All-trans retinoic acid (RA) is a critical signaling molecule and its concentration is tightly regulated. Several P450 enzymes including CYP26A1, CYP2C8, and CYP3A4 have been proposed to be responsible for RA clearance in the liver but their quantitative importance has not been demonstrated. To determine the contribution of CYP26A1 to hepatic clearance of RA, CYP26A1 protein was quantified in 37 human liver microsomes (HLMs). CYP26A1 expression ranged from not detectable to 2.80pmol/mg microsomal protein. RA clearance by P450 enzymes abundant in human liver was measured in Supersomes. CYP2C8, CYP3A4, CYP3A5 and CYP3A7 metabolized RA with unbound K(m) values of 3.4-7.2microM and V(max) values of 2.3-4.9pmol/min/pmol P450, but were less efficient than CYP26A1 in clearing RA. Simulations performed for livers with varying P450 expression levels over a range of RA concentrations demonstrated that at both endogenous and therapeutic concentrations of RA, CYP26A1 is the primary enzyme responsible for 4-OH RA formation clearance. HLM incubation data showed that 4-OH RA formation velocity varied from 0.2 to 15.3pmol/min/mg microsomal protein and velocity in HLMs was significantly correlated (p<0.01) to CYP26A1, CYP3A4, and CYP3A5 protein content, but not to CYP2C8. When experimental data were scaled to in vivo clearances, the predicted hepatic clearance of RA (0.07L/min using combined Supersome data) was similar to the published in vivo clearance of RA. These findings suggest that CYP26A1 is the P450 isoform that should be targeted when designing RA metabolism blocking agents.
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