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Journal of Lipid Research, Vol 30, 207-218, Copyright © 1989 by Lipid Research, Inc.
Intestinal apolipoprotein synthesis and secretion in the suckling pig
DD Black and NO Davidson
Department of Pediatrics, University of Chicago, Pritzker School of Medicine, IL 60637.
The present studies report characterization of intestinal apolipoprotein
(apoLp) synthesis and secretion in the suckling pig. Lipoproteins (d less
than 1.006 g/ml) from mesenteric lymph were found to contain both apoB-100
and B-48, in addition to apoA-IV, E, A-I, and Cs. Lymph low density
lipoproteins (LDL) and high density lipoproteins (HDL) contained mainly
apoB-100 and apoA-I, respectively. Analysis of core cholesteryl ester fatty
acid composition suggested filtration from plasma as the major source of
lymph LDL and HDL. Dual radioisotope labeling of intestinal and hepatic
apoLps in lymph, as well as immunoprecipitation of radiolabeled intestinal
mucosa, demonstrated intestinal synthesis of apoB-48, A-IV, and A-I. There
was no evidence for apoB-100 synthesis by intestinal mucosa. By contrast,
piglet liver synthesized apoB-100, E, A-I, and Cs, but not apoB-48. Newly
synthesized intracellular intestinal apoA-I was mainly (basic) isoform 1
(pI 5.58), while lymph and plasma HDL apoA-I were predominantly isoform 3
(pI 5.33), mature apoA-I. Lymph apoB (P less than 0.001) and apoA-I (P less
than 0.04) mass output increased significantly during lipid absorption.
Studies were subsequently conducted in fasting, fat- fed, bile-diverted,
and sham-operated animals to determine the role of both dietary and biliary
lipid in regulating intestinal apoLp biosynthesis. Proximal and distal
small intestinal loops were pulse- radiolabeled with [3H]leucine, and
apoB-48 and A-I were immunoprecipitated from cytosolic supernatants.
Although a proximal to distal gradient in intestinal synthesis rates for
both apoB and A-I was noted in all groups, the acute absorption of dietary
lipid did not significantly increase apoB or A-I synthesis in either
location. Complete removal of biliary lipid for 48 hr did not alter
synthesis rates in jejunum or ileum. These studies suggest that mesenteric
lymph apoLps in the suckling pig are derived both by filtration from plasma
and by direct secretion from the intestine. Physiologic regulation of
intestinal apoA-I and B-48 synthesis rates appears to be independent of
luminal lipid availability.

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Copyright © 1989 by the American Society for Biochemistry and Molecular Biology.
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