Abstract
Aldehyde oxidase‐1 (AOX‐1) is a cytosolic xenobiotic‐metabolizing enzyme that has gained increasing interest because of its emerging role in drug metabolism and toxicity. It is a catalyst of carbazeran 4‐oxidation, resulting in the formation of 4‐oxo‐carbazeran. However, cytochrome P450 is also a catalyst of oxidative reactions. Therefore, we tested the hypothesis that cytochrome P450 and AOX‐1 catalyze carbazeran 4‐oxidation in the endoplasmic reticulum and cytosol, respectively. Incubations containing human liver microsomes with or without NADPH yielded 4‐oxo‐carbazeran, but to a lesser extent than that in incubations containing human liver cytosol. Cytochrome P450 inhibitors (1‐aminobenzotriazole and ketoconazole) did not decrease the extent of carbazeran 4‐oxidation in liver microsomes, whereas an AOX‐1 inhibitor (hydralazine) decreased it in liver cytosol. Human recombinant CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP3A4, and CYP3A5 did not catalyze carbazeran 4‐oxidation, in contrast to the catalysis by recombinant AOX‐1. The extent of carbazeran 4‐oxidation in incubations containing liver microsomes was attributed to the presence of AOX‐1 in the microsomal preparations, as indicated by the decrease in carbazeran 4‐oxidation by an AOX‐1 inhibitor (hydralazine) and the detection of AOX‐1 protein in liver microsomes (at a level that was up to one‐third of that in liver cytosol). Cytosolic sulfotransferases‐catalyzed dehydroepiandrosterone sulfation increased with increasing amount of microsomes, indicating that the two lots of hepatic microsomes are contaminated with not only AOX‐1, but also other cytosolic enzymes. An isomer of 4‐oxo‐carbazeran, putatively identified as 5‐hydroxy‐carbazeran or 8‐hydroxy‐carbazeran, was detected in incubations containing carbazeran and CYP1A2, CYP2D6, CYP3A4, or CYP3A5. The 5‐hydroxy‐carbazeran or 8‐hydroxy‐carbazeran peak was detected in human liver microsomal incubation and the peak area increased with increasing incubation time. The 5‐hydroxy‐carbazeran or 8‐hydroxy‐carbazeran peak was decreased by a pan‐CYP inhibitor (1‐aminobenzotriazole), but not by an AOX‐1 inhibitor (hydralazine). The addition of 1‐aminobenzotriazole increased the 4‐oxo‐carbazeran formation, indicating a shunting of carbazeran metabolism to AOX‐1 in the presence of the CYP inhibitor. In conclusion, carbazeran is a dual substrate of AOX‐1 and cytochrome P450. However, AOX‐1, but not cytochrome P450, catalyzes carbazeran 4‐oxidation.Support or Funding InformationSingapore Ministry of Education Academic Research Fund Tier 1 [Grant R‐148‐000‐218‐112 to AJL]This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.
Published Version
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