Cysteine desulphydrase activity in higher plants - evidence for the action of l- and d-cysteine specific enzymes

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 myM aminooxyacetic acid (AOA), emission from C-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, C-cysteine desulphydrase activity was more than one order of magnitude higher than L-cysteine desulphydrase activity; L-cysteine desulphydrase was inhibited by 100 myM 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 myM 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. (IFU