The role of the sites for ATP of the Ca2+-ATPase from human red cell membranes during Ca2+-phosphatase activity |
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Authors: | Ariel J. Caride Alcides F. Rega Patricio J. Garrahan |
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Affiliation: | Departamento de Química Biològica, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Junín 956, 1113 Buenos Aires Argentina |
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Abstract: | 1. In the presence of ATP, the Ca2+ pump of human red cell membranes catalyzes the hydrolysis of phosphate. The requirement for ATP of the Ca2+- activity was studied in relation to the two classes of site for ATP that are apparent during Ca2+ -ATPase activity. 2. (a) The for ATP as activator of the Ca2+ - extrapolated at 0 mM PNPP is equal to the of the Ca2+ -ATPase. (b) PNPP competes with ATP and its effectiveness is the same regardless the nucleotide acts as the substrate of the Ca2+ -ATPase or as activator of the Ca2+ -. 3. PNPP at the high-affinity site does not substitute for ATP as activator of the Ca2+ -. 4. At ATP concentrations that almost saturate the high-affinity site, Ca2+ - activity increases as a function of PNPP along an S-shaped curve, while Ca2+ -ATPase activity is partially inhibited along a curve of the same shape and apparent affinity. The fraction of Ca2+ -ATPase activity which is inhibited by PNPP is that which results from occupation of the low-affinity site by ATP. 5. Activation of the Ca2+ -ATPase by ATP at the low-affinity site is associated with inhibition of the Ca2+ - activity. Both phenomena take place with the same apparent affinity and along curves of the same shape. 6. Experimental results suggest that: (a) the Ca2+ - activity depends on ATP at the high-affinity site; (b) PNPP is hydrolyzed at the low-affinity site; (c) Ca2+ -ATPase activity at the high-affinity size persists during Ca2+ - activity. |
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Keywords: | (Human erythrocyte) EGTA PNPP |
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