Studies on aminoglycoside antibiotics: enzymic mechanism of resistance and genetics

The kanamycin inactivating enzyme, 3'-phosphotransferase and 6'-acetyltransferase were first found in 1967 and on the basis of the enzymic mechanism of resistance a new research approach to the development of active useful derivatives was explored. The enzymic mechanism of resistance was conclusively confirmed by the synthesis of 3'-deoxykanamycin A and 3',4'-dideoxykanamycin B which did not undergo inactivation by 3'-phosphotransferase and inhibited the growth of resistant strains. Besides APH(3') and AAC(6') described above, the following enzymes were found to be involved in the mechanism of resistance to aminoglycosides: APH(3''), APH(5''), APH(6), APH(2''), AAC(3), AAC(2'), AAD(3''), AAD(2''), AAD(4'), AAD(6). Not only the removal of the group which undergoes the enzyme reaction but also the modification of the group binding to the enzyme has also given active derivatives such as amikacin etc. The substrate specificity of the enzymes, enzymes in the immobilized state, and the application of proton and 13C nmr for structure determination of reaction products are reviewed. It was noticed that all enzymes involved in resistance contain adenosine- and aminoglycoside-binding sites. These enzymes were thus suggested to be mainly different primarily in the positional relationships between these binding sites. It suggests a close evolutionary relationships of these enzymes. The role of these enzymes in the biosynthesis of aminoglycoside antibiotics is discussed and a general mode of the biosynthesis of aminoglycosides is proposed: a gene or gene set involved in biosynthesis of 2-deoxystreptamine which has no cytotoxicity is widely distributed and the deoxystreptamine produced is transformed to the final products.