EFFECT OF TEMPERATURE ON MG2+-DEPENDENT AND NA+-K+ ATPASES OF A RAT BRAIN MICROSOMAL PREPARATION

1. 1. The effects of temperature on the Mg2+-dependent and Na+K+ ATPase of a microsomal preparatiion from rat brain was studied, and the two enzyme systems were found to posses markedly differing temperature sensitivities. 2. 2. An Arrhenius plot for the Mg2+-dependent ATPase showed a discontinuity at about 20°C, with activation energies at 16·8 Kcal mole-1 for the range 1-20°C, and 7·0 Kcal mole-1 for 20-37°C. 3. 3. The Arrhenius plot for the Na+K+ ATPase is curvilinear and no enzyme activity was recorded below 5°C. The activation energy varied between 11 Kcal mole-1 (27-37°C) and 80 Kcal mole-1 (6-10†C). It is concluded that the Mg2+-dependent and Na+K+ ATPase are separate enzyme systems. 4. 4. The Na+K+ ATPase/Mg2+ ATPade activity ratio varied from 0 (below 5°C) to 1·3 (40°C). 5. 5. The results are compared with those obtained in the presence of strophanthin G, and their significance with respect to poikilotherms and hiberting homoiotherms is discussed. © 1968.