Protein Synthesis in Mammalian Tissues with Special Reference to Precursor Pools.

(1) The nature of the pool of free amino acids that act as immediate precursors for protein synthesis has been investigated under conditions in vivo. (2) Rats were infused with [U-14C] glycine for up to 6 hours during which time the specific radioactivity of free glycine in plasma and tissue approached a constant value. Free serine also became labelled. The ratio of the specific radioactivities of serine and glycine in the total protein pool of liver, heart, diaphragm, gastrocnemius, kidney, brain and jejunum was compared with the ratio in the free amino acid pool of these tissues and also of plasma. In all cases the ratio in protein was closer to the ratio in the tissue free pool. (3) Infusion of L-[U-14C] serine resulted in labelling of free glycine. In almost every case the ratio of the specific radioactivity of glycine to serine in the tissue protein was closer to the ratio in the free amino acid pool of the tissue. (4) After infusion of L-[U14C] glycine, the relative specific radioactivities of serine and glycine in plasma albumin, liver ferritin and total liver protein were similar to the ratio in the.liver free amino acids but differed from both arterial and hepatic portal plasma. However, the ratios in albumin, liver ferritin and total liver protein in themselves showed small but statistically significant differences. (5) Comparisons were made of the fractional synthesis rates of the tissue proteins from calculations which assumed either the tissue or the plasma free serine and glycine to be the precursor. These synthesis rates were also compared with others calculated after the L-[U14C] tyrosine. (6) The results showed that the plasma amino acids were not utilised directly in protein synthesis but suggested that the precursor pool was an intracellular compartment. There were also indications that more than one intracellular pool may be involved in protein synthesis.