Effect of cations on the human creatine kinase isoenzymes.

We report our findings on the effect of Ca2+, K+, Na+, Fe2+, Mn2+, Zn2+, and Cu2+ on the activity of creatine kinase isoenzymes derived from human tissues. The only inhibitory cation was Ca2+, and Dixon plots of the data indicate the inhibition to be competitive for the Mg2+ in the reaction assay mixture. This effect of Ca2+ on all three isoenzymes is completely reversed by including Mg2+ (10 mmol/L) and ethylenediaminetetraacetate (2 mmol/L) in the assay mixture. Our findings suggest that it is unnecessary to add chelators to the patient's sample.