PHYTOCHELATINS PROTECT PLANT ENZYMES FROM HEAVY-METAL POISONING

Challenging Rauvolfia serpentina suspension cells with sub-phytotoxic levels (100-mu-M) CdCl2 containing Cd-109(2+) and subsequent gel filtration of the carefully prepared cell-free extracts shows that the bulk ( > 97%) of the radioactivity is present as a phytochelatin complex. Less than 3% is associated with the high molecular weight fraction, no Cd2+ is found in free-form and there was no indication of the occurrence of a metallothionein complex. It is concluded that heavy metal ions entering cells at sub-lethal concentrations are totally complexed by phytochelatins (and to a much lesser extent to some high molecular weight proteins). It was shown that a series of metal-sensitive plant enzymes such as alcohol dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, nitrate reductase, ribulose-1,5-diphosphate carboxylase and urease tolerate Cd2+ in the form of a phytochelatin complex from 10- to 1000-fold the amount as compared with the free metal ion. Free phytochelatin peptides reactivate metal-poisoned nitrate reductase in vitro up to 1000-fold better than chelators such as glutathione and citrate, showing the extraordinary sequestering potential of these peptides.