ATPase varieties

The gradients of H, Na, and other cations and anions established by ATPases and other energy sources can be used for secondary active transport of various substrates. The best-understood systems use Na or gradients to transport amino acids and sugars in certain cells. Many of these systems operate as symports, with the ion and the transported amino acid or sugar moving in the same direction (that is, into the cell). In antiport processes, the ion and the other transported species move in opposite directions. (For example, the anion transporter of erythrocytes is an antiport.) Proton symport proteins are used by E. coU and other bacteria to accumulate lactose, arabinose, ribose, and a variety of amino acids. E. coli also possesses Na -symport systems for melibiose as well as for glutamate and other amino acids. 

The cause of Wilson disease was also revealed in 1993, when it was reported that a variety of mutations in a gene encoding a copper-binding P-type ATPase were responsible. The gene is estimated to encode a protein of 1411 amino acids, which is highly homologous to the product of the gene affected in Menkes disease. In a manner not yet fully explained, a nonfunctional ATPase causes defective excretion of copper into the bile, a reduction of incorporation of copper into... 

There are many ligands and group-specific reagents that have been demonstrated to alter the properties of H,K-ATPase, and which are not clinically useful. For example, there is a variety of chemicals that have been used in studies on structure-function relations of H,K-ATPase and that inhibit the enzyme in vitro by modification of its amino [49,67,158], sulfhydryl [95,165,166] or carboxyl groups [140]. 

Additional studies have also demonstrated that the application of other disulphides, such as oxidized dithiothreitol and cystine, produce similar effects to GSSG over an identical concentration range (Haddock et al., 1991). Therefore the Na/K ATPase activity may be modified by disulphides derived from a variety of species. 

These data demonstrate that both GSH and GSSG have profound effects on Na/K ATPase activity and may act in concert to modify enzyme activity during oxidant stress. However, it should be recognized that the steric conformation of an isolated enzyme preparation in a chemically buffered solution may be considerably different to the native enzyme located in a dynamic lipid bilayer. For this reason, these investigations have been extended to include a variety of preparations in which the Na/K pump is in its native environment. 

ABC transporters are involved in both uptake and excretion of a variety of substrates from ions to macromolecules. Whereas export systems of this type are present in all kingdoms of life, import systems are exclusively found in prokaryotes. ABC transporters are minimally composed of two hydrophobic membrane embedded components and two ATPase units. 

A variety of in vitro assays are available to identify compounds as substrates and inhibitors of P-gp. These assays, which have been reviewed elsewhere in great detail [20-24], can be classified into three general categories (1) transport, (2) accumulation/ efflux and (3) ATPase activity [20-28]. It is important to note that these in vitro model systems can be adapted for measuring the interaction of dmgs with other important drug transporter systems [22]. 

AAA ATPases associated with a variety of cellular activities E(MFP)AB 87(122) 112(159) 1BMF... 

A variety of automatic voltage clamp devices with special modifications have been extensively utilized in electrophysiological studies of /sc in several ocular tissues including the amphibian corneal epithelium [42] and human fetal retinal pigment epithelium [43, 44], as well as non-ocular tissues like the rat tracheal epithelium [45], A strong temperature dependency and inhibitory effect of serosally instilled ouabain on the rabbit conjunctival /sc are characteristic of active ion transport driven by Na+/K+-ATPases in the conjunctiva [6, 7],... 

The six ATPases belong to the rather large family of AAA ATPases (for ATPases Associated with a variety of cellular Activities) whose eukaryotic members include the motor protein dynein, the membrane fusion factor NSF, and the chaperone... 

Proteasome and RC subunits are subjected to a variety of post-translational modifications including phosphorylation, acetylation, myristoylation, and even O-glycosylation [136-140]. In yeast all seven a-subunits are acetylated as well as two yS-subunits. Since acetylation of the N-terminal threonine in an active j8-subunit would poison catalysis, it has been suggested that the propeptide extensions function to prevent acetylation [130]. Three members of the S4 ATPase subfamily (S4,... 

Ankyrins are intracellular proteins that associate with a variety of transport proteins including Na+ channels, Na +, K+ ATPase, Na +, Ca2+ exchange, Ca2+ release channels. 

In animal cells, Na+K+ ATPase maintains the differences in cytosolic and extracellular concentrations of Na+ and K+, and the resulting Na+ gradient is used as the energy source for a variety of secondary active transport processes. 

As an example of an asymmetric membrane integrated protein, the ATP synthetase complex (ATPase from Rhodospirillum Rubrum) was incorporated in liposomes of the polymerizable sulfolipid (22)24). The protein consists of a hydrophobic membrane integrated part (F0) and a water soluble moiety (Ft) carrying the catalytic site of the enzyme. The isolated ATP synthetase complex is almost completely inactive. Activity is substantially increased in the presence of a variety of amphiphiles, such as natural phospholipids and detergents. The presence of a bilayer structure is not a necessary condition for enhanced activity. Using soybean lecithin or diacetylenic sulfolipid (22) the maximal enzymatic activity is obtained at 500 lipid molecules/enzyme molecule. With soybean lecithin, the ATPase activity is increased 8-fold compared to a 5-fold increase in the presence of (22). There is a remarkable difference in ATPase activity depending on the liposome preparation technique (Fig. 41). If ATPase is incorporated in-... 

The whole question of the specificity was reopened with the discovery that E. coli phosphatase, contrary to an earlier statement (114), hydrolyzed a variety of polyphosphates including metaphosphate of average chain length 8 (97). It was subsequently reported that partially purified phosphatases from several mammalian tissues had appreciable PPi-ase activity at pH 8.5 (115). This was confirmed (116) and extended to include ATPase and fluorophosphatase activities (117). Proof that the same enzyme is responsible for the monoesterase and PPi-ase activities was afforded by heat inactivation studies, cross inhibition experiments, and inhibition of PPi-ase activity by L-phenylalanine, a specific inhibitor of intestinal phosphatase. It was also found that calf intestinal phosphatase couid be phosphorylated by 32P-PP and the number of sites so labeled agreed with the number of active sites determined with a monoester substrate using a stopped-flow technique (118). It would seem that the main reason for the confusion with regard to the PPi-ase activity results from the inclusion of Mg2+ in the assay. This stimulates the monoesterase activity but almost completely inhibits PPi-ase activity (117). 

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