EFFECTS OF MG2+ ON ACTIVATION OF THE (NA+ + K+)-DEPENDENT ATPASE BY NA+1

The effects of MgCl2 on Na activation of three different enzymatic reactions catalyzed by a rat brain (Na + K)-dependent ATPase (adenosine 5′-triphosphatase) were studied. For the Na+-dependent ATPase reaction measured with 6 μm ATP, the K0.5 for Na increased from 0.4 to 1.7 mm as the MgCl2 concentration was raised from 50 to 2000 μm; the half-maximal effect occurred at a free Mg2+ concentration near 0.8 mm. By contrast, with 3 mm ATP and 3 mm MgCl2 the K0.5 for Na was again 0.4 mm, but further addition of 2 mm MgCl2 then had little effect on the K0.5 for Na. For the Na-dependent phosphorylation of the enzyme, measured with 6 μm ATP, the K0.5 for Na increased similarly, from 0.2 to 0.8 mM, as the MgCl2 concentration was raised from 50 to 2000 μm, but for the (Na + K)-dependent ATPase reaction the K0.5 for Na was 13 mm and increased by only one-third as the MgCl2 concentration was raised. The K0.5 for K was also little affected by changes in MgCl2 concentration. Finally, with 3 mm ATP and 3 mm MgCl2 the K0.5 for Na in the (Na + K)-dependent ATPase reaction decreased to 5 mm. These observations are considered in terms of an enzyme having high-affinity and low-affinity substrate sites, with occupancy of the low-affinity sites modifying Na activation differently, depending both on the specific reaction catalyzed and on whether occupancy is by free Mg2+ or by Mg-ATP. © 1979.