An amperometric method was applied for real-time monitoring of intracellular redox enzyme activity. Baker's yeast (Saccharomyces cerevisiae) cells were immobilized on platinum microband electrodes and mediated anodic currents were measured. The currents were observed in the absence and in the presence of glucose as a source of reducing equivalents, NADH and NADPH. 2-Methyl-1,4-naphthoquinone (menadione, vitamin K-3) and water soluble 2-methyl-1,4-naphthoquinone sodium bisulfite (menadione sodium bisulfite MSB) were compared as artificial electron acceptors for their ability to transduce internal cellular redox activity into electrode current. It was found that hydrophobic menadione was superior to its water-soluble bisulfite derivative for probing whole intact cells. (C) 2003 Elsevier B.V. All rights reserved.