CRABPs Alter -Retinoic Acid Metabolism by CYP26A1 via Protein-Protein Interactions.

Cellular retinoic acid binding proteins (CRABP1 and CRABP2) bind -retinoic acid (RA), the active metabolite of vitamin A, with high affinity. CRABP1 and CRABP2 have been shown to interact with the RA-clearing cytochrome P450 enzymes CYP26B1 and CYP26C1 and with nuclear retinoic acid receptors (RARs). We hypothesized that CRABP1 and CRABP2 also alter RA metabolism and clearance by CYP26A1, the third key RA-metabolizing enzyme in the CYP26 family. Based on stopped-flow experiments, RA bound CRABP1 and CRABP2 with K values of 4.7 nM and 7.6 nM, respectively. The unbound RA K values for 4-OH-RA formation by CYP26A1 were 4.7 ± 0.8 nM with RA, 6.8 ± 1.7 nM with holo-CRABP1 and 6.1 ± 2.7 nM with holo-CRABP2 as a substrate. In comparison, the apparent k value was about 30% lower (0.71 ± 0.07 min for holo-CRABP1 and 0.75 ± 0.09 min for holo-CRABP2) in the presence of CRABPs than with free RA (1.07 ± 0.08 min). In addition, increasing concentrations in apo-CRABPs decreased the 4-OH-RA formation rates by CYP26A1. Kinetic analyses suggest that apo-CRABP1 and apo-CRABP2 inhibit CYP26A1 (K = 0.39 nM and 0.53 nM, respectively) and holo-CRABPs channel RA for metabolism by CYP26A1. These data suggest that CRABPs play a critical role in modulating RA metabolism and cellular RA concentrations.