Functional Instability of Skeletal Muscle Ribosomes after Protein Restriction of Rats

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Abstract
The capacity for polyphenylalanine synthesis of skeletal muscle ribosomes seemed unimpaired after feeding rats 3% or 20% protein in the diet, whereas endogenous protein synthesis was lower after feeding 3% protein. Furthermore, when ribosomal particles were pretreated, differences in activity between the two groups of rats were obtained. Pretreatment involved centrifugation for 18 hours at 55,000 × g through a buffered density gradient between 0.45 M and 1.1 M sucrose, at 0.15 M KCl and 1 mM MgSO, in the case of subunits; or 0.15 M KCl and 5 mM MgSO4 in the case of nondissociated ribosomes. The preparations obtained from rats fed 3% protein showed a higher sensitivity towards pretreatment, and thus reduced activity was observed in the following functions. 1. Binding of phenylalanyl-tRNA to 40 S subunits; 2. Stabilization of the complex by 60 S subunits; 3. Binding of [3H]polyuridylic acid to 40 S subunits; 4. Reassociation of 60 S with 40 S subunits and capacity of the recombined system to synthesize polyphenylalanine; 5. Activity of nondissociated ribosomes for polyuridylic acid-directed incorporation of phenylalanine into protein.