Hara S, Hara Y, Nakao T, Nakao M
Biochim Biophys Acta. 1981 Jun 9;644(1):53-61. doi: 10.1016/0005-2736(81)90057-2.
Showdomycin inhibited pig brain (Na+ + K+)-ATPase with pseudo first-order kinetics. The rate of inhibition by showdomycin was examined in the presence of 16 combinations of four ligands, i.e., Na+, K+, Mg2+ and ATP, and was found to depend on the ligands added. Combinations of ligands were divided into five groups in terms of the magnitude of the rate constant; in the order of decreasing rate constants these were: (1) Na+ + Mg2+ + ATP, (2) Mg2+, Mg2+ + K+, K+ and none, (3) Na+ + Mg2+, Na+, K+ + Na+ and Na+ + K+ + Mg2+, (4) Mg2+ + K+ + ATP, K+ + ATP and Mg2+ + ATP, (5) K+ + Na + + ATP, Na+ + ATP, Na+ + K+ + Mg2+ + ATP and ATP. The highest rate was obtained in the presence of Na+, Mg2+ and ATP. The apparent concentrations of Na+, Mg2+ and ATP for half-maximum stimulation of inhibition (KS0.5) were 3 mM, 0.13 mM and 4 MicroM, respectively. The rate was unchanged upon further increase in Na+ concentration from 140 to 1000 mM. The rates of inhibition could be explained on the basis of the enzyme forms present, including E1, E2, ES, E1-P and E2-P, i. e., E2 has higher reactivity with showdomycin than E1, while E2-P has almost the same reactivity as E1-P. We conclude that the reaction of (Na+ + K+)- ATPase proceeds via at least four kinds of enzyme form (E1, E2, E1 . nucleotide and EP), which all have different conformations.
秀多霉素以假一级动力学抑制猪脑（Na⁺ + K⁺）-ATP酶。在四种配体（即Na⁺、K⁺、Mg²⁺和ATP）的16种组合存在的情况下，研究了秀多霉素的抑制速率，发现其取决于添加的配体。根据速率常数的大小，配体组合被分为五组；按速率常数递减顺序排列如下：（1）Na⁺ + Mg²⁺ + ATP，（2）Mg²⁺、Mg²⁺ + K⁺、K⁺和无，（3）Na⁺ + Mg²⁺、Na⁺、K⁺ + Na⁺和Na⁺ + K⁺ + Mg²⁺，（4）Mg²⁺ + K⁺ + ATP、K⁺ + ATP和Mg²⁺ + ATP，（5）K⁺ + Na⁺ + ATP、Na⁺ + ATP、Na⁺ + K⁺ + Mg²⁺ + ATP和ATP。在Na⁺、Mg²⁺和ATP存在的情况下获得了最高速率。抑制作用达到半最大刺激时Na⁺、Mg²⁺和ATP的表观浓度（KS0.5）分别为3 mM、0.13 mM和4 μM。当Na⁺浓度从140 mM进一步增加到1000 mM时，速率不变。抑制速率可以根据存在的酶形式来解释，包括E1、E2、ES、E1-P和E2-P，即E2与秀多霉素的反应性高于E1，而E2-P与E1-P的反应性几乎相同。我们得出结论，（Na⁺ + K⁺）-ATP酶的反应至少通过四种具有不同构象的酶形式（E1、E2、E1·核苷酸和EP）进行。
