Metal concentration, metabolism

A variety of responses can be initiated by the direct interaction of metals with cellular components. Membrane damage and enzyme inhibition are examples of such a metal effect. Above a certain threshold concentration of metals in the cell, its physiological state is irreversibly changed (Van Assche et al., 1988). This response is reflected by an increase in capacity (activity under non-limiting substrate and coenzyme concentrations) of certain enzymes. This increase in capacity is generally called enzyme induction. These secondary, indirect effects of metals are considered to play an important role in the stress metabolism induced by toxic metal concentrations. 

As we have seen, trace metals are involved as cofactors of metalloenzymes and proteins, in all general metabolic processes of phytoplankton, including photosynthesis and respiration, and in assimilation of macronutrients. The vertical profiles of trace metal concentrations in open oceans (Figure 10.9) are like those of macronutrients that is, they show surface depletion resulting from algal uptake and partial release at greater depth due to mineralization. 

For many years, one way to solve these drawbacks has been the identification and isolation of new biocatalysts with the desired characteristics. The most common procedure for isolation and identification of biocatalysts has also been applied to microbial populations living in harsh environments such as hot springs, abyssal hot vents or geothermal power plants. These microorganisms, known as extremophiles can be foimd in environments of extreme temperature, ionic strength, pH, pressure, metal concentrations or radiation levels. These natural reservoirs of extremozymes directly offer new activities and extreme stabilities but these microorganisms also offer novel metabolic pathways. Table 10.11 summarizes the identified extremoz5mies found in some extremophiles. 

Field studies on the microbial communities of boreal coniferous forest humus exposed to environmental stress showed that the structure of the microbial community was influenced by changes in humus pH and metal concentrations at levels where few or no effects were evident on microbial biomass or metabolic activity. Changes in the relative proportions of gram-negative and gram-positive bacteria, including actinomycetes, occurred as well as adaptation to the environmental disturbance in question. Increased metal tolerance of the humus bacterial community resulted partly from a change in microbial species composition (Pennanen, 2001). 

Ionic strength Gaseous O2 concentration Gaseous CO2 concentration Dissolved O2 concentration Dissolved CO2 concentration Carbon source concentration Nitrogen source concentration Metabolic product concentration Minor metal concentration Nutrient concentration ... 

Brown MT and Depledge MH (1998) Determinations of trace metal concentrations in marine organisms. In Langston WJ and Bebianno MJ, eds. Metal metabolism in aquatic environments. Chapter 7, pp. 185-217. Chapman Hall, London. 

Because of the large gradients in trace metal concentrations between the open ocean and coastal waters, oceanic phytoplankton species have evolved the ability to grow at much lower available concentrations of iron, zinc, and manganese. In doing so they have been forced to rearrange their metabolic architecture (e.g., in the case of iron-rich protein complexes involved in photosynthesis) or to switch from scarce elements to more abundant ones in some critical metalloenzymes (e.g., Ni and Mn replacement of Fe in the antioxidant enzyme superoxide dismutase). 

Mammalian cells need some metals as nutrients to maintain functions such as proliferation, metabolism, and differentiation. Optimal metal concentrations vary with the types of cell. For example, estimates of the free zinc ion intra cellular concentration are 5-10 pM for pheochromocytoma (PC-12) cells, 170 pM for primary human monocytes and 350 pM for lymphocytes [6]. In addition, the regulating mechanism of metals is also diverse in cells. Mammalian cells have two ways for metal uptake and ejection one is endocytosis, another one is through metal transporters that can transport specified metal(s) selectively. Several kinds of transporters are known. Zinc transporters have two families ZnT (Slc30a family) effuses zinc ion from cytoplasm and ZIP (Slc39a family) infuses it into cytoplasm... 

Correlations between well-known diseases and changes in trace metal concentrations in serum, cells, or tissue have been detected to an increasing extent. Therefore trace metal determinations may be useful in different diseases, even in the prodromal stage, to prevent trace metal deficiency. In Crohn s disease the serum concentration of zinc and possibly of copper and iron is diminished, metabolic pathways are affected, and partial dermatitis occurs. Zinc supplementation removes skin lesions [52]. In acute and chronic liver diseases serum zinc is reduced and in consequence of biliary hypoexcretion the serum concentrations of manganese and copper are high. 

While metal uptake through the root is the first important step in hyperaccumulation, most of the metal is stored in the above-ground parts. Studies of cellular metal compartmentation have shown that in most hyperaccumulators the metal is sequestered preferentially into compartments where they can not damage metabolic processes, e.g., photosynthesis as a very cadmium-sensitive vital function of plants (see Chapter 13). Therefore, it is important for hyperaccumulators to keep the metal concentration in the cytoplasm of mesophyll cells as low as possible. 

On the other hand, organisms can also produce chelating agents to acquire metals that are necessary for certain metabolic functions. These chelating agents are often extremely specific for a given metal and are used to "collect" metals from solution or maintain a desired concentration of metals inside the cell. 

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