9-Cis beta-carotene as a precursor of retinol isomers in chicks.

The possibility that 9-cis beta-carotene serves as a precursor for retinol in general, and particularly for 9-cis retinol, was studied in chicks, in which the 9-cis beta-carotene is preferentially accumulated. One week old chicks were fed a synthetic diet supplemented with 9-cis and all-trans beta-carotene mixtures, in which the 9-cis isomer content was 9, 19.1, 31.2, 40.8, 57.7 or 77.0%. It was found that an elevation in the dietary 9-cis beta-carotene level led to a decrease in the hepatic retinol stores, despite the increased level of total and 9-cis beta-carotene in the liver. HPLC chromatography revealed the presence of all-trans, 13-cis and 9-cis retinol in the livers. A high 9-cis beta-carotene diet brought about a significant, but very small, elevation in the part of the cis retinol isomers relative to the all-trans isomer. It was concluded that 9-cis beta-carotene has a low activity as a precursor of retinol in general. This decreased activity is not due to a low availability of the isomer to the enzymatic cleavage. It seems also that 9-cis beta-carotene is not an efficient precursor of 9-cis retinol.