The intestine as a source of apolipoprotein A1.

The major apoprotein of rat mesenteric lymph chylomicrons has been isolated and characterized and shown to be identical to apoprotein A1 (apo A1) isolated from serum high density lipoprotein (HDL). During intestinal lipid absorption, active synthesis of apo A1 was demonstrated by radioactive amino acid incorporation into lymph chylomicron A1 as well as lymph HDL. Immunofluorescence studies of intestinal epithelium demonstrated a marked increase in apo A1 fluorescence, confirming an active synthesis of this apoprotein during lipid absorption. Quantitative immunoelectrophoretic methods were used to measure apo A1 in lymph and peripheral blood during various conditions designed to estimate the quantitative importance of intestinal apo A1 to the levels of circulating lipoproteins. During lipid feeding there was an increase in lymph apo A1 that was associated with lymph lipoproteins (50%) of density less than 1.006 g/ml whereas in basal lymph most apo A1 (85%) was in the lipoproteins of density greater than 1.006 g/ml. Lipid feeding in animals without lymph fistulas resulted in a significant increase in serum apo A1 levels; biliary diversion, designed to eliminate intestinal lipoproteins of density less than 1.006 g/ml, resulted in a significant decrease in serum apo A1 levels. These studies demonstrate that the intestine actively synthesizes apo A1 and is a significant source of this apoprotein for circulating lipoproteins.