Bc. Onisko et al., METABOLISM OF CYCLOATE IN RADISH LEAF - METABOLITE IDENTIFICATION BY PACKED CAPILLARY-FLOW FAST-ATOM-BOMBARDMENT TANDEM MASS-SPECTROMETRY, Biological mass spectrometry, 23(10), 1994, pp. 626-636
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.