mechanism‐based inhibition of cytochromes. The mechanistic details for the formation of reactive nitroso intermediate and its coordination with heme‐iron, forming metabolic‐intermediate complex (MIC), were explored in detail using quantum chemical analysis. This study revealed that the MIC formation from a primary amine favors a proton shuttle pathway, and is a highly exothermic process, involving eight different steps and preferably takes place on the doublet spin surface of Cpd I.