Abstract: On the basis of the electrocatalytic response of tyrosinase to catechol, an enzyme electrode, which was prepared by immobilizing tyrosinase in silica sol-gel, was employed to investigate the inhibitory effect of sinapic acid and its derivatives, the components in the chinese traditional medicine, Semen sinapis. This electrode exhibited a fast and steady state response to catechol, with a linear range from 1.0×10^-7 to 6.0×10^-5 mol/L and a sensitivity of 60.93 nA·L/μmol. The kinetic process of the chronoamperometric response to catechol, performed in the absence and in the presence of sinapine, sinapic acid and sinapic-acid methyl ester, was analyzed. Sinapic acid was known to show the highest inhibition ability, followed by sinapic-acid methyl ester and sinapine. Further studies reaveal that the inhibition mechanism of sinapine corresponds to the noncompetitive one, while those of sinapic acid and sinapic-acid methylester can be modeled as the mixed one. Using the Lineweaver-Burk linearization and a secondary one, the inhibition parameters (inhibition constant and coefficient of 50% inhibition) were calculated.

Key words: Tyrosinase, Chemically modified electrode, Sinapic acid, Sinapic-acid methyl ester, Sinapine