Potential importance of metal-ligand interactions in enzyme assays demonstrated with the assay cocktail for phosphoenolpyruvate carboxylase.

Assay cocktail modification such as addition of effector can cause inadvertent changes in the concentration of other metalligand and free species. In some cases, e.g. in the assay for phosphoenolpyruvate carboxylase under a limiting-substrate condition, unintentional changes in substrate concentration are significant and confound an interpretation based solely on the total concentration of each component. A cautionary argument is developed on the basis of examples from the current literature.