Immobilisation of lactate dehydrogenase on poly(aniline)-poly(acrylate) and poly(aniline)-poly(vinyl sulphonate) films for use in a lactate biosensor

The immobilisation of enzymes on an electrode surface, in such a manner that they retain both substrate specificity and high levels of catalytic activity, is of great importance in bioelectrochemistry. This includes areas such as the development of enzyme-catalysed fuel cell electrodes, biosensors and other biotechnological applications. We have investigated the catalytic activity of hexahistidine tagged variants of lactate dehydrogenase (EC 1.1.1.27) from the thermophile Bacillus stearothermophilus both in solution and when immobilised on poly(aniline)-poly(acrylate) (PANi-PAA) or poly(aniline)-poly(vinyl sulphonate) (PANi-PVS) composite films. Both the C- and N-terminally tagged enzymes are readily immobilised on the modified electrode and catalyse the conversion of lactate and NAD⁺ to pyruvate and NADH. The NADH that is generated can be readily oxidised at the PANi-modified electrode surface.In solution, the activity of the C-tagged enzyme (LDH-CHis) was some 30% less that of the wild-type under comparable conditions, whereas the N-tagged enzyme was found to possess essentially the same activity as the wild-type. However, when the enzymes were immobilised on PANi-PAA and PANi-PVS the C-tagged enzyme films showed a higher NADH-dependent current than the wild-type LDH whilst the N-tagged enzyme had the highest of the three. In addition, the C-tagged enzyme film appeared more stable than the wild-type LDH-PANi film. A novel immobilisation chemistry of the enzyme is proposed to account for these observations.