Membranes, cell, ATPase inhibitors function

ATP-dependent uptake of copper into everted membrane vesicles from cells expressing ecCopA could be demonstrated. Transport was inhibited by the classical P-type ATPase inhibitor vanadate. Dithiothreitol, a strong reductant, was required for ecCopA-catalyzed Cu uptake, suggesting that the substrate of ecCopA is Cu(l). Thus the function of ecCopA resembles that of the En. hirae CopB ATPase by functioning as a copper efflux pump in vivo when excess copper is present in the cytoplasm (Rensing et al., 2000). 

Mercury is a reactive element and its toxicity is probably due to interaction with proteins. Mercury has a particular affinity for sulphydryl groups in proteins and consequently is an inhibitor of various enzymes such as membrane ATPase, which are sulphydryl dependent. It can also react with amino, phosphoryl and carboxyl groups. Brain pyruvate metabolism is known to be inhibited by mercury, as are lactate dehydrogenase and fatty acid synthetase. The accumulation of mercury in lysosomes increases the activity of lysomal acid phosphatase which may be a cause of toxicity as lysosomal damage releases various hydrolytic enzymes into the cell, which can then cause cellular damage. Mercury accumulates in the kidney and is believed to cause uncoupling of oxidative phsophorylation in the mitochondria of the kidney cells. Thus, a number of mitochondrial enzymes are inhibited by Hg2+. These effects on the mitochondria will lead to a reduction of respiratory control in the renal cells and their functions such as solute reabsorption, will be compromised. 

Like thallous chloride [ Tl" ], the cationic technetium complex accumulates in the viable myocardial tissue proportional to blood flow. Studies using cultures of myocardial cells have shown that uptake is not dependent on the functional capability of the so-dium/potassium pump (Maublant et al. 1988). Cationic membrane transport inhibitors did not affect 1-min Tc-MIBI uptake kinetics when cells were preincubated for 1 min in solutions containing saturating concentrations of quabain (100 pM), a so-dium/potassium ATPase inhibitor (Piwnica-Worms et al. 1990). 

ATPases participate directly in various transport and motile functions. Of significance for this review is a manganese-specific ATPase found in rat brain [146], and Mn(II) which interacts specifically with other membrane-bound ATPases from brain and heart cells, including phospholipid-dependent ATPase activity exhibited by protein kinase C [147-149]. Interestingly hydrolysis of the cholinesterase-inhibitor Soman (isopropyl methyl-phosphonyl fluoride) is catalyzed by a manganese-dependent enzyme isolated from clonal neuroblastoma cells [150]. 

Asokan and Cho [83] reviewed the distribution of pH environments in the cell. Much of what is known in the physiological literature was determined using pH-sensitive fluorescent molecules and specific functional inhibitors. The physiological pH in the cytosol is maintained by plasma membrane-bound H+-ATPases, ion exchangers, as well as the Na+/K+-APTase pumps. Inside the organelles, pH microenvironments are maintained by a balance between ion pumps, leaks, and internal ionic equilibria. Table 2.1 lists the approximate pH values of the various cellular compartments. 

Flavonoids can affect the function of plasma membrane transport Na+- and K+-ATPase, mitochondrial ATPase, and Ca2+-ATPase. The Mg2+-ectoATPase of human leukocytes is inhibited by quercetin, which acts as a competitor of ATP binding to the enzyme. The sarcoplasmic reticulum Ca2+-ATPase of muscle is effectively inhibited by several flavonoids that were also active inhibitors of antigen-induced mast cell histamine release. 

Membrane-Based Assays Membranes prepared from cells expressing transporters have been widely used to study the function of ABC efflux pumps and to identify their substrates or inhibitors. Currently, there are two major membrane-based assays the ATPase assay and the membrane vesicular transport (uptake) assay. Compared to the cell-based assay, the membrane-based assay has several advantages including (1) the assay can be used to characterize the effect of a xenobiotic on one specific efflux transporter (2) the assay can be easily employed in a high throughput mode (3) membranes are easy to be maintained after preparation and (4) the assay is easy to conduct. 

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