Bioavailability and Sequestration

Many different mechanisms to mediate heavy metal resistance in prokaryotes exist, and have been carefully reviewed elsewhere (Ralston and O Halloran 1990 O Halloran 1993). In eukaryotes, this job is done by metallothioneins which sequester heavy metal ions and make them unavailable to exert toxic effects (reviewed by Hamer 1986 Andrews 1990). In this regard, heme-hepoxin induces both heme oxygenase and MT expression in human promyelocytic leukaemia cells and mouse hepatoma cells (Alam et al. 1989). The authors suggest that MT may sequester the nontoxic metal zinc, which would otherwise compete with Fe for occupation of sites on Fe-dependent regulatory proteins such as the IRE-BP. Thus, it is possible that sequestration of metals by MT could mediate, not only the direct toxic effect of such metals (reviewed in Chap. 1, this volume), but 

The availability of unbound, essential metals such as zinc and copper would not be without toxic consequences. As with iron, low molecular weight zinc complexes can catalyse biopolymer hydrolysis (O Halloran 

1993) copper is far more toxic, with the ability to participate in Fenton reactions to produce free radicals, lipid peroxidation, and cellular damage (Rowley and Halliwell 1983). Control of availability is likely to be essential. 

Other metals such as calcium and iron are tightly controlled with respect to subcellular localization and intracellular metal concentrations at various stages of the cell cycle. In the absence of detailed information on the cell biology of zinc (Vallee and Falchuk 1993), it is likely that a similar level of control is required. 

LPS induction of the respiratory burst is accompanied by increased MT mRNA and protein accumulation in these cells, and, intriguingly. 

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Sorption, bioavailability, and sequestration are interrelated phenomena affecting the transport and ultimate environmental fate of organic contaminants... 

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