Absorption of retinol from the retinol:retinol-binding protein complex by small intestinal gut sheets from the rat.

Absorption of retinol by the absorptive cells of the small intestine is a necessary first step in the metabolism of vitamin A. Previous studies had suggested that absorption of retinol from the retinol:retinol-binding protein complex (retinol:RBP) might be receptor mediated. We investigated the specificity of this process by determining the absorption of retinol from retinol:RBP by small intestinal sheets obtained from the suckling rat. We found that the absorption of retinol from retinol:RBP was a saturable process. Unlike the absorption of free retinol, absorption of retinol from retinol:RBP was not inhibited by N-ethylmaleimide. The absorption was specifically competable by unlabeled retinol:RBP and by both apo- and holocellular retinol-binding protein, but not by beta-lactoglobulin. This suggests that a specific mechanism is present for the absorption of retinol bound to RBP.