MICROSOMAL RETINAL SYNTHESIS - RETINOL VS HOLO-CRBP AS SUBSTRATE AND EVALUATION OF NADP, NAD AND NADPH AS COFACTORS

Holo-CRBP (cellular retinol binding protein) is recognized specifically by an NADP-dependent microsomal retinol dehydrogenase and protects retinol from conversion into retinal by NAD and NADPH dependent dehydrogenases. The synthesis of retinal from free retinol is catalyzed by both NADP- and NAD-dependent pathways, with the former being the preferred one (K(m) of 4 vs. 22-mu-M for retinol, and V(max)/K(m) of 33 vs. 9, respectively). NADPH does not support quantitatively significant retinal synthesis from physiological concentrations of retinol or holo-CRBP, if an NADPH regenerating system is used to prevent NADP formation.