Abstract
A recent X-ray crystal structure of a rabbit cytochrome P450 2B4 (CYP2B4)-ticlopidine complex indicated that the compound could be modeled with either the thiophene or chlorophenyl group oriented toward the heme prosthetic group. Subsequent NMR relaxation and molecular docking studies suggested that orientation with the chlorophenyl ring closer to the heme was the preferred one. To evaluate the predictive value of these findings, the oxidation of ticlopidine by reconstituted CYP2B4 was studied and compared with CYP2B6, in which the thiophene portion of the molecule likely orients toward the heme. In vitro incubation of ticlopidine with both enzymes yielded the same set of metabolites: 7-hydroxyticlopidine (M1), 2-oxoticlopidine (M2), 5-(2-chlorobenzyl)thieno[3,2-c]pyridin-5-ium metabolite (M3), 5-(2-chlorobenzyl)thieno[3,2-c]pyridin-5-ium metabolite (M4), ticlopidine N-oxide (M5), and ticlopidine S-oxide dimer, a dimerization product of ticlopidine S-oxide (M6). The rates of metabolite formation deviated markedly from linearity with time, consistent with the known inactivation of CYP2B6 by ticlopidine. Fitting to a first-order equation yielded similar rate constants (kobs) for both enzymes. However, the amplitude (Rmax) of M1 and M6 formation was 4 to 5 times higher for CYP2B6 than CYP2B4, indicating a greater residence time of ticlopidine with its thiophene ring closer to heme in CYP2B6. In contrast, CYP2B4 formed M4 and M5 in more abundance than CYP2B6, indicating an alternate orientation. Overall, the results suggest that the preferential orientation of ticlopidine in the active site of CYP2B4 predicted by X-ray crystallography and NMR studies is unproductive and that ticlopidine likely reorients within CYP2B4 to a more productive mode.
Footnotes
This work was supported by the National Institutes of Health National Institute of Environmental Health Sciences [Grant ES003619] (to J.R.H.).
Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
doi:10.1124/dmd.110.037101.
↵ The online version of this article (available at http://dmd.aspetjournals.org) contains supplemental material.
-
ABBREVIATIONS:
- P450
- cytochrome P450
- CYP2B6
- an N-terminal truncated and modified and C-terminal 4-His-tagged form of cytochrome P450 2B6
- CYP2B4
- an N-terminal truncated and modified and C-terminal His-tagged form of cytochrome P450 2B4 with a substitution of His226 to Tyr
- M2
- 2-oxoticlopidine
- 7-EFC
- 7-ethoxy-4-trifluoromethylcoumarin
- Cymal-5
- 5-cyclo-hexylpentyl-β-d-maltoside
- CPR
- recombinant NADPH-cytochrome P450 reductase
- Ni-NTA
- nickel-nitrilotriacetic acid
- DTT
- dithiothreitol
- HPLC
- high-performance liquid chromatography
- MS
- mass spectrometry
- ESI
- electrospray ionization
- MS/MS
- tandem mass spectrometry
- TSOD
- ticlopidine S-oxide dimer
- amu
- atomic mass units.
- Received November 8, 2010.
- Accepted December 14, 2010.
- Copyright © 2011 by The American Society for Pharmacology and Experimental Therapeutics
DMD articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|