A, analysis of retinoic acid conversion in RetSat-expressing cells. Tet-induced HEKK-RetSat or untransfected cells were incubated overnight with pure all-trans-retinoic acid (>90% pure by HPLC, assayed before incubation). Following incubation, retinoic acid was extracted and analyzed by reverse- phase HPLC System II. The appearance of 13,14-dihydroretinoic acid isomers was monitored at 290 nm (expected 25–30 min after injection). The peak numbers represent 13-cis-retinoic acid (1), 9,13-di-cis-retinoic acid (2), 9-cis-retinoic acid (3), and all-trans-retinoic acid (4). B, mixture of isomers of synthetic standards of 13,14-dihydroretinoic acid were examined by reverse-phase HPLC System II in order to establish the product elution profile. The inset shows the spectra of the different isomers of 13,14-dihydroretinoic acid. *, an unrelated compound. The experiment was performed in triplicate samples and repeated.