Creatine kinase: reassessment of optimal concentrations for adenosine-5'-diphosphate and magnesium

Whereas univariate studies led to an European agreement for the choice of optimal reagent concentrations of 2 mmol/L for ADP and 10 mmol/L for Mg2+ for determining creatine kinase (EC 2.7.3.2) activity in serum, whatever its isoenzyme pattern, the results of our bivariate study led us to recommend higher optimal concentrations: 4.1 to 4.7 mmol/L for ADP and 22 mmol/L for Mg2+. The zone of maximal activity was in fact a broad plateau such that more than 99% of maximal enzyme activity was attained at ADP concentrations between 3 and 5 mmol/L and Mg2+ concentrations between 17 and 26 mmol/L. Under these new conditions the maximum activity measured was modestly increased (about 10%) over the previously recommended method but the assay could be expected to be more resistant to the variations of ADP and Mg2+ concentrations. It may become necessary to modify the European recommended method.