SOURCE OF AMINO-ACIDS FOR TRANSFER-RNA 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 (SA(e)) and tissue (SA(i)) compartments, exceeded that in acylated-tRNA (SA(t)). In most tissues, SA(e) and SA(i) 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 SA(e) and SA(i) over time. More metabolically active tissues, such as GI and liver had the greatest differences between these compartments. The difference between SA(e) and SA(i) for both leucine and phenylalanine were due to SA(i) decreasing faster than SA(e), 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.