Theoretical investigation of differences in nitroreduction of aristolochic acid I by cytochromes P450 1A1, 1A2 and 1B1.

OBJECTIVES: The herbal drug aristolochic acid (AA) derived from Aristolochia species has been shown to be the cause of aristolochic acid nephropathy (AAN), Balkan endemic nephropathy (BEN) and their urothelial malignancies. One of the common features of AAN and BEN is that not all individuals exposed to AA suffer from nephropathy and tumor development. One cause for these different responses may be individual differences in the activities of the enzymes catalyzing the biotransformation of AA. Thus, the identification of enzymes principally involved in the metabolism of AAI, the major toxic component of AA, and detailed knowledge of their catalytic specificities is of major importance. Human cytochrome P450 (CYP) 1A1 and 1A2 enzymes were found to be responsible for the AAI reductive activation to form AAI-DNA adducts, while its structurally related analogue, CYP1B1 is almost without such activity. However, knowledge of the differences in mechanistic details of CYP1A1-, 1A2-, and 1B1- mediated reduction is still lacking. Therefore, this feature is the aim of the present study.

METHODS: Molecular modeling capable of evaluating interactions of AAI with the active site of human CYP1A1, 1A2 and 1B1 under the reductive conditions was used. In silico docking, employing soft-soft (flexible) docking procedure was used to study the interactions of AAI with the active sites of these human enzymes.

RESULTS: The predicted binding free energies and distances between an AAI ligand and a heme cofactor are similar for all CYPs evaluated. AAI also binds to the active sites of CYP1A1, 1A2 and 1B1 in similar orientations. The carboxylic group of AAI is in the binding position situated directly above heme iron. This ligand orientation is in CYP1A1/1A2 further stabilized by two hydrogen bonds; one between an oxygen atom of the AAI nitro-group and the hydroxyl group of Ser122/Thr124; and the second bond between an oxygen atom of dioxolane ring of AAI and the hydroxyl group of Thr497/Thr498. For the CYP1B1:AAI complex, however, any hydrogen bonding of the nitro-group of AAI is prevented as Ser122/Thr124 residues are in CYP1B1 protein replaced by hydrophobic residue Ala133.

CONCLUSION: The experimental observations indicate that CYP1B1 is more than 10× less efficient in reductive activation of AAI than CYP1A2. The docking simulation however predicts the binding pose and binding energy of AAI in the CYP1B1 pocket to be analogous to that found in CYP1A1/2. We believe that the hydroxyl group of S122/T124 residue, with its polar hydrogen placed close to the nitro group of the substrate (AAI), is mechanistically important, for example it could provide a proton required for the stepwise reduction process. The absence of a suitable proton donor in the AAI-CYP1B1 binary complex could be the key difference, as the nitro group is in this complex surrounded only by the hydrophobic residues with potential hydrogen donors not closer than 5 Å.