Cell-free Studies of Protein Synthesis with Skeletal Muscle from Normal and Potassium-depleted Rats

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Abstract
In a series of experiments, the effects of an inadequate dietary potassium intake on in vivo and in vitro aspects of protein synthesis in skeletal muscle of weanling rats were evaluated. In vivo uptake of 3H-leucine into mixed muscle proteins was significantly reduced by feeding a deficient potassium diet (0.05% K) when compared with incorporation for age control rats given an adequate diet (0.28% K). Cell-free reaction mixtures were employed to assess the influence of the dietary potassium deficiency on the protein synthetic machinery of skeletal muscle. Cell-free protein synthesis was higher for the system prepared from potassium-depleted rat muscle, due to an alteration in the pH 5 enzyme fraction. However, the synthetic activity of the ribosomes from potassium-depleted rats was equal to or lower than that of ribosomes prepared from well-nourished rats when assayed in the presence of the pH 5 enzyme fraction from normal muscle or liver. The sedimentation profile of ribosomes in sucrose gradients was similar for well-nourished and potassium-depleted rats. Incubation of the pH 5 enzyme fractions with deacylated liver tRNA suggests that the increased stimulation of protein synthesis by the pH 5 enzyme fraction of potassium-depleted rat muscle was unrelated to differences in the aminoacyl ligase activity of this fraction in the preparations from normal and potassium-depleted muscle. Addition of tRNA to the incubation mixture stimulated protein synthesis for the homologous cell-free system from muscle of normal rats but did not significantly change incorporation in the system prepared from potassium-depleted rat muscle. It is hypothesized that a factor(s) which participates in the elongation of the polypeptide chain may be released from the ribosomes in vivo or during their isolation as a consequence of the dietary potassium deficiency and that this factor(s) is subsequently precipitated during preparation of the pH 5 enzyme fraction.