Quantification of Postprandial Triglyceride-Rich Lipoproteins in Healthy Men by Retinyl Ester Labeling and Simultaneous Measurement of Apolipoproteins B-48 and B-100

Abstract

Abstract The metabolism of chylomicrons, very-low-density lipoprotein (VLDL), and their remnants in the postprandial state was studied in normolipidemic healthy men by measuring apoB-48 and apoB-100 and retinyl palmitate (RP) in fractions of triglyceride-rich lipoproteins after a mixed meal type of oral fat load supplemented with vitamin A. ApoB-48 was present at low concentrations in the fasting plasma samples in most subjects and increased in response to the test meal in Svedberg’s flotation rate (Sf) >20 lipoprotein fractions. Concomitantly, the level of Sf 60 to 400 apoB-100 (large VLDL) had doubled at 3 hours and returned to baseline at 9 hours. The number of apoB-48–containing lipoprotein particles did not exceed 20% of the total number of apoB-containing lipoproteins contained in Sf 12 to 400 fractions at any time point after fat intake. The peak plasma level of RP was delayed compared with the peak plasma concentration of apoB-48, suggesting that retinyl ester labeling of chylomicrons is questionable as a means of quantifying postprandial triglyceride-rich lipoproteins of intestinal origin. Approximately 2000 and 4000 RP molecules were carried in each chylomicron particle in the 3- and 6-hour samples, respectively, in contrast to the remnant fractions in which 100 to 600 RP molecules were found for each lipoprotein particle. The limited RP exchange between lipoprotein particles indicates that the smaller intestinal lipoproteins do no originate primarily from larger Sf >400 chylomicron particles but instead are secreted directly into the Sf 20 to 400 fraction and subsequently converted to smaller chylomicron remnants. This notion was further substantiated by injection of RP-labeled postprandial plasma into fasting subjects with subsequent tracing of RP in fractions of triglyceride-rich lipoproteins.