The aminoacyltransferase I-catalysed binding of phenylalanyl-tRNA (unfractionated Escherichia coli B tRNA acylated with radioactive phenylalanine and 19 non-radioactive amino acids) to skeletal-muscle ribosomes from diabetic rats was less than that to ribosomes from normal rats when the Mg2+ concentration was low (7.5mm); whereas just the reverse was true when the concentration of the cation was higher (15mm). Thus the Mg2+ dependency of aminoacyltransferase I-catalysed binding of phenylalanyl-tRNA to ribosomes from normal and diabetic rats paralleled the effect of Mg2+ concentration on synthesis of polyphenylalanine reported before. During incubation at 7.5mm-Mg2+ phenylalanyl-tRNA was bound only to ribosomes bearing nascent peptidyl-tRNA. There are fewer such ribosomes in a preparation from the muscle of diabetic animals because diabetic animals synthesize less protein in vivo. Thus the difference in polyphenylalanine synthesis in vitro is adequately explained by the difference in enzyme-catalysed binding of phenylalanyl-tRNA to ribosomes, however, the basis of the difference in protein synthesis in vivo is still unknown.

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