ATP LEVELS MODIFY ACTIVATION OF NA PUMP BY EXTERNAL CATIONS IN SQUID AXONS

IT has been proposed that, during the cycle of ATP hydrolysis, the (Na+K)ATPase goes through a 'closed' or 'occluded' conformation1. This would be produced on the active site for K translocation following dephosphorylation, and, while it lasts, the dephosphorylating cation would remain attached to the enzyme, preventing rephosphorylation. The stability of this conformation varies with the dephosphorylating cations (the more stable complex follows the sequence T1+> Rb+> K +> Cs+> NH4+>Li +). In addition, the complexes are less stable with increase in the ATP concentration1. Flux experiments in human red cells2-4 and squid axons5,6 have produced some evidence in line with this. Unfortunately, in these cases one cannot be sure if the observed effects were solely the consequence of changes in ATP or were influenced by simultaneous modifications of the ADP and inorganic phosphate content of the cells. We have avoided such difficulties here by using squid dialysed axons, and have found an increased effectiveness of NH4+ over K and Rb, and of K over Rb, in activating the Na pump when internal ATP is reduced. This followed the same pattern described for their effects on the levels of phosphoenzyme. Our results suggest the existence of an ATP-sensitive occluded conformation of the Na pump formed after dephosphorylation.