ORGAN SPECIFICITY OF RAT SODIUM-ACTIVATED AND POTASSIUM-ACTIVATED ADENOSINE-TRIPHOSPHATASE

We compared several Na, K-ATPase preparations from various organs of the rat. The brain Na, K-ATPase differed from the enzymes of other organs in its pH dependence and responses to ouabain and N-ethylmaleimide in spite of similarities in the kinetic parameters of activation by Na+, K+, Mg2+, and ATP. The optimum pH of the brain Nal-enzyme was at 7.4 to 7.5 at 37°C. The Lubrol extract of this brain enzyme preparation showed a lower optimum pH of 6.6. When the Lubrol extract of the brain was fractionated with (NH4)2SO4, the activity of the precipitate in the neutral pH region was restored. On the other hand, the optimum pH of the kidney Nal-enzyme was slightly affected by Lubrol and ammonium sulfate treatments (pH 7.5→7.3). The brain enzyme (K1/2=0.9 μM) showed about 100-fold higher sensitivity to ouabain than the enzymes from other organs (K1/2=100μM) in the presence of 120 mM Na+ and 10 mM K+. In a Hill plot of the ouabain inhibition, the former failed to give a linear relationship, while the latter gave a straight line with a Hill coefficient of 1.0. The effect of K+ on the brain enzyme-ouabain interaction led us to consider that the brain enzyme might have two components as regards ouabain affinity, high and low affinity components. The time course of N-ethylmaeimide inhibition of the brain enzyme was rapid and biphasic, while the kidney enzyme showed only a slow phase following pseudo-first order kinetics. ATP protected the kidney enzyme activity completely against N-ethylmaleimide inhibition, but the protection of the brain enzyme activity by ATP was only partial. We divided rat Na,K-ATPases into two groups, the brain type, which is restricted to the central nervous system, and the kidney type, which is found in most organs. © 1979, by the Japanese Biochemical Society.