Phenazine compounds have been used as simple organic catalysts for the rapid oxidation of NADH. Although positively charged phenazine compounds are highly water-soluble, their uncharged, reduced forms are not. We report the use of a zwitterionic phenazine compound, 3-(1-methoxyphenazin-5-ium-5-yl)propane-1sulfonate (mPPS), as an organic catalyst and electron mediator for the electrochemical oxidation of NADH and detection of glucose and lactate dehydrogenase (LDH). mPPS is highly stable in phosphate buffered saline (pH 7.4). Furthermore, the negatively charged, reduced form of mPPS is highly water-soluble. The detection limits of glucose and LDH in artificial serum were approximately 0.02 mM and 2 ng/mL, respectively.