Metal defence mechanisms

The metal defence mechanisms can be classified in two main categories ... 

As an indirect effect of increased metal uptake, the physiological state of the cell can alter and defence mechanisms can be induced. Phytochelatin (metal binding proteins) synthesis and induction of free radical quenching enzymes and metabolites were frequently observed. Especially the latter can protect membranes against oxidative breakdown. 

Early biological effects Initial reactions towards the presence of contaminants Defence mechanisms are a special case of early biological effects. They are either adaptive or defensive reactions and do not represent actual toxic damage. Extrusion of chemicals outside cells by protein pumps Induction of metal-binding proteins or biotransformation enzymes for elimination... 

Th + or AP" " induced a precipitate to form in all Bradyrhizobium and Sinorhi-zobium cultures tested, which suggested a defense mechanism based on metal precipitation by extracellular polymers (Santamaria et al., 2003). Among the metals tested, only Fe " ", Ap+, and Th were able to induce the formation of precipitate. AP+ is probably the natural soil component against which this defence mechanism could be directed, and a different defence mechanism based on extracellular aluminium precipitation within a gelatinous residue has been described for P. fluorescens (Appanna and St. Pierre, 1996). However, tliis polymer was composed mainly of phosphatidylethanolamine. While metal binding to extracellular polymers and bacterial surfaces have been proposed as the reason for increased metal resistance of biofilm-growing bacteria, this proposed defense mechanism involved the physical removal of the capsule after metal binding (Santamaria et al., 2003). 

The third section of the periodic table comprises transition metals or the d-group elements, which are involved in many biological defence mechanisms against toxic radicals. As in chemical catalysis the same metals are active in biochemical systems. Surprisingly these redox mediators are involved in both, the generation and the removal of reactive oxygen species. 

The efficient removal of O2 and H2O2 vvill diminish OH formation and therefore antioxidant defence systems have evolved to limit their accumulation. Enzymic and low molecular weight antioxidants exist to scavenge free radicals as self-protection mechanisms. Some proteins exhibit antioxidant properties because they chelate transition-metal catalysts. The significance of antioxidants in relation to inflammatory joint disease is discussed below. 

Various mechanisms have been developed by plants to alleviate stress imposed by exposure to toxic metal concentrations. Although the biochemical and molecular bases of adaptation and resistance of plant communities to high metal levels will be discussed extensively in Chapter 3, a survey summarising the possible anti-metal stress defence strategies and mechanisms forms an essential part of this chapter on toxicology. Every plant species, whether it is resistant or not to high metal concentrations, possesses defence responses against increased metal concentrations in the cell indeed. 

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