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Biosynthesis of Low Density Lipoprotein by Cell-free Preparations of Rat Intestinal Mucosa

From the ¹ From the Laboratory for Gastrointestinal Research, McGill University Clinic, Royal Victoria Hospital, and Laboratory for Nutrition and Metabolic Research, Jewish General Hospital, Montreal, Canada

The capacity of cell-free preparations of rat small intestinal mucosa to synthesize and release lipoproteins was investigated. Palmitate-1-¹⁴C was administered intragastrically and 30 min later labeled cell-free fractions from homogenates of intestinal mucosa were prepared. The fractions (whole homogenate, mitochondria, and microsomes) were incubated in fortified media containing ³H-labeled amino acids, and lipoproteins of various densities were separated, after the addition of carrier rat plasma, by density centrifugation, by polyanion precipitation, and by immunoprecipitation. All fractions incorporated radioactivity into the lipid and protein moieties of the medium lipoproteins. The activity of the microsomal fraction accounted for most of the activity of the whole homogenate. Presence of postmicrosomal supernatant or pH 5 fraction, GTP, ATP, and ATP-generating system was essential for optimal activity of the microsomal fraction. The ratio of ¹⁴C:³H of microsomal (solubilized by LiCl) and medium low density lipoprotein (LDL) isolated by ultracentrifugation, by precipitation with polyanions, or by immunoprecipitation remained within reasonably narrow limits.

The identity of microsomal and medium LDL with that of plasma LDL was demonstrated by immunoelectrophoresis, double immunodiffusion, and peptide mapping.

The results indicate that the cellular equivalent of the microsomal fraction may be the major site of synthesis and association of the lipid and protein moieties of LDL. They further indicate that the intestinal mucosa may contribute significantly to the level of plasma LDL and that through the incorporation of LDL apoprotein into chylomicrons and very low density lipoprotein it can be of central importance in the transport of exogenous and endogenous lipid.

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