Production of β-Lipoprotein by Intestine in the Rat

Abstract Evidence for the production of circulating β-lipoprotein and endogenous triglycerides by the intestine was obtained from an examination of intestinal lymph in control rats and in rats fed orotic acid as 2% of the diet. The lipoproteins in lymph were characterized by immunochemical methods, electrophoresis, preparative ultracentrifugation, and lipid analysis. In control rats on a fat-free diet, intestinal lymph contained higher concentrations of β-lipoprotein and triglycerides than did plasma, whereas the lymph concentration of α-lipoprotein was lower than in plasma. In rats fed orotic acid the intestinal lymph content of β-lipoprotein and triglycerides was as high as in the controls, although the plasma concentrations of both β-lipoprotein and triglycerides were less than 5% of normal. In control as well as in orotic acid-fed rats, all intestinal lymph β-lipoprotein and 95% of lymph total fatty acids were recovered in a very low density lipoprotein fraction (density less than 1.006 g per ml) with pre-β electrophoretic mobility. α-Lipoprotein was found in the fraction of density greater than 1.006 g per ml and in the very low density lipoprotein after partial delipidation. Data from the present experiments and earlier experiments with Triton WR-1339 indicate that on a fat-free diet approximately 10% of the plasma triglyceride is derived from the intestine and reaches the blood via lymph in association with very low density lipoprotein. Chylomicrons appeared in the lymph of orotic acid-fed as well as control rats following a fat meal. Previous work has shown that the livers of orotic acid-fed rats are unable to produce β-lipoprotein or to release triglycerides into the circulation. Thus when β-lipoprotein is produced by the intestine but not by the liver, normal transport of exogenous as well as endogenous triglycerides is observed from the intestine into lymph but not from the liver into blood.