Hepatic protein synthesis in the sheep

Rates of protein synthesis for the liver, plasma albumin and total plasma protein were quantified in sheep either offered a supra-maintenance 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]glycine, [1-C-13]leucine and [H-2(5)]phenylalanine. Blood and plasma samples were removed at timed intervals from the hepatic portal and hepatic veins plus the aorta. Enrichments of the free amino acids (AA) were determined in all blood and plasma samples as was the protein-bound AA in an apolipoprotein B100 extract. Protein-bound phenylalanine enrichments were also measured in albumin and total protein from plasma plus samples from liver biopsies. The apolipoprotein B100 enrichments agreed well with those of the free AA in hepatic (and hepatic portal) plasma but were lower than for arterial free AA and greater than liver homogenate free AA. This adds support to the concept that export proteins may preferentially use AA directly from extracellular sources. Intake had no significant effect on constitutive liver protein synthesis and the values agreed well with those obtained by other isotopic approaches. There were, however, significant declines, based on hepatic venous free phenylalanine enrichment, at the lower intake in both the fractional (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, there was a smaller reduction in total plasma protein mass (145 v. 151 g, P=0.035) with no observed significant difference in kinetic parameters. Albumin synthesis was calculated to account for a maximum of 17 % of total liver protein synthesis in the fed condition and this may fall to 8 % during moderate fasts.