Identification of circulating apolipoproteins synthesized by rat small intestine in vivo.

Apolipoproteins synthesized by intestine were identified in mesenteric lymph and plasma collected from an animal preparation in which exposure to [3H]leucine was restricted to the mucosal epithelial cells of a fat-transporting small intestinal segment in situ. The intestine released labeled chylomicrons and very low density lipoproteins only into lymph but labeled high density lipoproteins into both plasma and lymph. Of the 3H incorporated into individual apolipoproteins, separated by polyacrylamide gel electrophoresis, 25% was recovered in apolipoprotein B (apoB), 50% in apoA-I, 21% in apoA-IV, and 4% in C apolipoproteins, nearly all in apoC-II and apoC-III-0. Only traces of radioactivity were associated with the arginine-rich protein and none was detected in apoA-II. Labeled apoB was found in chylomicrons and very low density lipoproteins and labeled apoA-I and apoA-IV in all lipoproteins. Evidence suggested that substantial amounts of labeled apoA-IV were also present in the lipoprotein-free lymph fraction (d greater than 1.21 g/ml), further indicating that apoA-IV is a major apolipoprotein product in intestine. Thus, the pattern of apolipoprotein synthesis by intestine is notably different from that reported for liver.