Amino acids suppress intracellular protein degradation in rat liver during parenteral nutrition

The effects of variations in the amino acid supply on the rates of synthesis and degradation of liver proteins and on the rate of synthesis and secretion of plasma proteins were investigated. Rats were nourished by infusion of total parenteral nutrition solutions containing four different levels (0, 1.65, 3.3 and 6.6%) of amino acids for 7 d. The fractional rate of total protein synthesis in the liver was determined by injecting a flooding dose of [3H]phenylalanine. The proportion of newly synthesized proteins retained and exported by the liver was estimated by injecting a tracer dose of [14C]leucine and then measuring the protein radioactivity remaining in the liver and present in the plasma after secretion was completed. The rate of plasma albumin synthesis was significantly lower in the 0 and 1.65% amino acid groups than in the other groups. The fractional synthesis rates of liver domestic proteins, however, were essentially the same in rats administered all levels of amino acids except for the 0% amino acid group, which showed a slightly higher value than the other groups. The fractional degradation rates of liver domestic proteins, calculated as the difference between the fractional synthesis rate of liver domestic proteins and the net gain of liver proteins, were found to be inversely related (r = -0.999, P < 0.05) to the level of amino acids in infusion solutions up to 3.3% amino acids. It was concluded that protein degradation plays the predominant role in the regulation of liver protein mass.