CA2+ REQUIREMENT FOR NA+ STIMULATED MG2+-ATPASE IN NUCLEI ISOLATED FROM EMBRYONIC HEART

Nuclei isolated from embryonic chick heart contain a Mg2+-activated ATPase, which hypothetically plays some role in energy requiring reactions such as nuclear membrane transport and/or nucleoprotein synthesis. In most preparations, the enzyme can be stimulated by the addition of Na+. In other preparations no effect or inhibition occurs. Studies with Ca2+-EGTA buffers indicate that this variability is dependent on the initial Ca2+ concentration of the preparation. Na+ stimulation requires the presence of about 0.01 to 0.10 mM free Ca2+. At concentrations above or below this range, Na+ causes inhibition of Mg2+-ATPase. Basic Mg2+-ATPase does not appear to require Ca2+ for maximal activity. Assuming the presence of a given level of Ca2+ and Na+ in the nuclear environment, the nuclear Mg2+-ATPase activity is probably depressed under physiological conditions. A mechanism which controls the level of free Ca2+ could then govern the rate of energy release from ATP. © 1968.