As stated earlier, synthesis of PCs is induced by most heavy metals, including the multiatomic anions [74,76], in most of the higher plants [73,83]. It has also been observed that the enzyme involved in its synthesis, PC synthetase, needs the presence of heavy metals for its activation; a crude preparation of the enzyme from S. vulgaris was activated best by Cd2+, and by Ag+, Bi3+, Pb2+, Zn2+, Cu2+, and Au+ in decreasing order . No activation of the enzyme was detected by the metals of the hard-acceptor category including Al3+, Ca2+, Fe3+, Mg2+, Mn2+, Na+, and K+. The trend of activation observed by Grill et al. , however, was not observed for PC synthetase from tobacco cells, except that Cd was the most effective activator, followed by Ag+. The activation by Cu2+ was next to Ag+, and Pb2+, Zn2+, and Hg2+ produced only weak stimulation of the enzyme activity . Thus, although the enzyme has a rather nonselective domain for binding with metals, it is mostly activated by heavy metals.
This strongly suggests that intracellular metabolism of heavy metals, other than Cd as well, might be largely mediated through PCs. The view also stems from the fact that the heavy metal ions that activate PC synthetase in vitro are also able to induce PC synthesis in vivo , with one exception: Ni2+ induced PC synthesis in vivo, but did not activate PC synthetase activity in vitro. Furthermore, the indication of possible involvement of PC in heavy metal tolerance also comes from genetic evidence; in addition to being sensitive to Cd2+, phytochelatin-deficient cad1 mutants of Arabidopsis are also sensitive to Hg2+ , and GSH-deficient strains of S. pombe show reduced tolerance to Pb2+ as well as showing no tolerance to Cd2+ .
PC synthetase activity is detected mostly in roots, but not in leaves or fruits [61,101]. The constitutive presence of PC synthetase in roots suggests an important role of PCs in metal detoxification. Because plants assimilate various metal ions from soil, the first organ exposed to these ions is the root. Localization of PC synthetase to roots and stems probably provides an effective means of restricting the heavy metals to these organs by chelation in the form of Cd-PC complexes. It has been demonstrated that PCs are able to protect enzymes from heavy metal poisoning in vitro [65,132]; many metal-sensitive plant enzymes (rubisco, nitrate reductase, alcohol dehydrogenase, glycerol-3-phosphate dehydrogenase, and urease) were more tolerant to Cd in the form of a Cd-PC complex compared with the free metal ion. Free PCs could reactivate the metal-poisoned enzymes (nitrate reductase poisoned by Cd-acetate) in vitro more effectively than other chelators such as GSH or citrate .
Recognition of PCs as the chelators of heavy metals in general and protectors of plants against their toxic effect, however, requires careful consideration. For instance, in tobacco cells not selected for metal tolerance, BSO increased the toxicity of Cd but not of Zn or Cu, as if the control of sequestration differed between the metals . This may, of course, be true, but has not been properly demonstrated. Second, although most of the heavy metals are able to induce synthesis of PCs in plants, only a few of them (Cd, Cu, and Ag) form complexes with the peptides . Recently, As has been reported to form complexes with PCs in arsenate-tolerant Holcus lanatus . In fact, PC-metal complex formation has been reported mostly for Cd. A few reports of PC forming complexes with Cu are also available [74,75,135], and formation of PC-Zn complexes has been observed in cells (of Rauvolfia) grown in micronutrient concentration of Zn . PCs have also been reported to form complexes with Hg and Pb in vitro [136,137].
Nevertheless, genetic evidence is that PCs are involved in tolerance to these metals; PC-deficient cad1 mutants of Arabidopsis are also sensitive to Hg2+ , and GSH-deficient strains of S. pombe show reduced tolerance to Pb2+ . Thus, although the involvement of PCs in making plants resistant to heavy metals other than Cd cannot be overlooked, more information is required on their induction by individual heavy metals in different plant species. Also, information is required on the formation of PC-metal complexes and cellular localization of the metals (individual) and Apo-PC and metal-PC complexes before the functional significance of PCs known for Cd can be generalized for all heavy metals.
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Detoxification is something that is very important to the body, but it is something that isn't understood well. Centuries ago, health masters in the East understood the importance of balancing and detoxifying the body. It's something that Western medicine is only beginning to understand.