Kinetic Evaluation of Aminoethylisothiourea on Mushroom Tyrosinase Activity |
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Authors: | Shu-Bai Li Hua-Li Nie Hai-Tao Zhang Yong Xue Li-Min Zhu |
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Institution: | (1) College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, 201620, China; |
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Abstract: | This study demonstrates that aminoethylisothiourea (AET), a potent inhibitor of inducible nitric oxide synthase, is an irreversible
competitive inhibitor of mushroom tyrosinase by chelation to the active site of tyrosinase when l-3,4-dihydroxyphenylalanine was assayed spectrophotometrically. The spectrophotometric recordings of the inhibition of tyrosinase
by AET were characterized by the presence of a lag period prior to the attainment of an inhibited steady-state rate. The lag
period corresponded to the time in which AET was reacting with the enzymatically generated o-quinone. Increasing AET concentrations provoked longer lag periods as well as a concomitant decrease in the tyrosinase activity.
Both lag period and steady-state rate were dependent on AET, substrate, and tyrosinase concentrations. The inhibition of diphenolase
activity of tyrosinase by AET showed positive kinetic cooperativity which arose from the protection of both substrate and
o-quinone against inhibition by AET. The UV-visible spectrum of a mixture of tyrosinase and AET exhibited a characteristic
shoulder peak ascribed to the chelation of AET to the active site of tyrosinase. Moreover, the presence of copper ions only
partially prevented but not reverted mushroom tyrosinase inhibition when CuSO4 was added to the assay medium on tyrosinase activity. |
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