SPECIFICITY OF THE RETINOL TRANSPORTER OF THE RAT SMALL-INTESTINE BRUSH-BORDER

The uptake of vitamin A (all-trans-retinol) by the absorptive cell of the small intestine is the necessary first step in its utilization by the organism and appears to involve a specific carrier that operates b y facilitated diffusion. We investigated the specificity of that proce ss by determining the absorption of all-trans-, 13-cis-, and 9-cis-ret inol, 3-dehydroretinol, and retinal (vitamin A aldehyde) by gut sheets from the small intestine of suckling rats. We found that radiolabeled all-trans-retinol and 3-dehydroretinol were absorbed at similar rates and that approximately 60% of the total absorption could be competed for by unlabeled all-trans-retinol. A similar level of inhibition coul d be achieved for all-trans-retinol absorption by treating the intesti nal sheets with N-ethylmaleimide. The noncompetable, noninhibitable co mponent of all-trans-retinol absorption corresponded to the total abso rption rate for 13-cis- and 9-cis-retinol and retinal. Additionally, w e found that the relative rates of transport of these retinoids were u nrelated to their relative affinities for the abundant absorptive cell retinoid carrier protein, cellular retinol-binding protein, type II, and were not driven by esterification. This confirms that the absorpti on of retinol is facilitated by a transporter and establishes that it is specific for the all-trans alcohol forms of vitamin A.