The transport of vitamin A and carotenoids in human plasma

Abstract

The hydrocarbon carotenoids, β-carotene and lycopene, are transported exclusively by the serum lipoproteins. About 75% of these carotenoids are found in the low-density S f 3–9 lipoproteins, while the remainder are found in the high-density α-lipoproteins. Lutein, the carotenoid alcohol, is more widely distributed between the serum proteins, although a major fraction of the serum lutein is transported by the low- and high-density lipoproteins. It is suggested that the free hydroxyl group facilitates lutein association with other serum proteins. Vitamin A alcohol is not transported by a serum lipoprotein but a protein with a density greater than 1.21 g./ml. This protein is apparently not serum albumin, since albumin isolated by two different experimental procedures did not contain significant amounts of vitamin A alcohol. Vitamin A esters, during the period of active absorption, are concentrated in the low-density S f 10–100 lipoproteins. Experimental results from several studies, which suggest that the reticuloendothelial system is the site not only for the removal of vitamin A esters from plasma but also for the conversion of S f 10–100 to S f 3–9 lipoproteins, are summarized.