Aminoacyl-tRNAs are the biologically active substrates for peptide bond formation in protein synthesis. The stability of the acyl linkage in each aminoacyl-tRNA, formed through an ester bond that connects the amino acid carboxyl group with the tRNA terminal 3′ -OH group, is thus important. While the ester linkage is the same for all aminoacyl-tRNAs, the stability of each is not well characterized, thus limiting insight into the fundamental process of peptide bond formation. Here, we show, by analysis of the half-lives of 12 of the 22 natural aminoacyl-tRNAs used in peptide bond formation, that the stability of the acyl linkage is effectively determined only by the chemical nature of the amino acid side chain. Even the chirality of the side chain exhibits little influence. Proline confers the lowest stability to the linkage, while isoleucine and valine confer the highest, whereas the nucleotide sequence in the tRNA provides negligible contribution to the stability. We find that, among the variables tested, the protein translation factor EF-Tu is the only one that can protect a weak acyl linkage from hydrolysis. These results suggest that each amino acid plays an active role in determining its own stability in the acyl linkage to tRNA, but that EF-Tu overrides this individuality and protects the acyl linkage stability for protein synthesis on the ribosome.

Kidneys from normal, diabetic-nonketotic and ketotic Chinese hamsters were homogenized, fractionated and assayed for beta-glucosidase, and beta-galactosidase activities. The kidneys of the ketotic animals were enlarged but the protein content in each subcellular fraction was similar in all three groups of animals. beta-Glucosidase was found chiefly in the soluble fraction and no difference was observed in these animals. beta-Galactosidase was distributed in both cytoplasmic and particulate fractions; difference in the specific activity of beta-galactosidase between control and ketotic animals was found in nuclear, lysosomal-mitochondrial, microsomal and soluble fractions.