Effects of dietary protein on composition and turnover of apoproteins in plasma lipoproteins of rabbits

Abstract

The hypercholesterolemia and atherosclerosis produced in rabbits by feeding cholesterol-free, semipurified diets are due to the use of casein as the protein component of such diets and can be prevented by replacing the casein with isolated soy protein. To investigate the reasons for the differing effects of these dietary proteins on plasma cholesterol levels, plasma lipoproteins were isolated from rabbits fed semipurified diets containing either casein or isolated soy protein, labeled with 125I, and reinjected into rabbits fed one or other of these two diets. 125I-labeled apoproteins of intermediate density lipoprotein, isolated from rabbits on either diet, turned over more rapidly in rabbits fed soy protein compared with those fed casein. 125I-labeled apoproteins of very low density lipoprotein from rabbits fed soy protein were transferred to high density lipoprotein more rapidly than those from rabbits fed casein. In this case, the results were determined primarily by the diet fed to the donor rabbits, but the diet fed to the recipients also appeared to have some influence. The apoproteins of plasma lipoproteins from rabbits fed casein or soy protein were separated by isoelectric focussing and tentatively identified by comparison of their isoelectric points with those of apoproteins from human plasma lipoproteins. The concentration of apoprotein E was markedly increased in the very low density and intermediate density lipoproteins of casein-fed rabbits, and apoprotein C was also increased in the very low density lipoprotein of rabbits fed casein, compared with those fed soy protein. Effects of dietary proteins on plasma cholesterol may be secondary to their effects on the composition and metabolism of the protein components of plasma lipoproteins.