Serum and intracellular retinol in the equine.

1. Serum and intracellular distribution of retinol was determined in equines maintained on four levels of vitamin A intake. 2. The form of retinol transported in serum was determined by gel filtration and chromatography to be a complex of retinol bound to a protein of molecular weight (MW) of approximately 20000, which was in turn complexed probably with prealbumin to yield a complex with a MW of 75000 to 80000. 3. Increasing dietary vitamin A levels enhanced the concentration of lipoprotein-bound retinyl esters in the plasma. 4. Vitamin A in the liver cytosol was found predominantly as retinyl esters in a lipid-protein aggregate of MW approximately 2 X 10(6) and hydrated density of 1.063-1.111. In the kidney and adrenal gland, two lipid-protein entities were found with MW of approximately 1.8 X 10(6) and 1.7 X 10(6) respectively. These fractions contained approximately 40 and 20% lipid respectively and had densities of 1.063-1.111 and approximately 1.21. 5. All lipid-protein aggregates were associated with retinyl palmitate hydrolase activity and guanidine treatment released a 15000 MW material, presumably intracellular retinol-binding protein. 6. Increasing dietary vitamin A enhanced the proportion of retinol in the 1.7 X 10(6) fraction. 7. Findings in equine plasma and liver resemble previous observations in other species. The characterization of two new lipid-protein aggregates in equine kidney and adrenal glands, which have hydrolase activity, may be important in intracellular retinal transport and metabolism, especially in animals subjected to high intakes of vitamin A.