title: Chromium(III)ATP inactivating (Na+ + K+)-ATPase supports Na+-Na+ and Rb+-Rb+ exchanges in everted red blood cells but not Na+,K+ transport.
abstract: The chromium(III) complex of ATP, an MgATP complex analogue, inactivates (Na+ + K+)-ATPase by forming a stable chromo-phosphointermediate. The rate constant k2 of inactivation at 37 degrees C of the beta, gamma-bidentate of CrATP is enhanced by Na+ (K0.5 = 1.08 mM), imidazole (K0.5 = 15 mM) and Mg2+ (K0.5 = 0.7 mM). These cations did not affect the dissociation constant of the enzyme-chromium-ATP complex. The inactive chromophosphoenzyme is reactivated slowly by high concentrations of Na+ at 37 degrees 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+ ions hindered the reactivation with a Ki of 70 microM. Formation of the chromophosphoenzyme led to a reduction of the Rb+ binding sites and of the capacity to occlude Rb+. The beta, gamma-bidentate of chromium(III)ATP (Kd = 8 microM) had a higher than the alpha, beta, gamma-tridentate of chromium(III)ATP (Kd = 44 microM) or the cobalt tetramine complex of ATP (Kd = 500 microM). The beta, gamma-bidentate of the chromium(III) complex of adenosine 5'-[beta, gamma-methylene]triphosphate also inactivated (Na+ + K+)ATPase. Although CrATP could not support Na+, K+ exchange in everted vesicles prepared from human red blood cells, it supported the Na+-Na+ and Rb+-Rb+ exchange. It is concluded that CrATP opens up Na+ and K+ 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'-[beta, gamma-methylene]triphosphate. Because the beta, gamma-bidentate of chromium ATP is recognized better than the alpha, beta, gamma-tridentate, it is concluded that the triphosphate site recognizes MgATP with a straight polyphosphate chain and that the Mg2+ resides between the beta- and the gamma-phosphorus. The enhancement of inactivation by Mg2+ and Na+ may be caused by conformational changes at the triphosphate site.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D002412:Cations; D004789:Enzyme Activation; D004793:Enzyme Reactivators; D006801:Humans; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism; D008274:Magnesium / Q000494:pharmacology; D011188:Potassium / Q000097:blood* / Q000494:pharmacology; D011868:Radioisotopes; D012413:Rubidium / Q000097:blood*; D012756:Sheep; D012964:Sodium / Q000097:blood* / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors* / Q000097:blood; D013552:Swine
chemical_list: D002412:Cations; D004793:Enzyme Reactivators; D007473:Ion Channels; D011868:Radioisotopes; D000255:Adenosine Triphosphate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D008274:Magnesium; D012413:Rubidium; D011188:Potassium