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 by facilitated diffusion. We investigated the specificity of that process by determining the absorption of all-trans-, 13-cis-, and 9-cis-retinol, 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 could be achieved for all-trans-retinol absorption by treating the intestinal sheets with N-ethylmaleimide. The noncompetable, noninhibitable component of all-trans-retinol absorption corresponded to the total absorption rate for 13-cis- and 9-cis-retinol and retinal. Additionally, we found that the relative rates of transport of these retinoids were unrelated 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 absorption of retinol is facilitated by a transporter and establishes that it is specific for the all-trans alcohol forms of vitamin A.