Abstract
The effects of Mg 2+ on the hepatic microsomal Ca 2+-sequestering system was tested. Ca 2+-ATPase activity and Ca 2+ uptake were both dependent on the concentration of free Mg 2+, reaching maximum levels at 2 mM. The effects of Mg-ATP were also influenced by the concentration of free Mg 2+, being maximally effective at a ratio of 1:1. The results suggest that Mg 2+ influences Ca 2+ sequestration at various steps, namely in addition to forming the substrate of the Ca 2+-ATPase reaction, Mg-ATP, Mg 2+ stimulates the reaction at an additional step, as indicated by its stimulatory effect on the Ca 2+-ATPase reaction and on Ca 2+ uptake, even at optimal Mg-ATP levels. The stimulatory effect of Mg 2+ was evident at various pH levels tested, and it was nucleotide specific. The stimulatory effect of Mg 2+ might be exerted at the dephosphorylation step of the enzymatic reaction or at an other, yet undetined, site. The results demonstrate a plural effect of Mg 2+ on the hepatic microsomal sequestration system. This indicates that, depending on its magnitude, changes in Mg 2+ distribution might influence cytosolic Ca 2+ levels.
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