We determined and characterized the Mg2+-dependent, Ca2+-stimulated ATPase (Ca-ATPase) activity in cell plasma membranes from the myometrium of pregnant women, and compared these characteristics to those of the active Ca2+-transport already demonstrated in this tissue. Similarly to the Ca2+-transport system, the Ca2+-ATPase is Mg2+-dependent, stimulated by calmodulin, and inhibited by vanadate. The Km for Ca2+ activation is 0.40 m M, very similar to that found for active calcium transport, i.e. 0.25 m M. Consequently, this Ca2+-ATPase can be responsible for the active calcium transport across the plasma membranes of smooth muscle cells., F. Carrera, T. Proverbio, R. Marín, F. Proverbio., and Obsahuje bibliografii
The inhibitory effect of 2 % ethanol (400 mM) in the incubation medium on several characteristics of the Na+-ATPase of basolatcral plasma membranes from rat kidney proximal tubular cells was investigated. Ethanol did not change the Km of the enzyme for Mg2+, ATP or Na + ; it did not change either the optimal pH or temperature values of the incubation medium for the enzyme to act and finally, it did not affect the apparent energy of activation of the enzyme. It was also found that 2 % ethanol produced stronger inhibition of the ATPase when it is in an activated or stimulated state, than when it is working at its lower basal level. The presented results can be explained by assuming that 2 % ethanol in the incubation medium inhibits Na +-ATPase activity by affecting the enzyme structure as well as its activating mechanism.