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Biochimica et biophysica acta, 1971-12, Vol.249 (2), p.345-351
Ort / Verlag
Netherlands: Elsevier B.V
Erscheinungsjahr
1971
Link zum Volltext
Quelle
Elsevier Journal Backfiles on ScienceDirect (DFG Nationallizenzen)
Beschreibungen/Notizen
1.
1. A Ca
2+-activated ATPase of isolated membranes of guinea pig placenta is characterized.
2.
2. This enzyme is preferentially activated by calcium ions. In the presence of 5 mM ATP, maximal enzyme activity being obtained at 3–5 mM Ca
2+. The maximal rate of ATP hydrolysis varies between 15 and 22 μmoles P
i per mg protein in 30 min.
3.
3. Mg
2+ also activates the enzyme, but always to a lesser degree than Ca
2+. Mn
2+, but not Sr
2+, activates the enzyme. At optimal Ca
2+ concentration, addition of Mg
2+ is always inhibitory. Ca
2+ and Mg
2+ seem to act on the same site.
4.
4. The enzyme does not require Na
+ or K
+ for activation by Ca
2+.
5.
5. The optimal pH for Ca
2+ activation of the enzyme lies between 8.2 and 8.5; at pH 7.1 and 9.5 only 50 % of maximal activation occurs.
6.
6. Addition of increasing amounts of Na
2H
2EDTA leads to progressive decreases of activity, complete inhibition occurring at 5 mM when the incubation fluid contains 5 mM Ca
2+.
7.
7. Ethacrynic acid inhibits the enzyme but ouabain does not.
8.
8. Other triphosphates may serve as substrate; but the
ν
max
for Na
2ATP is highest.
9.
9. We suggest that this enzyme aids in maintaining the uphill gradient of Ca
2+ between maternal and fetal circulation.