Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
10796099 | Biochimica et Biophysica Acta (BBA) - Bioenergetics | 2005 | 9 Pages |
Abstract
The bidentate complex of ATP with Cr3+, CrATP, is a nucleotide analog that is known to inhibit the sarcoplasmic reticulum Ca2+-ATPase and the Na+,K+-ATPase, so that these enzymes accumulate in a conformation with the transported ion (Ca2+ and Na+, respectively) occluded from the medium. Here, it is shown that CrATP is also an effective and irreversible inhibitor of the plasma membrane Ca2+-ATPase. The complex inhibited with similar efficiency the Ca2+-dependent ATPase and the phosphatase activities as well as the enzyme phosphorylation by ATP. The inhibition proceeded slowly (T1/2 = 30 min at 37 °C) with a Ki = 28 ± 9 μM. The inclusion of ATP, ADP or AMPPNP in the inhibition medium effectively protected the enzyme against the inhibition, whereas ITP, which is not a PMCA substrate, did not. The rate of inhibition was strongly dependent on the presence of Mg2+ but unaltered when Ca2+ was replaced by EGTA. In spite of the similarities with the inhibition of other P-ATPases, no apparent Ca2+ occlusion was detected concurrent with the inhibition by CrATP. In contrast, inhibition by the complex of La3+ with ATP, LaATP, induced the accumulation of phosphoenzyme with a simultaneous occlusion of Ca2+ at a ratio close to 1.5 mol/mol of phosphoenzyme. The results suggest that the transport of Ca2+ promoted by the plasma membrane Ca2+-ATPase goes through an enzymatic phospho-intermediate that maintains Ca2+ ions occluded from the media. This intermediate is stabilized by LaATP but not by CrATP.
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Authors
Otacilio C. Moreira, Priscila F. Rios, Hector Barrabin,