COMPARISON OF THE METABOLISM OF [1,2,6,7-H-3(N)]CHOLESTERYL OLEATE, CHOLESTERYL [9,10-H-3]OLEATE, AND CHOLESTERYL [1-C-14]OLEATE LABELED LIPOPROTEINS IN THE RAT

The intravascular metabolism of sterol labeled [1,2,6,7-H-3(N)]cholesteryl oleate and acyl labeled cholesteryl [9,10-H-3]oleate and cholesteryl [1-C-14]oleate was compared in the rat, an animal species without plasma cholesteryl ester transfer activity (CETA). In a first series of studies, the metabolism of sterol labeled [1,2,6,7-H-3(N)]cholesteryl oleate and acyl labeled cholesteryl [1-C-14]oleate was compared, and the two tracers had identical plasma clearance rates when incorporated into human low density lipoproteins (LDL). The H-3 sterol labeled cholesteryl ester (CE), however, had a plasma clearance rate lower than the C-14 acyl labeled CE when incorporated into rat alpha- and beta-migrating LDL and human or rat high density lipoproteins (HDL). Unesterifed H-3 cholesterol reappeared in the plasma whereas the C-14 radioactivity in the plasma remained associated with the CE. In a second set of studies, LDL and HDL were radiolabeled with cholesteryl [9,10-H-3]oleate and cholesteryl [1-C-14]oleate: Large amounts of H-3 radioactivity that were dialyzable and not associated with the lipoprotein CE reappeared in the plasma during the kinetic studies. The two tracers had identical plasma disappearance rates when the plasma samples were dialyzed. The results of these studies indicate that the nature of the tracer used to trace lipoprotein CE can affect the estimated kinetic parameters of plasma CE.