Leucine pools in normal and dystrophic chicken skeletal muscle cells in culture

The specific radioactivity of [3H]Leu in the extracellular, intracellular, and Leu-tRNA pools of normal (white leghorn) and dystrophic (line 307) embryonic chick breast muscle cultures was analyzed as a function of equilibration time and extracellular Leu concentration (0.05-5 mM). The primary results were the following 1) [3H]Leu equilibrated to a constant specific radioactivity in the intracellular and Leu-tRNA pools within 2 min after addition to both normal and dystrophic cultures. 2) After equilibration, the extracellular [3H] Leu specific radioactivity in dystrophic cell culture medium was lower than that of medium exposed to normal cells (especially at low Leu concentrations), probably because of increased release of unlabeled Leu from the dystrophic cells as a result of faster protein breakdown. Accordingly, the specific radioactivities in the intracellular and the Leu-tRNA pools were also lower in dystrophic cells. 3) At 5 mM extracellular Leu, the specific radioactivity in the Leu-tRNA pool was approximately 40% lower than the specific radioactivity in the intracellular pool in both normal and dystrophic cells. Thus, high concentrations of extracellular Leu cannot be used to "flood out" reutilization of unlabeled Leu (released by protein degradation) during protein synthesis. 4) At 5.0 mM extracellular Leu, the specific radioactivity of [3H]Leu in the intracellular pool was comparable to that in the extracellular pool in normal and dystrophic cells; however, the specific radioactivity of Leu-tRNA (i.e. the immediate precursor to protein synthesis) was only 55-65% of the extracellular specific radioactivity in normal and dystrophic cells. In conclusion, reutilization of Leu from protein degradation is higher in dystrophic muscle cell cultures than in normal muscle cell cultures, and accurate rates of protein synthesis in cell cultures can only be obtained if specific radioactivity of amino acid in tRNA is measured.