Mouse RetSat catalyzes the saturation of the C13-C14 doublebond of all-trans-retinol to produce all-trans-13,14-dihydroretinol81 (Figure 1). A related enzyme in zebrafish catalyzes the saturationof the C7-C8 double bond in addition to the C13-C14 doublebond of all-trans-retinol to produce both 8 and all-trans-7,8-dihydroretinol. 2 Further oxidation of 8 and of all-trans-7,8-dihydroretinol by retinol dehydrogenases and then by retinaldehydedehydrogenase enzymes leads to formation of all-trans-13,14-dihydroretinoic acid 9 and all-trans-7,8-dihydroretinoic acid, com-pounds whose levels are exquisitely controlled in vivo by theenzymes that catalyze their synthesis and breakdown.2,3 Both 9 andall-trans-7,8-dihydroretinoic acid are highly selective agonists inactivating the retinoic acid receptor (RAR) but not the retinoid Xreceptor (RXR). 3,4 Because 13,14-dihydroretinoids are chiral com-pounds it is important to determine their absolute configuration andevaluate how the different enantiomers interact with bindingproteins, receptors, and enzymes. In this study we investigated theabsolute configuration of biologically derived 8 and consequentlyof 9 and evaluated the activation of RAR by the enantiomers of 9