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Abstract
2,5,2′,5′-Tetrachlorobiphenyl (TCB) induced type I binding spectra with cytochrome P450 (P450) 2A6 and 2A13, with Ks values of 9.4 and 0.51 µM, respectively. However, CYP2A6 oxidized 2,5,2′,5′-TCB to form 4-hydroxylated products at a much higher rate (∼1.0 minute−1) than CYP2A13 (∼0.02 minute−1) based on analysis by liquid chromatography–tandem mass spectrometry. Formation of 4-hydroxy-2,5,2′,5′-TCB by CYP2A6 was greater than that of 3-hydroxy-2,5,2′,5′-TCB and three other hydroxylated products. Several human P450 enzymes, including CYP1A1, 1A2, 1B1, 2B6, 2D6, 2E1, 2C9, and 3A4, did not show any detectable activities in oxidizing 2,5,2′,5′-TCB. Cynomolgus monkey CYP2A24, which shows 95% amino acid identity to human CYP2A6, catalyzed 4-hydroxylation of 2,5,2′,5′-TCB at a higher rate (∼0.3 minute−1) than CYP2A26 (93% identity to CYP2A6, ∼0.13 minute−1) and CYP2A23 (94% identity to CYP2A13, ∼0.008 minute−1). None of these human and monkey CYP2A enzymes were catalytically active in oxidizing other TCB congeners, such as 2,4,3′,4′-, 3,4,3′,4′-, and 3,5,3′,5′-TCB. Molecular docking analysis suggested that there are different orientations of interaction of 2,5,2′,5′-TCB with the active sites (over the heme) of human and monkey CYP2A enzymes, and that ligand interaction energies (U values) of bound protein-ligand complexes show structural relationships of interaction of TCBs and other ligands with active sites of CYP2A enzymes. Catalytic differences in human and monkey CYP2A enzymes in the oxidation of 2,5,2′,5′-TCB are suggested to be due to amino acid changes at substrate recognition sites, i.e., V110L, I209S, I300F, V365M, S369G, and R372H, based on the comparison of primary sequences.
Footnotes
- Received August 2, 2016.
- Accepted September 12, 2016.
T.S., K.K., S.T., and M.K. were supported in part by grants from the Ministry of Education, Science, and Culture of Japan and the Ministry of Health and Welfare of Japan. N.K., S.U., H.Y., and F.P.G. were also supported partly by a Health and Labor Scientific Research Grant [Grant H27-food-designated-017], the Japan Society for the Promotion of Science Grant-in-Aid for Young Scientists B [Grant 15K18934] and Scientific Research [Grant 26460206], and by the United States Public Health Service [Grant R01 GM118122], respectively.
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- Copyright © 2016 by The American Society for Pharmacology and Experimental Therapeutics
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