ISSN:
1471-4159
Quelle:
Blackwell Publishing Journal Backfiles 1879-2005
Thema:
Medizin
Notizen:
Abstract: With a partially purified, membrane-bound (Ca + Mg)-activated ATPase preparation from rat brain, the K0.5 for activation by Ca2+ was 0.8 p μm in the presence of 3 mm-ATP, 6 mm-MgCl2, 100 mM-KCI, and a calcium EGTA buffer system. Optimal ATPase activity under these circumstances was with 6-100 μm-Ca2+, but marked inhibition occurred at higher concentrations. Free Mg2+ increased ATPase activity, with an estimated K0.5, in the presence of 100 μm-CaCl2, of 2.5 mm; raising the MgCl2 concentration diminished the inhibition due to millimolar concentrations of CaCl2, but antagonized activation by submicromolar concentrations of Ca2+. Dimethylsulfoxide (10%, v/v) had no effect on the K0.5 for activation by Ca2+, but decreased activation by free Mg2+ and increased the inhibition by millimolar CaCl2. The monovalent cations K+, Na+, and TI+ stimulated ATPase activity; for K+ the K0.5 was 8 mm, which was increased to 15 mm in the presence of dimethylsulfoxide. KCI did not affect the apparent affinity for Ca2+ as either activator or inhibitor. The preparation can be phosphorylated at 0°C by [γ-32P]-ATP; on subsequent addition of a large excess of unlabeled ATP the calcium dependent level of phosphorylation declined, with a first-order rate constant of 0.12 s−1. Adding 10 mm-KCI with the unlabeled ATP increased the rate constant to 0.20 s−1, whereas adding 10 mm-NaCl did not affect it measurably. On the other hand, adding dimethyl-sulfoxide slowed the rate of loss, the constant decreasing to 0.06 s−1. Orthovanadate was a potent inhibitor of this enzyme, and inhibition with 1 μm-vanadate was increased by both KCI and dimethylsulfoxide. Properties of the enzyme are thus reminiscent of the plasma membrane (Na + K)-ATPase and the sarcoplasmic reticulum (Ca + Mg)-ATPase, most notably in the K+ stimulation of both dephosphorylation and inhibition by vanadate.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1111/j.1471-4159.1981.tb05301.x
Permalink