In Vitro Binding of Retinol to Rat-Tissue Components

The high-speed supernatant fraction of rat liver, lung, kidney, testis, and intestinal mucosa contains a component capable of binding [(3)H]retinol in vitro when binding is analyzed by sucrose density gradient centrifugation or gel filtration. This binding component can be distinguished from one identified in rat serum. Whereas the tissue component sediments in the 2S region of sucrose gradients, the serum component sediments in the 4.6S region. Molecular weight estimations by gel filtration indicate molecular weights of 16,000 and 67,000 for the tissue and serum binding components, respectively. Unlabeled retinol, but not retinoic acid, competes for the binding of [(3)H]retinol in tissue cytosols. Competition for the binding of [(3)H]retinol by unlabeled retinal has also been observed in tissue cytosols, but may result from the in vitro reduction of retinal to retinol. Unlabeled retinol, retinal, and retinoic acid fail to compete for the binding of [(3)H]retinol in serum under the conditions used. The tissue binding component (testis) is sensitive to digestion with Pronase, but not with RNase or DNase, indicating a protein nature for this component.