In Na, K-ATPase reaction, Na
+ is indispensable to the formation of phosphointermediate, and K
+ promotes dephosphorylation of phosphointermediate. It is known that Li
+, Rb
+, and Cs
+, which are homologs of K
+ and NH
4+, have a similar action to that of K
+, but the details are not well understood. In the current study we examined the effects of the homologs of K
+ and NH
4+ on Na, K-ATPase reaction and hydrolysis of
p-nirophenyl phosphate (
p-NPP), a partial reaction of Na, K-ATPase, using Na, K-ATPase purified from rabbit brains. In addition, since some previous reports suggested that inhibition of Na, K-ATPase activity by cisplatin may be the cause of renal failure, we examined the effects of cisplatin on the Na, K-ATPase activity which was dependent on the homologs of K
+ and NH
4+.
The homologs of K
+ and NH
4+ promoted ATPase activity in the presence of Na
+ and Mg
2+, and the order of the promoting effect and the affinity was K
+> NH
4+ = Cs
+= Rb
+ > Li
+. As no additive promotion of ATPase activity was observed by the addition of any of these ions in the presence of K
+, the effects of these ions were thought to be similar to that of K
+.
All the ions except Li
+ activated hydrolysis of
p-NPP as partial reaction of Na, K-ATPase. On the other hand, Li
+ showed a promoting effect, as Na
+ does, on ATPase activity in the absence of K
+, whereas other homologs of K
+ and NH
4+ did not. Cisplatin inhibited ATPase activity irrespective of the presence of K
+, or homologs of K
+ or NH
4+. Furthermore, cisplatin inhibited
p-NPP hydrolysis activated by the homologs of K
+ or NH
4+ and ATPase activity activated by Na
+ or Li
+.
These findings suggest that all the homologs of K
+ and NH
4+ have an action on Na, K-ATPase activity, similar to that of K
+, and only Li
+ has both Na
+- and K
+- like effects, and that cisplatin inhibits ATPase and
p-NPPase activities activated by the homologs of K
+ and NH
4+.
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