Photoisomerization of retinoic acids in ethanol under room light: a warning for cell biological study of geometrical isomers of retinoids.

Photoisomerization of all-trans-retinoic acid and the geometrical isomers [9-cis-retinoic acid, 11-cis-retinoic acid, 13-cis-retinoic acid and 9,13-di-cis-retinoic acid] in ethanol and their biological effects on F9 teratocarcinoma cells were analyzed. The rates of photoisomerization of the retinoic acids illuminated by fluorescent lamps (1,200 lx) increased in inverse proportion to their concentrations. When the ethanolic solution of all-trans-retinoic acid (10(-5) M) was kept under illuminated condition, the equilibrium mixture of the geometrical isomers of retinoic acid [all-trans-retinoic acid 25%, 9-cis-retinoic acid 10%, 11-cis-retinoic acid 10%, 13-cis-retinoic acid 30%, 9,13-di-cis-retinoic acid 5% and unidentified compound 20%] formed at around 30 min. The apparent velocity of the photoisomerization was approximately 8 x 10(-7) mol/L.min. Equilibrium mixtures with similar compositions were obtained by the photoisomerization of other geometrical isomers. The geometrical isomers produced by the photoisomerization possessed significantly different biological effects in the induction of differentiation of F9 cells into parietal endoderm-like cells: activities of 9-cis-retinoic acid (ED50, 8 x 10(-7) M), 11-cis-retinoic acid (ED50, 8 x 10(-7) M), and 13-cis-retinoic acid (ED50, 8 x 10(-7) M) were approximately 1/10 of all-trans-retinoic acid (ED50, 8 x 10(-8) M), and activity of 9,13-di-cis-retinoic acid (ED50, 1 x 10(-5) M) was 1/100 of the level of all-trans-retinoic acid. Further, the retinoic acids acted with each other additively on F9 cells.