Description:
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<jats:list-item><jats:p>The chromium(III) complex of ATP, an MgATP complex analogue, inactivates (Na<jats:sup>+</jats:sup>+ K<jats:sup>+</jats:sup>)‐ATPase by forming a stable chromo‐phosphointermediate. The rate constant <jats:italic>k</jats:italic><jats:sub>2</jats:sub> of inactivation at 37°C of the β,γ‐bidentate of CrATP is enhanced by Na<jats:sup>+</jats:sup> (<jats:italic>K</jats:italic><jats:sub>0.5</jats:sub>= 1.08 mM), imidazole (<jats:italic>K</jats:italic><jats:sub>0.5</jats:sub>= 15 mM) and Mg<jats:sup>2+</jats:sup> (<jats:italic>K</jats:italic><jats:sub>0.5</jats:sub>= 0.7 mM). These cations did not affect the dissociation constant of the enzyme‐chromium‐ATP complex.</jats:p></jats:list-item>
<jats:list-item><jats:p>The inactive chromophosphoenzyme is reactivated slowly by high concentrations of Na<jats:sup>+</jats:sup> at 37°C. The half‐maximal effect on the reactivation was reached at 40 mM NaCl, when the maximally observable reactivation was studied. However, 126 mM NaCl was necessary to see the half‐maximal effect on the apparent reactivation velocity constant. K<jats:sup>+</jats:sup> ions hindered the reactivation with a <jats:italic>K</jats:italic><jats:sub>i</jats:sub> of 70 μM.</jats:p></jats:list-item>
<jats:list-item><jats:p>Formation of the chromophosphoenzyme led to a reduction of the Rb<jats:sup>+</jats:sup> binding sites and of the capacity to occlude Rb<jats:sup>+</jats:sup>.</jats:p></jats:list-item>
<jats:list-item><jats:p>The β,γ‐bidentate of chromium(III)ATP (<jats:italic>K</jats:italic><jats:sub>d</jats:sub>= 8 μM) had a higher affinity than the α,β,γ‐tridentate of chromium(III)ATP (<jats:italic>K</jats:italic><jats:sub>d</jats:sub>= 44 μM) or the cobalt tetramine complex of ATP (<jats:italic>K</jats:italic><jats:sub>d</jats:sub>= 500 μM). The β,γ‐bidentate of the chromium(III) complex of adenosine 5′‐[β, γ‐methylene]triphosphate also inactivated (Na<jats:sup>+</jats:sup>+ K<jats:sup>+</jats:sup>)ATPase.</jats:p></jats:list-item>
<jats:list-item><jats:p>Although CrATP could not support Na<jats:sup>+</jats:sup>, K<jats:sup>+</jats:sup> exchange in everted vesicles prepared from human red blood cells, it supported the Na<jats:sup>+</jats:sup>‐Na<jats:sup>+</jats:sup> and Rb<jats:sup>+</jats:sup>‐Rb<jats:sup>+</jats:sup> exchange.</jats:p></jats:list-item>
<jats:list-item><jats:p>It is concluded that CrATP opens up Na<jats:sup>+</jats:sup> and K<jats:sup>+</jats:sup> channels by forming a relatively stable modified enzyme‐CrATP complex. This stable complex is also formed in the presence of the chromium complex of adenosine 5′‐[β,γ‐methylene]triphosphate. Because the β, γ‐bidentate of chromium ATP is recognized better than the α, β, γ‐tridentate, it is concluded that the triphosphate site recognizes MgATP with a straight polyphosphate chain and that the Mg<jats:sup>2+</jats:sup> resides between the β‐ and the γ‐phosphorus. The enhancement of inactivation by Mg<jats:sup>2+</jats:sup> and Na<jats:sup>+</jats:sup> may be caused by conformational changes at the triphosphate site.</jats:p></jats:list-item>
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