HEPATIC PROTEIN-SYNTHESIS IN THE SHEEP - EFFECT OF INTAKE AS MONITORED BY USE OF STABLE-ISOTOPE-LABELED GLYCINE, LEUCINE AND PHENYLALANINE

Rates of protein synthesis for the liver, plasma albumin and total pla sma protein were quantified in sheep either offered a supra-maintenanc e intake or fasted for 3 d. The technique of continuous infusion over a 12 h period was employed with the simultaneous infusion of [1-C-13]g lycine, [1-C-13]leucine and [H-2(5)]phenylalanine. Blood and plasma sa mples were removed at timed intervals from the hepatic portal and hepa tic veins plus the aorta. Enrichments of the free amino acids (AA) wer e determined in all blood and plasma samples as was the protein-bound AA in an apolipoprotein B100 extract. Protein-bound phenylalanine enri chments were also measured in albumin and total protein from plasma pl us samples from liver biopsies. The apolipoprotein B100 enrichments ag reed well with those of the free AA in hepatic (and hepatic portal) pl asma but were lower than for arterial free AA and greater than liver h omogenate free AA. This adds support to the concept that export protei ns may preferentially use AA directly from extracellular sources. Inta ke had no significant effect on constitutive liver protein synthesis a nd the values agreed well with those obtained by other isotopic approa ches. There were, however, significant declines, based on hepatic veno us free phenylalanine enrichment, at the lower intake in both the frac tional (3.4 v. 4.7 % per d; P=0.024) and absolute (2.4 v. 4.2 g/d; P=0 .011) synthesis rates of albumin, which matched the estimated decrease in total plasma albumin content (52 v. 67 g, P<0.01). In contrast, th ere was a smaller reduction in total plasma protein mass (145 v. 151 g , P=0.035) with no observed significant difference in kinetic paramete rs. Albumin synthesis was calculated to account for a maximum of 17 % of total liver protein synthesis in the fed condition and this may fal l to 8 % during moderate fasts.