Physiological mechanisms of nutrient transport: Vitamin A and retinol-binding protein

Biologically active metabolites of retinol (vitamin A) affect growth, differentiation, and survival of cells. Retinol-binding protein (RBP) is a specific circulatory transport protein for retinol and participates its delivery to cells. Based on evidence for receptor-mediated endocytosis of RBP, experiments were carried out to further characterize (a) the RBP receptor and (b) the fate of endocytosed RBP in a human epithelial cell line. Additional experiments were performed with mice to examine (c) possible age-related changes in tissue accumulation of injected RBP. The results provide the first size characterization of an RBP receptor in these cells, a 157 kDa species, and the first evidence for recycling of endocytosed RBP. Age-related decreases in transport were suggested by a 23-27% decrease in the accumulation of exogenous RBP in old vs. young tissues (early post-injection times). Additional work on related transport mechanisms was carried out for RBP and the iron carrier transferrin.