Roles of maize cytochrome P450 (CYP) enzymes in stereo-selective metabolism of hexabromocyclododecanes (HBCDs) as evidenced by in vitro degradation, biological response and in silico studies

In vitro biotransformation of HBCDs by maize cytochrome P450 (CYP) enzymes, responses of CYPs to HBCDs at protein and transcription levels, and in silico simulation of interactions between CYPs and HBCDs were investigated in order to elucidate the roles of CYPs in the metabolism of HBCDs in maize. The results showed that degradation reactions of HBCDs by maize microsomal CYPs followed the first-order kinetics and were stereo-selective, with the metabolic rates following the order (-)γ- > (+)γ- > (+)α- > (-)α-HBCD. The hydroxylated metabolites OH-HBCDs, OH-PBCDs and OH-TBCDs were detected. (+)/(-)-α-HBCDs significantly decreased maize CYP protein content and inhibited CYP enzyme activity, whereas (+)/(-)-γ-HBCDs had obvious effects on the induction of CYPs. HBCDs selectively mediated the gene expression of maize CYPs, including the isoforms of CYP71C3v2, CYP71C1, CYP81A1, CYP92A1 and CYP97A16. Molecular docking results suggested that HBCDs could bind with these five CYPs, with binding affinity following the order CYP71C3v2 < CYP81A1 < CYP97A16 < CYP92A1 < CYP71C1. The shortest distances between the Br-unsubstituted C atom of HBCD isomers and the iron atom of heme in CYPs were 4.18-11.7 Å, with only the distances for CYP71C3v2 being shorter than 6 Å (4.61-5.38 Å). These results suggested that CYP71C3v2 was an efficient catalyst for degradation of HBCDs. For (+)α- and (-)γ-HBCDs, their binding affinities to CYPs were lower and the distances to the iron atom of heme in CYPs were shorter than their corresponding antipodes, consistent with the in vitro experimental results showing that they had shorter half-lives and were more easily hydroxylated. This study provides solid evidence for the roles of maize CYPs in the metabolism of HBCDs, and gives insight into the molecular mechanisms of the enantio-selective metabolism of HBCDs by plant CYPs.