Phytochemistry, 1987, Vol.26(6), pp.1583-1589
Emission of hydrogen sulphide in response to D -cysteine by leaf discs of cucurbit plants or cultured tobacco cells was considerably smaller than in response to L -cysteine. Whereas hydrogen sulphide emission from L -cysteine was inhibited by 100 μM aminooxyacetic acid (AOA), emission from D -cysteine was unaffected. These results from in vivo studies were found to be inconsistant with the L - and D -cysteine desulphydrase activities measured in crude homogenates. In vitro , D -cysteine desulphydrase activity was more than one order of magnitude higher than L -cysteine desulphydrase activity; L -cysteine desulphydrase was inhibited by 100 μM AOA to a smaller, D -cysteine desulphydrase to a higher extent than in vivo . Cystine lyase activity, which may interfere in the cysteine desulphydrase assay, was not found. In cucurbit leaves, the differences between in vivo and in vitro experiments can partially be explained by differences in the influx of L - and D -cysteine into the leaf discs. Influx of L -cysteine proceeded at a rate about four times higher than the influx of D -cysteine; it was inhibited by 100 μM AOA, whereas influx of D -cysteine was unaffected. Subcellular distribution of L - and D -cysteine desulphydrase was analysed in cultured tobacco cells. Both enzyme activities were found to be soluble. The D -cysteine activity was predominantly localized in the cytoplasm whereas L -cysteine activities were also found in chloroplasts and mitochondria. The L -cysteine desulphydrase in the cytoplasmic fraction may entirely be due to broken chloroplasts and mitochondria. Inhibitor studies with ammonium, pyruvate, AOA and O-acetylserine revealed considerable differences between L - and D -cysteine desulphydrase activity and between L -cysteine desulphydrase activity in chloroplasts and mitochondria. Therefore, the present data suggest that degradation of L - and D -cysteine are catalysed by different enzymes in different compartments of the cell.
Cucurbita Pepo ; Cucurbitaceae ; Nicotiana Tabacum ; Solanaceae ; Cysteine Catabolism ; Cysteine Desulphydrase ; D-Cysteine ; Hydrogen Sulphide ; L-Cysteine ; Subcellular Localization ; Sulphur Metabolism ; Botany
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