Polyphenylalanine synthesis and binding of phenylalanyl transfer ribonucleic acid by ribosomes from muscle of normal and diabetic rats.

Ribosomes from muscle of diabetic rats synthesized less polyphenylalanine than normal ribosomes at low magnesium concentrations (5 to 10 mM), but they were more active than normal ribosomes at high magnesium concentrations (12.5 to 20 mM). The magnesium dependence of binding of phenylalanyl transfer RNA to normal and diabetic ribosomes closely paralleled the synthesis of polyphenylalanine. A portion of the radioactivity associated with ribosomes when binding of 14C-Phe-tRNA was assayed cochromatographed, after hydrolysis, with phenylalanine; the remainder did not migrate from the origin. At 7.5 m&r magnesium, the greater amount of 14C-Phe-tRNA bound to normal ribosomes was accounted for by the formation of origin material. The origin material was identified as a peptide with carboxyl-terminal I%-phenylalanine. Removal of peptidyltRNA by incubation with puromycin abolished the difference in the ability of normal and diabetic ribosomes to synthesize polyphenylalanine and to bind Phe-tRNA. We suggest that only ribosomes that have peptidyl-tRNA can bind Phe-tRNA at low magnesium concentrations. Since normal ribosomes contain more peptidyl-tRNA than diabetic ribosomes, they would bind more Phe-tRNA and synthesize more polyphenylalanine at low magnesium concentration.