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 syn thesis and degradation of liver proteins and on the rate of synthesis and secretion of plasma proteins were investigated. Rats were nourishe d 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 f ractional rate of total protein synthesis in the liver was determined by injecting a flooding dose of [H-3]phenylalanine. The proportion of newly synthesized proteins retained and exported by the liver was esti mated by injecting a tracer dose of [C-14]leucine and then measuring t he protein radioactivity remaining in the liver and present in the pla sma after secretion was completed. The rate of plasma albumin synthesi s was significantly lower in the 0 and 1.65% amino acid groups than in the other groups. The fractional synthesis rates of liver domestic pr oteins, however, were essentially the same in rats administered all le vels of amino acids except for the 0% amino acid group, which showed a slightly higher value than the other groups. The fractional degradati on rates of liver domestic proteins, calculated as the difference betw een the fractional synthesis rate of liver domestic proteins and the n et 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.