METABOLISM OF CYCLOATE IN RADISH LEAF - METABOLITE IDENTIFICATION BY PACKED CAPILLARY-FLOW FAST-ATOM-BOMBARDMENT TANDEM MASS-SPECTROMETRY

The metabolism of cycloate, a thiocarbamate herbicide, was investigate d in mature radish leaf. Twelve new metabolites were identified by liq uid chromatographic/mass spectrometric analysis using fast atom bombar dment and packed capillary liquid chromatography columns. Full-scan an d tandem mass spectrometric methods were employed. Application of the on-column focusing technique resulted in identifications with injectio ns of as little as 15 ng of metabolite (29 ppb in radish). This inject ion technique allows the practical use of packed capillary liquid chro matography/mass spectrometry in sample-limited applications. Cycloate is oxidized to several ring-hydroxylated isomers that are subsequently glucosylated and esterified with malonic acid. Cycloate is also conju gated with glutathione. Metabolic hydrolysis of the glutathione conjug ate formed a cysteine conjugate that is further metabolized by amidati on with either malonic or acetic acid. Transamination of the cysteine conjugate gave a thiolactic acid derivative. Metabolites were also ide ntified that were the result of both ring-hydroxylation and conjugatio n with glutathione. One of these, an N-acetylcysteine conjugate, is th e first report of a mercapturic acid in plants. The structures of two of the new metabolites were confirmed by chemical synthesis.