Biosynthesis of 9-cis-retinoic acid from 9-cis-beta-carotene in human intestinal mucosa in vitro.

The role of 9-cis-beta-carotene (9-cis-beta-C) as a potential precursor of 9-cis-retinoic acid (9-cis-RA) has been examined in human intestinal mucosa in vitro. By using HPLC, uv spectra, and chemical derivatization analysis, both 9-cis-RA and all-trans-retinoic acid (all-trans-RA) have been identified in the postnuclear fraction of human intestinal mucosa after incubation with 9-cis-beta-C at 37 degrees C. The biosynthesis of both 9-cis-RA and all-trans-RA from 9-cis-beta-C was linear with increasing concentrations of 9-cis-beta-C (2-30 microM) and was linear with respect to tissue protein concentration up to 0.75 mg/ml. Retinoic acid was not detected when a boiled incubation mixture was incubated in the presence of 9-cis-beta-C. The rate of synthesis of 9-cis- and all-trans-RA from 4 microM 9-cis-beta-C were 16 +/- 1 and 18 +/- 2 pmol/hr/mg of protein, respectively. However, when 2 microM all-trans-beta-C was added to the 4 microM 9-cis-beta-C, the rate of all-trans-RA synthesis was increased to 38 +/- 6 pmol/hr/mg of protein, whereas the rate of 9-cis-RA synthesis remained the same. These results suggest that 9-cis-RA is produced directly from 9-cis-beta-C. Furthermore, incubations of either 0.1 microM 9-cis- or all-trans-retinal under the same incubation conditions showed that 9-cis-RA could also arise through oxidative conversion of 9-cis-retinal. Although only 9-cis-RA was detected when 9-cis-RA was used as the substrate, the isomerization of the all-trans-RA to 9-cis-RA cannot be ruled out, since both all-trans-RA and trace amounts of 9-cis-RA were detected when all-trans-retinal was incubated as the substrate. These data indicate that 9-cis-beta-C can be a source of 9-cis-RA in the human. This conversion may have a significance in the anticarcinogenic action of beta-C.