Dietary Amino Acids Are the Preferential Source of Hepatic Protein Synthesis in Piglets
Abstract
Abstract To investigate the utilization of dietary amino acids for hepatic protein synthesis, seven female pigs ( 28 d old, 7.5 kg) were implanted with catheters in a carotid artery, the jugular and portal veins, and the stomach. A portal flow probe was also implanted. The pigs were fed a high protein diet once hourly and infused intragastrically with U- 13 Calgal protein for 6 h. Amino acid labeling was measured in arterial and portal blood, in the hepatic free and protein-bound pools and in apolipoprotein B-100 (apoB-100), albumin and fibrinogen. The isotopic enrichments of apoB-100–bound U- 13 Cthreonine, leucine, lysine and phenylalanine were 33, 100, 194 and 230% higher than those of their respective hepatic free amino acid pools ( P < 0.01). Using the labeling of apoB-100 to estimate that of the protein synthetic precursor, the fractional rate of hepatic protein synthesis was 42 ± 2%/d. Between 5 and 8% of the dietary tracer amino acids was used for hepatic protein synthesis. In contrast to the small intestinal mucosa, in which the majority of the metabolized amino acids were apparently catabolized, protein synthesis utilized from 48% (threonine) to 90% (lysine) of the hepatic uptake of tracer amino acids. It appears that hepatic protein synthesis consumes nutritionally significant quantities of dietary essential amino acids in first pass and that extracellular, especially portal, essential amino acids are channeled to hepatic protein synthesis in the fed state. stable isotopes hepatic amino acid metabolism hepatic protein synthesis metabolic compartmentation pigs Footnotes ↵ 1 This work is a publication of the USDA/ARS Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine and Texas Children's Hospital, Houston, TX. ↵ 2 Supported in part by National Institutes of Health grant RO1-HD33920 (D.G.B.) and by federal funds from the U.S. Department of Agriculture, Agricultural Research Service, Cooperative Agreement 58–6250–6-001. B.S. was supported in part by the Alexander von Humboldt-Stiftung. ↵ 3 The contents of this publication do not necessarily reflect the views or policies of the U.S. Department of Agriculture, nor does mention of trade names, commercial products, or organizations imply endorsement from the U.S. Government. ↵ 4 The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact. ↵ 5 To whom correspondence should be addressed. Manuscript received: February 2, 1998. Initial review completed: March 6, 1998. Revision accepted: April 26, 1998.