Distributions of retinoids, retinoid‐binding proteins and related parameters in different types of liver cells isolated from young and old rats

The levels of retinoids, retinol‐binding protein, cellular retinol‐binding protein, cellular retinoic‐acid‐binding protein, transthyretin and the activities of retinyl palmitate hydrolase and cholesteryl oleate hydrolase were determined in purified parenchymal, fat‐storing, endothelial and Kupffer cell preparations, and in liver homogenates from young adult (6‐month‐old) and old (36‐month‐old) rats. Retinoid levels were also determined in the plasma from young and old rats. Retinoid contents were determined by HPLC. The binding proteins and transthyretin were measured by specific radioimmunoassays; retinyl palmitate and cholesterol oleate hydrolases were measured by sensitive microassays. The retinoid content of both the liver homogenates and of the fat‐storing, and parenchymal cell preparations increased between 6 months and 36 months of age. The cellular distribution of retinoids was similar for the two age groups analyzed with the fat‐storing cells being the main retinoid storage sites in the rat liver. Concentrations of retinol‐binding protein and transthyretin were high in parenchymal cell preparations. Cellular retinol‐binding protein was enriched both in parenchymal and in fat‐storing cell preparations; the highest concentrations of cellular retinoic‐acid‐binding protein were present in fat‐storing cell preparations. No major differences were observed between the two age groups in the cellular concentrations and distributions of any of these binding proteins. High activity of cholesterol oleate hydrolase was measured in parenchymal and in Kupffer cell preparations; endothelial cell preparations also contained considerable activities. The distribution of this activity over the various cell types reflects their role in lipoprotein metabolism. Retinyl palmitate hydrolase activity was specifically enriched in parenchymal and in fat‐storing cell preparations, consistent with the roles of these cells in retinoid metabolism. No major differences were observed between the two age groups in the cellular distributions of the two hydrolase activities. This study indicates that no major changes occur in the retinoid‐related parameters analyzed with age, suggesting that rat liver retinoid metabolism does not change dramatically with age and that retinoid homeostasis is maintained.