ATPases characteristics

In addition to marked perifascicular atrophy, infarctlike areas are sometimes seen, and are also consistent with a microangiopathy. Muscle fibers which appear normal morphologically may show loss of myofibrillar ATPase activity from the center of the fibers this is also characteristic of muscle subject to ischemia. Such changes may be reversible, but more prolonged ischemia undoubtedly causes irreversible muscle necrosis. 

Solubilization of an active H,K-ATPase is also a prerequisite for reconstitution of the enzyme into liposomes. With these H,K-ATPase proteoliposomes it is then possible to study the transport characteristics of pure H,K-ATPase, without the interference of residual protein contamination that is usually present in native vesicular H,K-ATPase preparations. Rabon et al. [118] first reported the reconstitution of choleate or n-octylglucoside solubilized H,K-ATPase into phosphatidylcholine-cholesterol liposomes. The enzyme was reconstituted asymmetrically into the proteoliposomes with 70% of the pump molecules having the cytoplasmic side extravesicular. In the presence of intravesicular K, the proteoliposomes exhibited an Mg-ATP-dependent H transport, as monitored by acridine orange fluorescence quenching. Moreover, as seen with native H,K-ATPase vesicles, reconstituted H,K-... 

A novel substance with a bufadienolide-related chemical structure has been isolated from B. marinus skin. This substance is 3P-hydroxy-ll, 12-seco-5p, 14P-bufa-20, 22-dienolide-l 1, 14-olides-12oic acid (called marinoic acid) and shows the Na+/K+-ATPase inhibitory activity characteristic of the bufadienolides (Matsukawa, Akizawa, Morris, Butler Jr and Yoshioka 1996). 

ABC transporters are multidomain systems that translocate substrates across membranes. A common characteristic is the well-conserved ATP binding cassette (ABC) domain that couples ATP hydrolysis to transport. Members of this group of proteins constitute the largest superfamily of transport components, and they are found in all organisms from Archaea to humans. According to the work of Dassa, who developed a classification based on the ATPase components, the ABC systems can be divided into a number of subfamilies (for details see http //www.pasteur.fr/recherche/unites/pmtg/abc/) [136]. 

The characteristic Walker A and Walker B motifs that are involved in ATP binding [144] are always found in the ATPase or ABC domains. In addition, a signature motif, also called the LSGGQ motif, is typical of all bacterial ABC domains involved in binding-protein-dependent import. The signature motif is absent in other types of ATPases. 

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 red blood cell has no mitochondria and is totally dependent on anaerobic glycolysis for ATP. In pyruvate kinase deficiency, the decrease in ATP causes the erythrocyte to lose its characteristic biconcave shape and signals its destruction in the spleen. In addition, decreased ion pumping by Na /K -ATPase results in loss of ion balance and causes osmotic fragility, leading to swelling and lysis. 

One striking characteristic of the coupling ATPase of energy-transducing membranes, apart from the extraordinarily large number of different polypeptide subunits, is the existence of two different polypeptides involved in the response of the enzyme to the inhibitor oligomycin. One binds the inhibitor, the other, separated in space from the former by possibly as much as 10 to 15 A, confers oligomycin sensitivity to the entire enzyme complex. How could the transfer of information between these two polypeptide subunits and their concerted interaction with the ATPase proper be visualized ... 

Because three Na+ ions move outward for every two K+ ions that move inward, the process is electrogenic—it creates a net separation of charge across the membrane. The result is a transmembrane potential of —50 to —70 mV (inside negative relative to outside), which is characteristic of most animal cells and essential to the conduction of action potentials in neurons. The central role of the Na+K+ ATPase is reflected in the energy invested in this single reaction about 25% of the total energy consumption of a human at rest ... 

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