A RETINOL-BINDING PROTEIN RECEPTOR-MEDIATED MECHANISM FOR UPTAKE OF VITAMIN-A TO POSTIMPLANTATION RAT EMBRYOS

The major form of dietary vitamin A in blood is retinol bound to retin ol-binding protein (REP), but it is not clear how maternal retinol rea ches the embryo. We demonstrate here that uptake of radiolabeled retin ol from holo-RBP to the visceral yolk sac of cultured mt embryos exhib its receptor-mediated characteristics (K-m = 4.1 x 10(-7) M, V-max = 3 .0 x 10(-T2) mol/sec per gram yolk sac protein) and also demonstrate t he presence in the yolk sac endoderm of a protein previously described as an REP receptor. Furthermore, transport of radiolabeled retinol fr om the yolk sac to the embryo proper can be markedly decreased by inhi bition of visceral yolk sac REP synthesis. Thus embryonic accumulation of retinol depends on a relay of retinol from maternal REP to REP syn thesized in the yolk sac, via interaction with a visceral yolk sac REP receptor.