Electrocatalytic reduction of lipoic acid and electroenzymatic reduction of NAD(P)(+) for integrated dehydrogenase biosensors

Biosensors for the oxidized substrates of NAD(P)(+)-specific dehydrogenases demand the reductive recycling of the coenzymes. So far, suitable catalyst s for the corresponding two-electron transfer are not available. In the pre sent paper, this transport has been realized by a combined electrocatalytic al and electroenzymatic process. Lipoic acid has been reduced on graphite e lectrodes functionalized with Fe(II)-phthalocyanine in 95%, yield at - 1200 mV in phosphate buffer pH 7.0. With the electrocatalytically reduced produ ct, dihydrolipoic acid, lipoamide dehydrogenase could reduce NAD(+) in 20% yield and thioredoxin reductase NADP(+) in 18.4% yield. So far, the combine d electrocatalytic/electroenzymatic system has not yet been realized, mainl y because at the potential needed for the lipoic acid reduction, a parallel one-electron reduction of NAD(P)(+) was observed, implying the dimerizatio n of the coenzyme. (C) 2001 Elsevier Science B.V. All rights reserved.