Vitamin A uptake from retinol-binding protein in a cell-free system from pigment epithelial cells of bovine retina. Retinol transfer from plasma retinol-binding protein to cytoplasmic retinol-binding protein with retinyl-ester formation as the intermediate step.

摘要: We have investigated the steps by which retinol, released from plasma retinol-binding protein (RBP), enters cells and is accumulated for most part as a retinyl-ester, only small fraction of it being present complex with cytoplasmic (CRBP). For this purpose, we developed cell-free system composed membrane-enriched fractions bovine retinal pigment epithelium selectively incorporates exogenous vitamin A when presented retinol-RBP complex. Upon incubation in presence [3H]retinol-RBP, isolated membrane take up esterify retinol. 4-fold reduction total incorporation observed conditions specifically inhibit retinyl-ester formation, thus indicating that two processes retinol uptake esterification are functionally coupled. Evidence bound to receptor sharing functional structural similarities CRBP actual substrate esterification. Vitamin accumulation seems require allow recycling limited number free, membrane-associated, receptors. Mobilization stored membrane-bound mediated membrane-associated hydrolase activity controlled level apo-CRBP acts carrier Up 90% upon (11 microM) concomitant formation retinol-CRBP. The overall process, never needs leave its binding proteins, allows form regulated mobilization retinol-CRBP