| Research Abstract |
I have investigated the ion transport mechanism of Na^+, K^+-ATPase using oligomycin, which inhibits the Na^+ transport and the ATPase activity. I obtained the following findings. 1) Oligomycin inhibited only the Na^+ transport among the typical P-type-ATPases, i.e., Na^+, K^+-ATPase, H^+, K^+-ATPase and Ca^<2+>-ATPase. 2) Oligomycin had a binding domain in the N-terminal 200 amino acids of Na^+, K^+-ATPase α subunit, . This domain included the first and second transmembrane segments. The hydrophobic oligomycin, therefore, should bind somewhere in these segments, though I cannot identify its fine binding sites. From these results and other findings, which are an assumptive model of Na^+, K^+-ATPase and localization of the amino acids required for ion transport, I propose the following hypothesis ; the conformational change of the first-sixth transmembrane segments is accompanied with Na^+ transport. When Na^+ is transported, oligomycin disturb the change of the first and second segments, so that the transport is inhibited. When the conformation comes back to the original state, K^+ is transported. Oligomycin does not inhibit K^+ transport. According to this hypothesis, I have investigated oligomycin and Na^+ on K^+-dependent p-nitrophenyl phosphatase (pNPPase) activity, which is a partial reaction of Na^+, K^+-ATPase. When Na^+ : K^+ ratio is over10 : 1, oligomycin stimulated pNPPase activity. This result is unexpected, because it is expected that oligomycin increases the inhibitory effect of Na^+ on pNPPase activity. Under this experimental conditions, oligomycin increased the affinity for K^+ to Na^+, K^+-ATPase. This finding suggested that oligomycin affected the interaction between ions and K^+ binding (and/or Na^+ releasing site) at extracellular side.
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