Oxidation of NADH produced by a lactate dehydrogenase immobilised on poly(aniline)–poly(anion) composite films

The immobilisation of enzymes is important for applications in bioelectrochemistry such as biosensors or biofuel cells. In this paper we report the immobilisation of lactate dehydrogenase (LDH) on poly(aniline)–poly(acrylate) [PANi–PAA] and poly(aniline)–poly(vinylsulfonate) [PANi–PVS] composite films. Two genetically engineered forms of LDH (E.C.1.1.1.27) from Bacillus stearothermophilus, one with a poly(histidine) tag on the C-Terminus (LDH-CHis) the other with a poly(histidine) tag on the N-terminus (LDH-NHis), together with the wild type enzyme (WT-LDH) were studied. The LDH-CHis and LDH-NHis both have better affinity for the poly(aniline)–poly(anion) composite films than the WT-LDH. The immobilised LDH reduces the coenzyme NAD+ to NADH and oxidises the substrate, l-lactate, to pyruvate. The NADH produced is then oxidised at the poly(aniline)–poly(anion) composite films. The effects of buffer concentration, temperature, NAD+ concentration, enzyme immobilisation conditions, film thickness and electrode rotation rate on the catalytic current were all investigated.