L-MALATE-SENSING ELECTRODE BASED ON MALATE-DEHYDROGENASE AND NADH OXIDASE

An L-malate-sensing electrode was constructed from an oxygen electrode and a layer containing immobilized malate dehydrogenase (MDH) and NADH oxidase. MDH catalyses the dehydrogenation of L-malate by NAD+ and NADH oxidase catalyses the regeneration of NAD+ with the use of oxygen. The regeneration enables the L-malate oxidation to proceed efficiently even in a medium of neutral pH. At pH 8.0, L-malate in the concentration range 5-mu-M-1.5 mM can be measured. The relative standard deviation for the measurement is 1.2% (L-malate concentration, 0.2 mM; n = 10). The present L-malate-sensing electrode is stable for 8 weeks. A two-electrode sensor system consisting of the L-malate-sensing electrode and an L-lactate-sensing electrode based on lactate oxidase was prepared and applied to the simultaneous determination of the two components in wines.