Abstract
The activity of rat brain pyrophosphatase at various concentrations of added Mg ++ and inorganic pyrophosphate has been measured. In confirmation and extension of previous results the data show a pronounced inhibition of the enzyme action by pyrophosphate and a weaker inhibition by excess magnesium pyrophosphate. Concentrations of free and combined Mg ++ and pyrophosphate in the assay conditions were calculated by use of an apparent dissociation constant for magnesium pyrophosphate as determined by spectrophotometric measurements of the competition of pyrophosphate and ethylenediamine tetraacetate for Mg ++. Evidence is presented that the activation of rat brain pyrophosphatase by Mg ++ can be ascribed to a requirement of the enzyme for magnesium pyrophosphate for a substrate. The requirement for a much higher Mg ++ and pyrophosphate concentration for maximum activity under some conditions can logically result from the removal of the potent inhibition by free pyrophosphate and lack of inhibition by free Mg ++.
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