PEROXIDASE BASED AMPEROMETRIC BIOSENSORS FOR THE DETERMINATION OF GAMMA-AMINOBUTYRIC-ACID

This work presents the realization of enzymatic bioelectrodes, suitabl e for the determination of gamma-aminobutyric acid (GABA). The biosens ors are based on the catalytic activity of the enzymes gamma-aminobuty ric glutamic transaminase (GABA-T), succinic semialdehyde dehydrogenas e (SSDH) and horseradish peroxidase (HPO). The first two enzymes are u sually indicated by the general term ''GABASE''. All the biosensors pr esented in this work are realized by immobilizing the enzyme HPO on th e tip of an amperometric oxygen selective electrode: the resulting NAD PH-sensitive biosensor is used in combination with GABASE to determine the concentration of GABA in aqueous samples. Since SSDH depends on t he NADP(+)/NADPH equilibrium, it follows that, in the presence of HPO, the NADPH formed is oxidized to NADP, and the decrease in the concent ration of dissolved oxygen is proportional to the concentration of NAD PH and, in turn, to that of GABA. The experiments were performed eithe r with GABASE free in solution or co-immobilized with HPO on the surfa ce of the oxygen electrode. In the latter case, the immobilization of the three enzymes has been performed either on a single membrane or on two separated membranes. In both cases there is an almost perfect lin earity between the electrode signal and GABA concentration in the rang e 5.0x10(-5)-1.2x10(-3) M, with a lower detection limit of 2.0x10(-5) M; but the single-membrane biosensor showed a better overall performan ce, especially in terms of repeatability of measurements and lifetime of operation.