Plasma lipoprotein retinoids after vitamin A feeding in normal man: minimal appearance of retinyl esters among low-density lipoproteins

Abstract

Retinyl esters have been thought to be carried solely by lipoproteins of intestinal origin (chylomicrons and their catabolic derivatives). Recent reports, however, have indicated that there may be significant transfer of retinyl esters from chylomicrons to low-density lipoproteins (LDL) in vitro, and that in other species, substantial amounts of retinyl esters may appear in LDL. Since in man lipoproteins of intestinal origin are not considered to contribute to a quantitatively significant extent to circulating LDL, we have examined this issue further. The distribution of retinol and retinyl esters within the plasma lipoproteins of eight normal human volunteers was measured following the ingestion of vitamin A along with a mixed meal. Retinyl esters appeared in the chylomicrons and very-low-density lipoproteins. Small amounts of retinyl esters were also detected in the intermediate- and low-density lipoprotein (LDL) classes. Estimates of the masses of retinyl esters, however, indicated that 5% or less of chylomicron retinyl esters appeared in the LDL. These observations are consistent with orderly chylomicron delipidation and provide further evidence that chylomicron-derived components do not contribute directly or to a quantitatively significant extent to circulating LDL.