Sweadner K J
J Biol Chem. 1985 Sep 25;260(21):11508-13.
There are two isozymes of the Na,K-ATPase, which can be purified separately from rat renal medulla and brainstem axolemma. Here the basic kinetic properties of the two Na,K-ATPases have been compared in conditions permitting enzyme turnover. The two isozymes are half-maximally activated at different concentrations of ATP, the axolemma Na,K-ATPase having the higher affinity. They are half-maximally activated by Na+ and K+ at very similar concentrations but show differences in cooperativity toward Na+. The affinities of both isozymes for ATP and Na+ are affected in a qualitatively similar way by variations in the concentration of K+. Both isozymes transport 22Na+ and 42K+ in a ratio close to 3:2 in artificial lipid vesicles. The two isozymes differ most strikingly in the inhibition of ATPase activity by ouabain. The axolemma Na,K-ATPase has a high affinity for ouabain with positive cooperativity, while the renal medulla Na,K-ATPase has a lower affinity with negative cooperativity. It is likely that the cooperativity differences are due to kinetic effects, reflecting different rates of conformation transitions during enzyme turnover. The functional result of the contrasting cooperativities is that the difference in sensitivity to ouabain is amplified.
钠钾ATP酶有两种同工酶，可分别从大鼠肾髓质和脑干轴膜中纯化得到。在此，我们比较了两种钠钾ATP酶在允许酶周转的条件下的基本动力学特性。这两种同工酶在不同浓度的ATP下被半最大激活，轴膜钠钾ATP酶具有更高的亲和力。它们在非常相似的浓度下被Na⁺和K⁺半最大激活，但对Na⁺的协同性存在差异。两种同工酶对ATP和Na⁺的亲和力受K⁺浓度变化的影响在性质上相似。在人工脂质小泡中，两种同工酶转运²²Na⁺和⁴²K⁺的比例接近3:2。这两种同工酶在哇巴因对ATP酶活性的抑制作用方面差异最为显著。轴膜钠钾ATP酶对哇巴因具有高亲和力且呈正协同性，而肾髓质钠钾ATP酶具有较低亲和力且呈负协同性。协同性差异很可能是由于动力学效应，反映了酶周转过程中构象转变的不同速率。相反协同性的功能结果是对哇巴因敏感性的差异被放大。
