STABLE ISOTOPES IN THE STUDY OF BIOSYNTHETIC PATHWAYS

The use of stable isotopes in the elucidation of pathways of biosynthesis is char-actised by two particular features. In many cases, NMR, and in particular NMR, has been the method of isotope detection employed. Furthermore, in many cases the use of radioactive isotopes and their ease of detection has been combined with the structural definition achievable by the use of stable isotopes in conjunction with NMR. Although much of the reported work has utilised carbon-13 in combination with carbon-14 labelling, there are, nevertheless many instances of the use of nitrogen-15 and oxygen-18 in the study of biosynthetic pathways. 

The most rewarding procedure has involved the feeding of specifically Relabelled possible precursors to an appropriate culture followed by isolation of the metabolite(s). Screening of the product for radioactivity affords a preliminary indication of the precursors. Subsequent repetition with appropriately C-labelled precursors and examination of the product by NMR provides data on the exact location in which the precursors have been incorporated without the necessity for degradative studies. This information is adequate in many cases to deduce the metabolic pathway. In carbon-13 studies, both proton NMR using the satellite method, and C-NMR have been employed. 

Whilst there are a number of advantages in using a radioactive label in preliminary experiments to establish correct experimental conditions prior to the use of the more expensive stable isotope label, it is not essential. Indeed, in the case of nitrogen 5 and oxygen-18 studies, the absence of a suitable radioisotope rules out such a procedure. Furthermore, mass spectrometry may also 

Several useful reviews have appeared on the use of stable isotopes in biosynthetic studies [122,123,125,126,203—206], and on the instrumental methods employed [207]. The techniques will be illustrated in this section by a number of examples. 

Perhaps the most widely studied area, and at the same time the area of most interest to the medicinal chemist, is that of antibiotic biosynthesis. The widespread interest is no doubt related to the high incorporation of carbon-13 into bacterial or fungal metabolites and because of the high isolated yields. 

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