Mercury contamination during pH measurement and its effect on creatine kinase activity

Inhibition of activity of the enzyme creatine kinase occurs when small volumes of assay mixtures have their pH measured using a combination pH electrode prior to addition of the enzyme. This inhibition can be attributed to diffusion of ionic mercury from the calomel reference electrode through the saturated potassium chloride salt bridge and the ground glass-liquid junction to the test solution. The concentration of mercury accumulating in a solution by this process can be sufficient to affect the activity of an enzyme, and EDTA cannot be used successfully to scavenge mercury ion and avoid the inhibition.