Theoretical Investigation of Cytochrome P450 Enzyme-Mediated Biotransformation Mechanism of BHPF: Unveiling the Metabolic Safety Aspects of an Alternative to BPA

Fluorene-9-bisphenol (BHPF), emerging as an alternative to bisphenol A (BPA), is extensively utilized in industry and consumer goods. BHPF exhibits antiestrogenic effects and potential reproductive toxicity. Similar to BPA, BHPF can closely access the active site of the cytochrome P450 (CYP450) enzyme, interact with the Fe=O moiety, and potentially initiate metabolic reactions. Using density functional theory (DFT) calculations, we explored the mechanisms underlying BHPF activation using a CYP450 compound I (Cpd I) model, identifying several plausible products. Compared with the higher energy barriers associated with phenyl ring addition reactions, the formation of a phenoxyl-type radical through phenolic hydrogen atom abstraction, followed by OH rebound or radical coupling, represents an energetically favorable pathway. The OH rebound process yields three primary products: 9-(3,4-dihydroxyphenyl)-9-(4-hydroxyphenyl)fluorene (PRD1), semiquinone radical anion (PRD2), and 9-(4-hydroxyphenyl)fluorene carbocation (PRD3), constituting the major outcomes of the BHPF metabolic reaction. Importantly, a lipophilic ether metabolite, BHPF-O-BHPF (PRD4), formed through the coupling of phenoxyl radicals, reflects a widespread metabolic pathway observed in phenolic molecules. Despite constituting a minor proportion, the toxicity of this product necessitates increased attention. These findings contribute significantly to an enhanced understanding of the potential hazards associated with BHPF and other unknown chemical entities.