The cathodic current for the reduction of benzoquinone at a glassy carbon electrode was increased in the presence of nucleoside oxidase and inosine. This was attributed to the regeneration of benzoquinone from hydroquinone by the laccaselike reaction of the enzyme, the rate of which depended on the concentration of inosine in the solution. Accordingly, an enzyme electrode for nucleosides was constructed by immobilizing nucleoside oxidase behind a dialysis membrane on a carbon paste electrode containing p-benzoquinone or hydroquinone. The current response of the electrode to nucleosides was studied, and the results were discussed on the basis of the laccaselike activity of the enzyme. The nucleoside oxidase-modified electrode was tested as an amperometric enzyme electrode for nucleosides and could be used at least for three weeks.k7W WORDS: Biosensors, nucleosides, nucleoside oxidase.
A"TR0DUCTIONConsiderable attention has been given to the coupling of electrode reaction with an enzymatic reaction, in which the substrate of the enzyme can be oxidized or reduced electroenzymatically, i.e., by bioelectrocatalysis [ 1-31. Molecules serving as redox mediators are usually used to achieve the electron transfer coupling, and enzyme electrodes relying on this principle of current response, mediated amperometric enzyme electrodes, have been constructed (4, 51. A variety of redox molecules including redox polymers have been studied for the mediated electron transfer coupling based on glucose oxidase and other oxidoreductases [ 1-31. Similar coupling without mediators has also been reported between ordinary electrodes (such as carbon and metal surfaces), and several kinds of oxidoreductases [ 6-91.This article describes a new type of electroenzymatic coupling different from the above two coupling schemes, which rely on the unique properties of the enzyme used: nucleoside oxidase [NOD] from Pseudomom maltophiria LB-86 (10-131. NOD has a molecular weight of 'To whom correspondence should be addressed. 8 1992 VCH Publishers, Inc. 1040-0397/92/$3.50 + .25 891 892 Ikeda et al.