Glutathione metabolism and possible biological roles in higher plants

Synthesis of glutathione in plants seems to proceed in the same series of enzyme catalysed reactions observed in animal cells; the pathway of gluthathione degradation, however, has not yet completely been eludicated in plants. Whereas gamma-glutamylcyclotransferase and 5-oxo-prolinase activity seem to be involved in degradation of gluthathione in plants, the participation of a gamma-glutamyltranspeptidase is uncertain. First observations indicate a separation of glutathione synthesis and degradation by compartmentation in green cells. As the reaction catalysed by 5-oxo-prolinase is regulated by sulphur nutrition, it might be the rate limiting step in glutathione catabolism in plants as observed in animal cells. From investigations with plant tissue cultures and from transport studies, glutathione appears to be the main long-distance transport form of reduced sulphur translocated from mature leaves to the roots and to other parts of the plant. In addition, glutathione seems to be used a s a storage form of reduced sulphur in plant cells supplied with excess inorganic sulphur. Whereas a role of glutathione in the detoxification of pesticides can be regarded as definitely established, a function of this peptide in the detoxification of hydrogen peroxide in chloroplasts is evident from biochemical investigations only. Further studies are needed to show whether glutathione is indeed used as reductant for the detoxification of hydrogen peroxide in chloroplasts in vivo. (IFU)