PLASMA LIPOPROTEIN METABOLISM IN PERFUSED RAT LIVERS. II. TRANSFER OF FREE AND ESTERIFIED CHOLESTEROL INTO THE PLASMA*

Abstract

In a previous paper (1), the synthesis and entry into the plasma of the protein moiety of the different lipoprotein classes has been described.The present studies deal with the metabolism of the free and esterified cholesterol of the lipoproteins.Relatively little is known of the metabolism of the plasma lipids as they exist in the various lipo- protein fractions.Study of certain lipids is diffi- cult, if not impossible, because exchange occurs among different lipoprotein fractions, and useful observations can be made only with those lipids not undergoing exchange (2).Previous studies (3-5) have shown that the in vitro exchange of ester cholesterol is slow relative to that of free cholesterol.In the present study, we labeled free and esterified cholesterol of the plasma biosynthetically and measured their equilibration be- tween various lipoprotein fractions.After we established that there was virtually no exchange of esterified cholesterol in vitro, we examined the metabolism of this lipid fraction of the various lipoproteins. METHODSAnimals.Male rats of the Wistar strain 1 weighing 300 to 500 g were used as blood and liver donors in all studies except the two shown in Tables VI, VII, and VIII, in which Sprague-Dawley rats 2 were used.The standard diet was one of Rockland Farm mouse pellets.* Supported by U. S.