SYNTHESIS OF HIGH SPECIFIC ACTIVITY [H-3] 9-CIS-RETINOIC ACID AND ITSAPPLICATION FOR IDENTIFYING RETINOIDS WITH UNUSUAL BINDING-PROPERTIES

all-trans-Retinoic acid is known to bind to the retinoic acid receptor s (RARs) resulting in an increase in their transcriptional activity. I n contrast, recently identified 9-cis-retinoic acid (9-cis-RA), which is an additional endogenous RA isomer, is capable of binding to both R ARs and retinoid X receptors (RXRs). These distinct properties have ra ised questions as to the biological role governed by these two retinoi c acid isomers and the set of target genes that they regulate. Herein, we report the synthesis of high specific activity [H-3]-9-cis-RA and its application to study the ligand-binding properties of the various retinoid receptor subtypes. We examined the binding properties of RARs and RXRs for a series of synthetic retinoids and compared the ligand- binding properties of these arotinoid analogs with their ability to re gulate gene expression via the retinoid receptors in a cotransfection assay. The utilization of the [3H]-9-cis-RA competitive binding assay and the cotransfection assay has made it possible to rapidly identify important structural features of retinoids leading to increased select ivity for either the RAR or RXR receptor subtypes.