INVITRO UPTAKE OF VITAMIN-A FROM RETINOL-BINDING PLASMA-PROTEIN TO MUCOSAL EPITHELIAL-CELLS FROM MONKEYS SMALL-INTESTINE

Abstract

The in vitro uptake of retinol from its plasma carrier protein, the retinol-binding protein (RBP), to the cells of the cynomolgus monkey''s small intestine was studied. [3H]Retinol was readily delivered from the RBP to the cells without a concomitant cellular uptake of the RBP. The [3H]retinol accumulation in the cells was dependent on the temperature and was virtually abolished at 0.degree.. Several metabolic inhibitors could not impede the uptake process. The cellular [3H]retinol accumulation was linear for about 45 min and exhibited characteristic saturation kinetics. The uptake of [3H]retinol by the cells was inhibited by RBP containing unlabeled retinol, vitamin A-depleted RBP and Fab [antibody active] fragments against RBP. In contrast, free, unlabeled retinol and the metabolite form of RBP, lacking retinol and affinity for prealbumin, were inactive. There was a receptor for vitamin A on the cell surface which recognized the protein part of the protein ligand complex.