ENZYME-KINETICS AND THE ACTIVATION-ENERGY OF MG-ATPASE IN CARDIAC SARCOLEMMA - ADP AS AN ALTERNATIVE SUBSTRATE

Increasing concentrations of Mg within a range between 0.1-5.0 mmol/l step-by-step activated the Mg-dependent ATPase and ADPase in rat heart sarcolemma. Both Mg-dependent. activities were influenced by NaN3 in a similar way. Also, activation of both enzymes by their substrates, A DP and ATP, were affected by NaN3 in a similar mode. It appears that b oth enzyme activities are secured by the same system which is capable of ADP hydrolysis during ATP insufficiency. In the absence of NaN3 the enzyme revealed higher affinity to ATP than to ADP. The activation en ergy was lower for ATP hydrolysis. The above findings indicate that at non limiting concentrations of Mg2+ the enzyme is favoring ATP.