MEDIATED BIOELECTROCATALYSIS BASED ON NAD-RELATED ENZYMES WITH REVERSIBLE CHARACTERISTICS

Diaphorase (DI) works as an effective catalyst for the electrochemical oxidation and reduction of NAD with the aid of several quinones or fl avins as electron transfer mediators. The redox kinetics between DI an d mediators have been expressed by a Butler-Volmer-type equation. NAD- dependent L-lactate dehydrogenase (LDH) catalyzing the redox reaction between L-lactate and pyruvate was coupled to the DI-catalyzed NAD red ox system to achieve better understanding of mediated two-enzyme-linke d bioelectrocatalysis with reversible characteristics. Under the condi tions where the concentration polarization of NAD due to the DI-cataly zed electrochemical reaction is suppressed by the LDH reaction, the NA D concentration dependence of the catalytic current was expressed by a n approximate equation involving the enzyme kinetics between DI and NA D. The suppression of the NAD concentration polarization is also usefu l to observe steady-state catalytic waves of an uphill reaction betwee n DI and the mediator. The oxidation reaction involving the uphill ele ctron transfer from L-lactate to NAD(+) is susceptible to a inhibition from pyruvate due to the reversible characteristics of LDH. The prese nt knowledge has led to the strategy to realize a two-way bioelectroca talysis for the reduction of pyruvate and the oxidation of L-lactate. New potentiometry for the detection of the solution potential governed by the electrochemically inactive pyruvate/L-lactate redox couple has also been demonstrated based on the reversible characteristics of the DI-DLH-linked bioelectrocatalytic system. (C) 1998 Elsevier Science S .A. All rights reserved.