Effect of retinol status on retinol-binding protein biosynthesis rate and translatable messenger RNA level in rat...

Studies were conducted to explore the role of retinol in the control of the rate of synthesis of plasma retinol-binding protein (RBP) in the liver of the rat. Previous studies have shown that nutritional retinol status strongly influences RBP secretion from the liver cell. Both the in vivo relative rate of RBP synthesis and the in vitro translatable level of RBP-specific mRNA were examined in normal vitamin A, retinol-depleted, and retinol-repleted rats. The relative rate of RBP synthesis was estimated by measuring the extent of incorporation of [3H]leucine, [3H]lysine, and [3H]phenylalanine after a 12-min pulse label into immunoprecipitable RBP, relative to the incorporation of these amino acids into total liver trichloroacetic acid-precipitable protein. The level of translatable RBP-specific mRNA was quantitated in vitro by translation of rat liver poly(A+)RNA in the rabbit reticulocyte lysate protein-synthesizing system. The amount of newly synthesized RBP was determined relative to the amount of newly synthesized total protein. Both the relative rate of RBP synthesis (approximately 0.26%) and the translatable level of RBP-specific mRNA (approximately 0.14%) were found to be constant regardless of the retinol status of the rats. These results indicate that retinol, the molecule that RBP specifically binds and transports, does not appear to control the rate of synthesis of RBP or the translatable level of RBP-specific mRNA in the liver of the rat. Regulation of plasma RBP levels by retinol must be exercised at a locus beyond that of RBP synthesis.