REACTIVITIES OF AMPEROMETRIC ORGANIC-PHASE PEROXIDASE-MODIFIED ELECTRODES IN THE PRESENCE AND ABSENCE OF THIOUREA AND ETHYLENETHIOUREA AS INHIBITORS

The reactivities of amperometric horseradish peroxidase (HRP) electrod es in acetonitrile, methanol and acetone were studied by cyclic voltam metry, 1,1-Dimethylferrocene (DMFc) was used as a soluble electron tra nsfer mediator. The responses of the HRP electrode to butanone peroxid e (BTP) as a substrate, and thiourea (THU) and ethylenethiourea (ETU) as inhibitors, were measured. Michaelis-Menten analyses of the calibra tion curves obtained for BTP, THU and ETU in the solvents were carried out to determine the apparent Michaelis-Menten constant (K-m'), the l imiting current (I-max) and the apparent inhibition constant (K-i'). T he catalytic efficiency of the biosensor was determined as the ratio o f the peak current controlled by the enzyme catalytic reduction of BTP to that controlled by the diffusion of DMFc to the electrode surface (I-k/I-d). The values of the Michaelis-Menten kinetic parameters and I -k/I-d were used to assess the performance of the peroxide sensor in t he presence and absence of inhibitors. Our results indicate a decrease in sensor performance with increasing solvent hydrophilicity (methano l > acetonitrile > acetone). This has been attributed to the ability o f the hydrophilic solvents to desorb essential water of hydration from the enzyme active site. This removal of water decreases the rate of e nzymatic reaction and subsequently the catalytic performance of the se nsor.