The uptake of retinol has been investigated by incubating suspensions of isolated bovine retinal pigment epithelium plasma membranes in the presence of [3H]retinol-retinol-binding protein complex. After SDS gel electrophoresis of the labelled membranes, [3H]retinol is found to be associated with a protein band of molecular weight 16000. The binding process is saturable, specific and shows a linear dependence on the membrane concentration in the incubation mixture. A dissociation constant of the order of 10-9 has been calculated for the retinol-receptor interaction. These observations confirm previous data obtained with isolated pigment epithelium cells and suggest the existence of an intermediate step, at the membrane level, in the transfer of retinol from its plasma carrier to the cell cytoplasm. © 1981 Academic Press Inc. (London) Ltd.
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