Source of amino acids for tRNA acylation in growing chicks

Specific radioactivity in three amino acid compartments was examined in broiler chicks following a flooding dose of leucine or phenylalanine. In general, specific radioactivity of leucine and phenylalanine in deproteinated plasma (SAe) and tissue (SAi) compartments, exceeded that in acylated-tRNA (SAt). In most tissues, SAe and SAi rapidly reached a similar peak level by 5 min followed by a slow decline for the next 30 minutes. Many tissues (eg. GI tract, liver, skin, and thigh) failed to maintain equilibrium between SAe and SAi over time. More metabolically active tissues, such as GI and liver had the greatest differences between these compartments. The difference between SAe and SAi for both leucine and phenylalanine were due to SAi decreasing faster than SAe, indicating dilution with unlabelled amino acids from proteolysis. Plasma and tissue specific radioactivity overestimated tRNA specific radioactivity by as much as 5 and 2.8 fold using leucine or 2.7 and 1.4 fold using phenylalanine, respectively. These data suggest that intracellular compartmentation of protein metabolism and the coupling of protein degradation and synthesis occur, in vivo.