title: Visible spectra of type II cytochrome P450-drug complexes: evidence that "incomplete" heme coordination is common.
abstract: The visible spectrum of a ligand-bound cytochrome P450 is often used to determine the nature of the interaction between the ligand and the P450. One particularly characteristic form of spectra arises from the coordination of nitrogen-containing ligands to the P450 heme iron. These type II ligands tend to be inhibitors because they stabilize the low reduction potential P450 and prevent oxygen binding to the heme. Yet, several type II ligands containing aniline, imidazole, and triazole moieties are also known to be substrates of P450, although P450 binding spectra are not often scrutinized to make this distinction. Therefore, the three nitrogenous ligands aniline, imidazole, and triazole were used as binding spectra standards with purified human CYP3A4 and CYP2C9, because their small size should not present any steric limitations in their accessing the heme prosthetic group. Next, the spectra of P450 with drugs containing the three nitrogenous groups were collected for comparison. The absolute spectra demonstrated that the red-shift of the low-spin Soret band is mostly dependent on the electronic properties of the nitrogen ligand since they tended to match their respective standards, aniline, imidazole, and triazole. On the other hand, difference spectra seemed to be more sensitive to the steric properties of the ligand because they facilitated comparison of the spectral amplitudes achieved with the drugs versus those with the standard nitrogen ligands. Therefore, difference spectra may help reveal "weak" coordination to the heme that results from suboptimal orientation or ligand binding to more remote locations within the P450 active sites.
authors.affiliation: Department of Experimental and Clinical Pharmacology, University of Minnesota, College of Pharmacy, 7-115B Weaver-Densford Hall, 308 Harvard Street SE, Minneapolis, MN 55455, USA.
authors.forename: Charles W
authors.initials: CW
authors.lastname: Locuson
authors.forename: J Matthew
authors.initials: JM
authors.lastname: Hutzler
authors.forename: Timothy S
authors.initials: TS
authors.lastname: Tracy
mesh_terms: D000814:Aniline Compounds / Q000378:metabolism; D001189:Aryl Hydrocarbon Hydroxylases / Q000378:metabolism; D052117:Benzodioxoles / Q000737:chemistry; D001665:Binding Sites; D065729:Cytochrome P-450 CYP2C9; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D015725:Fluconazole / Q000378:metabolism; D006418:Heme / Q000737:chemistry*; D006801:Humans; D007093:Imidazoles / Q000378:metabolism; D017964:Itraconazole / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D008027:Light; D008956:Models, Chemical; D015394:Molecular Structure; D009249:NADP / Q000378:metabolism; D017711:Nonlinear Dynamics; D011485:Protein Binding; D011743:Pyrimidines / Q000737:chemistry; D011994:Recombinant Proteins / Q000378:metabolism; D012015:Reference Standards; D012044:Regression Analysis; D013057:Spectrum Analysis / Q000592:standards; D013426:Sulfaphenazole / Q000378:metabolism; D014230:Triazoles / Q000378:metabolism
chemical_list: D000814:Aniline Compounds; D052117:Benzodioxoles; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D007093:Imidazoles; D008024:Ligands; C517786:PH 302; D011743:Pyrimidines; D011994:Recombinant Proteins; D014230:Triazoles; D013426:Sulfaphenazole; C045575:1,2,4-triazole; D017964:Itraconazole; D006418:Heme; D009249:NADP; C029899:imidazole; D015725:Fluconazole; D003577:Cytochrome P-450 Enzyme System; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D001189:Aryl Hydrocarbon Hydroxylases; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; C023650:aniline