Origins and metabolism of the intracellular amino acid pools in rat liver and muscle

Abstract

1. 1. When [ 14C]amino acids were continuously administered to a rat by intravenous infusion, the specific activity of the liver and muscle intracellular free amino acid soon increased but even when the intracellular pool was maximally labeled, the specific activity was lower than the plasma specific activity. This observed intracellular dilution has been interpreted as being due to intracellular unlabeled protein breakdown. 2. 2. Protein catabolism in the normal fed rats contributed approx. 50% of the liver's and 30% of the muscle's intracellular amino acid pool. 3. 3. During the early stages of fasting as much as 90% of liver amino acid was derived from protein degradation, with a major fraction entering the plasma to supply amino acids for other tissues. After depletion of the labile liver protein, muscle protein breakdown has increased and supplied 65% of muscle intracellular amino acid pool, which in time maintained the plasma and tissue amino acid levels.