CYP26C1 Is a Hydroxylase of Multiple Active Retinoids and Interacts with Cellular Retinoic Acid Binding Proteins

Abstract

The clearance of retinoic acid (RA) and its metabolites is believed to be regulated by the CYP26 enzymes, but the specific roles of CYP26A1, CYP26B1, and CYP26C1 in clearing active vitamin A metabolites have not been defined. The goal of this study was to establish the substrate specificity of CYP26C1, and determine whether CYP26C1 interacts with cellular retinoic acid binding proteins (CRABPs). CYP26C1 was found to effectively metabolize all- retinoic acid (RA), 9--retinoic acid (9--RA), 13--retinoic acid, and 4-oxo-RA with the highest intrinsic clearance toward 9--RA. In comparison with CYP26A1 and CYP26B1, CYP26C1 resulted in a different metabolite profile for retinoids, suggesting differences in the active-site structure of CYP26C1 compared with other CYP26s. Homology modeling of CYP26C1 suggested that this is attributable to the distinct binding orientation of retinoids within the CYP26C1 active site. In comparison with other CYP26 family members, CYP26C1 was up to 10-fold more efficient in clearing 4-oxo-RA (intrinsic clearance 153 l/min/pmol) than CYP26A1 and CYP26B1, suggesting that CYP26C1 may be important in clearing this active retinoid. In support of this, CRABPs delivered 4-oxo-RA and RA for metabolism by CYP26C1. Despite the tight binding of 4-oxo-RA and RA with CRABPs, the apparent Michaelis-Menten constant in biological matrix () value of these substrates with CYP26C1 was not increased when the substrates were bound with CRABPs, in contrast to what is predicted by free drug hypothesis. Together these findings suggest that CYP26C1 is a 4-oxo-RA hydroxylase and may be important in regulating the concentrations of this active retinoid in human tissues.

Human cytochrome P450 enzymes 5–51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition – toxic effects and benefits

“…The reactions catalyzed (Fig. 23) are similar as for P450 26B1, although recent work by Zhong et al (2018) has defined some important differences in both the substrate and product profiles. Zhong et al (2018) reported that P450 26C1 has a specificty constant ( k cat / K m ) up to 10-fold higher than P450 26A1 or 26B1 in oxidizing all- trans -4-oxo retinoic acid.…”

Regulating Retinoic Acid Availability during Development and Regeneration: The Role of the CYP26 Enzymes

“…CYP26C1 also cleared RA metabolite 4-oxo-atRA more efficiently than the other two CYP26 enzymes. Overall, the metabolisation of RA by the CYP26 enzymes is up to 10 4 -fold higher than for other CYP enzymes shown to hydroxylate RA in the liver, such as CYP3A4, CYP3A5, CYP3A7, CYP2C8 and CYP2C22 [135,[137][138][139][140][141][142][143][144][145][146][147][148][149][150].…”

“…These are thought to be produced from the β-ionone ring of RA following multiple hydroxylations. These chemical forms are less biologically active than RA and undergo further glucuronation by UDP-glucuronosyltransferases (e.g., UTP2B7) to 4-O-β-glucuronide and are eventually eliminated from the cell (Figure 1) [139,143,144,146,[150][151][152].…”

Recent insights on the role and regulation of retinoic acid signaling during epicardial development

“…In zebrafish, however, the expression of zebrafish crabp2a and crabp2b is essential for hindbrain development and to ensure the robustness of RA-based patterning (Cai et al, 2012;Rackauckas, Schilling, & Nie, 2018;Schilling et al, 2012). Recent studies have revealed that both holo-CRABP1 and holo-CRABP2 deliver RA to CYP26B1 or CYP26C1 (Nelson et al, 2016;Zhong, Ortiz, Zelter, Nath, & Isoherranen, 2018). CRABP2 has been proposed to deliver RA to the nucleus for signaling while CRABP1 has been proposed to sequester RA and target it for degradation by CYP26 enzymes (Fiorella & Napoli, 1994).…”

“…CRABP2 has been proposed to deliver RA to the nucleus for signaling while CRABP1 has been proposed to sequester RA and target it for degradation by CYP26 enzymes (Fiorella & Napoli, 1994). Recent studies have revealed that both holo-CRABP1 and holo-CRABP2 deliver RA to CYP26B1 or CYP26C1 (Nelson et al, 2016;Zhong, Ortiz, Zelter, Nath, & Isoherranen, 2018). Meanwhile, unliganded apo-CRABPs inhibited the metabolism of RA by CYP26B1.…”