DIETARY AMINO-ACIDS ARE THE PREFERENTIAL SOURCE OF HEPATIC PROTEIN-SYNTHESIS IN PIGLETS

To investigate the utilization of dietary amino acids for hepatic prot ein synthesis, seven female pigs (28 d old, 7.5 kg) were implanted wit h 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-C-1 3]algal protein for 6 h. Amino acid labeling was measured in arterial and portal blood, in the hepatic free and protein-bound pools and in a polipoprotein B-100 (apoB-100), albumin and fibrinogen. The isotopic e nrichments of apoB-100-bound [U-C-13]threonine, leucine, lysine and ph enylalanine were 33, 100, 194 and 230% higher than those of their resp ective hepatic free amino acid pools (P < 0.01). Using the labeling of apoB-100 to estimate that of the protein synthetic precursor, the fra ctional rate of hepatic protein synthesis was 42 +/- 2%/d. Between 5 a nd 8% of the dietary tracer amino acids was used for hepatic protein s ynthesis. In contrast to the small intestinal mucosa, in which the maj ority of the metabolized amino acids were apparently catabolized, prot ein synthesis utilized from 48% (threonine) to 90% (lysine) of the hep atic uptake of tracer amino acids. It appears that hepatic protein syn thesis consumes nutritionally significant quantities of dietary essent ial amino acids in first pass and that extracellular, especially porta l, essential amino acids are channeled to hepatic protein synthesis in the fed state.