Extracellular chelators

Extracellular chelators are not only known for decreasing uptake fluxes. Under iron-limiting conditions, many organisms are known to release side-rophores that strongly complex and solubilise iron (see Chapter 6 of this... 

Pharmacological and toxicological studies have shown that demetallation can occur in vivo with deposition of free metal ions in bone and liver especially in situations where the chelate has a long residence time in the body. This is the case with non-extracellular chelates like organ-specific contrast agents or with intravascular compounds such as polymers. 

For a more detailed review and critical appraisal of the published data on metal complexation by micro-organisms and extracellular chelating compounds see Beveridge and Doyle (1989), Campbell (1994), Tessier et al. (1994), Beveridge et al. (1997) and Town et al. (1998). 

In a study of the absorption of inorganic mercury by the rat jejunum, Foulkes and Bergman (1993) found that while tissue mercury could not be rigorously separated into membrane-bound and intracellular compartments (as can the heavy metal cadmium), its uptake into the jejunum includes a relatively temperature-insensitive and rapid influx into a pool readily accessible to suitable extracellular chelators. A separate, slower and more temperature-sensitive component, however, leads to the filling of a relatively chelation-resistant compartment. Nonspecific membrane properties, such as surface charge or membrane fluidity, might account for mucosal mercury uptake (Foulkes and Bergman 1993). 

The extracellular domain of cadherins consists of a variable number of a repeated sequence of about 110 amino acids. This sequence is termed the cadherin repeat and resembles in overall structure, but not in sequence, the Ig like domains. The cadherin repeat is the characteristic motive common to all members of the cadherin superfamily. Classical and desmosomal cadherins contain five cadherin repeats, but as many as 34 repeats have been found in the FAT cadherin (see below). Cadherins are calcium-dependent cell adhesion molecules, which means that removal of Ca2+, e.g., by chelating agents such as EDTA, leads to loss of cadherin function. The Ca2+-binding pockets are made up of amino acids from two consecutive cadherin repeats, which form a characteristic tertiary structure to coordinate a single Ca2+ion [1]. 

Figure 11 depicts two characteristics of the Indo-l-detected calcium response. The intracellular Ca rise is primarily from intracellular stores because adding EGTA to chelate extracellular Ca does not inhibit the response (Figure 11, upper panel). 

Figure 7. Sensitivity of the FMLP-induced calcium signal to removal of extracellular calcium. Indo-l-loaded neutrophils were stimulated with 10 M FMLP in a medium of normal osmolality (320 mosmol/kg) and indo-1 fluorescence was recorded as described in Figure 6. Trace 1 Cells in a medium with normal calcium (1.5 mN). Trace 2 EGTA added to chelate extracellular calcium before stimulation extracellular calcium (1.5 milf) readded 70 s after stimulation. Trace 3 Cells in a medium with normal calcium EGTA added 70 s after stimulation to chelate extracellular calcium.

Human tears contain 0.5-1.1 mM Ca2+ [215], which may be the source of extracellular calcium for maintaining the integrity of the corneal and conjunctival intercellular junctions. Indeed, chelation of extracellular Ca2+ with 0.5% EDTA reduced transcorneal and transconjunctival resistance by 80 and 65%, respectively (Kompella, Kim, and Lee, unpublished observation). In 1991, Rojanasakul... 

Urinary lead levels have also been used to measure current exposure (Robinson 1974) but they are of questionable value as biomarkers of exposure because of the relatively low and fluctuating lead levels that are excreted in the urine (ACGIH 1986 Ibels and Pollock 1986 Jensen 1984). In contrast, the determination of urinary lead following a single injection of the chelating agent, calcium disodium EDTA, which mobilizes extracellular lead and produces increased urinary excretion of lead, is presumed to be indicative of an elevated body burden of lead (Cory-Slechta et al. 1987 Ibels and Pollock 1986 Janin et al. 1985). Children whose PbB levels are 45 pg/dL should not receive a provocative chelation... 

The Gd111 chelates, when used as contrast agents, are administered intravenously and distribute through the extracellular and intravascular spaces. Typical doses of the currently used, low molecular... 

Taylor There s no EGTA here for the simple reason that it would chelate some of the inhibitors we use, such as Gd3+. But with that said, we don t need to chelate extracellular Ca2+ with EGTA to prevent Ca2+ entry we get no detectable Ca2+ -entry whether we simply omit Ca2+ or replace it with EGTA. 

Moffett, J. W. and Brand, L. E. (1996). Production of strong, extracellular Cu chelators by marine cyanobacteria in response to Cu stress, Limnol. Oceanogr., 41, 388-395. 

Basal levels of Ca2+ in resting neutrophils are around 100 nM. Upon stimulation with soluble agonists such as fMet-Leu-Phe, PAF and LTB4, intracellular levels rise to about 1 pM within 20-30 s and then return to basal levels. Experiments in which neutrophils are suspended in media devoid of Ca2+ (and containing EGTA to chelate trace contaminants of this cation), show that although the initial rise in intracellular Ca2+ is unaffected, the elevation in concentration is not sustained when extracellular Ca2+ is absent. Thus, the initial rise in intracellular Ca2+ is due to mobilisation of intracellular stores Ca2+ influx is then activated in order to sustain these levels (Fig. 6.11). Addition of Ins 1,4,5-P3 to permeabilised neutrophils results in a rapid (within 2 s) rise in intracellular Ca2+, which then declines to basal levels, possibly as a result of the released Ca2+ returning to the intracellular stores. These intracellular vesicles must therefore have independent Ca2+ efflux and influx. 

Matrix metalloproteinases (MMPs) are a class of zinc- and calcium-dependent enzymes that are responsible for the metabolism of extracellular matrix proteins [27]. Increased activity of MMPs has been associated with pathological diseases such as arthritis, cancer, multiple sclerosis and Alzheimer s disease [28-31]. Therefore, they constitute an important group of drug targets. Their inhibition is accomplished by blocking the active site of the catalytic domain with ligands that contain hydroxamic or carboxylic acids to chelate the Zn metal. The identification of low molecular weight compounds that contain different scaffolds may lead to the development of a new class of specific inhibitors. 

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