Abstract
Microorganisms have wide capacity to produce various structurally unique and active secondary metabolites, which are used for drugs and bioproves for chemical biology study. We therefore have constructed a microbial metabolite fraction library by a systematic separation method. The fractions were analyzed by a PDA-LC/MS to reveal UV and mass spectra of each metabolite within the fraction. Based on the results for LC/MS analysis, we have developed a unique metabolite distribution map called an MP (microbial products) plot to efficiently distinguish characteristic metabolites from frequently appeared metabolites in addition to a usual spectral database. On the screening for structurally unique secondary metabolites by the spectral database and MP plot, two unknown peaks having the same UV and mass spectra were found in the fraction library of Streptomyces griseochromogenes JC82-1223. They were isolated by a single HPLC purification and the structures were determined to be spirotoamides A (1) and B (2), respectively. They had a 6,6-spiroacetal core structure and a carboxamide moiety. We speculated that spirotoamides were synthesized by type I polyketide synthases. A dihydroxylketone was expected to be a hypothetical precursor and it might be nonenzymatically cyclized to form spiroacetal structure. Then the transfer of amino group by a carboxamide synthase and hydroxylation by P450 enzymes might be involved in the formation of spirotoamides. These results showed that the construction of the microbial metabolite fraction library with combination of spectral database and MP plot is a promising method to discover structurally unique compounds.
