Metabolic stereoselectivity of cytochrome P450 3A4 towards deoxypodophyllotoxin: In silico predictions and experimental validation.

Deoxypodophyllotoxin is stereoselectively converted into epipodophyllotoxin by recombinant human cytochrome P450 3A4 (CYP3A4). Further kinetic analysis revealed that the Michaelis-Menten K(m) and V(max) for hydroxylation of deoxypodophyllotoxin by CYP3A4 at C7 position were 1.93 microM and 1.48 nmol/min/nmol, respectively. Deoxypodophyllotoxin was subjected to automated docking analysis in order to get better knowledge of the interaction between the CYP3A4 enzyme and the substrate, using the PatchDock algorithm with distance constraints. Automated docking showed that the beta-hydrogen atom at C7 position is in the most appropriate binding orientation at the site of oxidation. The docking results are consistent with the experimental data for the bioconversion of deoxypodophyllotoxin into epipodophyllotoxin by CYP3A4. In addition, the effects of five lignans, deoxypodophyllotoxin, epipodophyllotoxin, podophyllotoxin, demethylenedeoxypodophyllotoxin, and demethylenepodophyllotoxin, on CYP3A4 were compared in order to investigate the influence of the methylenedioxy group on the biotransformation process, to give insight into the mode of metabolization and to explain inhibitory activity of lignans.

Bioconversion of deoxypodophyllotoxin into epipodophyllotoxin in E. coli using human cytochrome P450 3A4.

Biotransformation of deoxypodophyllotoxin to epipodophyllotoxin by three major human hepatic enzymes, CYP1A2, CYP2C9 and CYP3A4, heterologously expressed in E. coli DH5alpha, was investigated. It was shown that CYP3A4 catalysed the hydroxylation of deoxypodophyllotoxin into epipodophyllotoxin in yields up to 90%. The structure of the metabolite was determined using HPLC-MS and HPLC-SPE-NMR techniques. There was no detectable production of epipodophyllotoxin or podophyllotoxin by CYP1A2 and CYP2C9 enzymes. The CYP3A4 enzyme shows a distinctly different reactivity to deoxypodophyllotoxin compared to the semi-synthetic derivatives etoposide and teniposide, which are degraded by 3-O-demethylation. These findings demonstrate a novel system for the production of 2,7'-cyclolignans, starting from the easily accessible deoxypodophyllotoxin.