Effects of calcium on Na+, K+-ATPase isolated from human placenta

It has been reported that Ca²⁺ can be either stimulatory or inhibitory of the activity of Na⁺,K⁺-ATPase obtained from different sources. The aim of the present work was to study the effect of increasing concentrations of Ca²⁺ on Na⁺,K⁺-ATPase purified from human placenta. The temperature dependence of Na⁺,K⁺-ATPase activity and of spectral parameters of a spin label paramagnetic maleimide derivative (MSL, 2,2,6,6-tetramethylpiperidin-1-oxyl-4-maleimide) were also investigated in absence and in presence of 2 mM Ca²⁺. Ca²⁺ affects positively the enzymatic activity and this effect is evident at all temperature tested. Maleimide spectral parameters are not affected by the presence of 2 mM Ca²⁺ showing that the microenvironments of SH groups are not modified following ion interactions.