Quantum chemical studies of anaerobic reductive metabolism of halothane by cytochrome P-450

Proposed pathways for anaerobic reductive metabolism of the widely used anesthetic halothane by cytochrome P-450s have been investigated using two all-valence semiempirical molecular orbital methods. Molecular conformations and electronic structure of halothane and intermediates in proposed one- and two-electron reductive metabolism were calculated and results used to explore the feasibility of each step in the pathway to observed metabolites. The results suggest facile bromide ion elimination with formation of a CHClCF₃ radical as a consequence of single electron reduction of halothane. Characterization of this radical species and its carbanion which would be formed by a second electron reduction suggests that either species could undergo reasonable nonenzymatic transformations leading to the formation of the two volatile metabolic products observed and the proposed trifluoroethylcarbene intermediate. This carbene, rather than CF₃CCl, is predicted to be preferentially formed by either a radical or ionic halide elimination reaction. Moreover, when bound as an axial ligand to the iron atom at the heme site in cytochrome P-450, a carbene complex is formed which gives calculated spectral characteristics consistent with those observed when excess halothane is added to reduced cytochrome P-450 or rat liver microsomes under anaerobic conditions.