An amperometric enzyme electrode for the detection of Ca2+ using NADP-dependent isocitrate dehydrogenase coupled to an N-methylphenazinium
tetracyanoquinodimethanide (NMPTCNQ) electrode is described. The elect
rode responds to Ca2+ in a linear manner and is unaffected by the pres
ence of Na+ (165 mM) and K+ (5 mM).