Stereospecificity of Aminoglycoside−Ribosomal Interactions

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Abstract
Aminoglycoside antibiotics bind to the A-site decoding region of bacterial rRNA causing mistranslation and/or premature message termination. Aminoglycoside binding to A-site RNA decoding region constructs is established here to be only weakly stereospecific. Mirror-image prokaryotic A-site decoding region constructs were prepared in the natural d-series and the enantiomeric l-series and tested for binding to a series of aminoglycosides. In general, aminoglycosides bind to the d-series decoding region constructs with 2−3-fold higher affinities than they bind to the enantiomeric l-series. Moreover, l-neamine, the enantiomer of naturally occurring d-neamine, was prepared and shown to bind approximately 2-fold more weakly than d-neamine to the natural series decoding region construct, a result consistent with weakly stereospecific binding. The binding of naturally occurring d-neamine and its synthetic l-enantiomer was further evaluated with respect to binding to prokaryotic and eukaryotic ribosomes. Here, weak stereospecifcity was again observed with l-neamine being the more potent binder by a factor of approximately 2. However, on a functional level, unnatural l-neamine proved to inhibit in vitro translation with significantly lower potency (approximately 5-fold) than d-neamine. In addition, both l- and d-neamine are bacteriocidal toward Gram-(−) bacteria. l-Neamine inhibits the growth of E. coli and P. aeruginosa with 8- and 3-fold higher MIC than d-neamine. Interestingly, l-neamine also inhibits the growth of aminoglycoside-resistant E. coli, which expresses a kinase able to phosphorylate and detoxify aminoglycosides of the d-series. These observations suggest that mirror-image aminoglycosides may avoid certain forms of enzyme-mediated resistance.