Uptake and cleavage of β-carotene by cultures of rat small intestinal cells and human lung fibroblasts

The purpose of the experiments reported here was to study the uptake kinetics of beta-carotene (BC) into rat small intestinal cells (hBRIE 380) in culture and the cleavage kinetics of BC to determine which of these two is the limiting process. Time and concentration dependence of uptake of BC into hBRIE 380 showed no saturation up to 25 μmol/L BC; uptake was barely reduced at 4° C even though conversion to retinol was virtually completely inhibited at that temperature. No evidence of a membrane receptor could be observed. Similar uptake kinetics, with a smaller amount taken up, were found with human lung fibroblast (strains W1-38 and HLF). Cleavage to retinol (17.3% of BC per 106 cells in 24 hr) and retinoic acid (RA) (5.3%) were observed, with an apparent KM of 9 μmol/L with respect to retinol. Both retinol and RA were identified by high-pressure liquid chromatography and derivatization. No retinyl esters were detectable under our experimental conditions. Conversion in W1-38 cells was also observed (7.8% retinol, 2.5% RA). We conclude that uptake of BC into rat hBRIE 380 cells and fibroblasts is passive and unregulated; conversion to retinol and RA is regulated, presumably through the cleavage enzyme(s). © 1992.