The ultraviolet irradiation of mushroom tyrosinase exhibited gradual loss in catalytic activity as a function of radiation dose. Destruction of tryptophanyl residues in the enzyme was studied chemically (determined using pDAB reagent) as well as by fluorescence measurement. The rate constant for the loss in fluorescence intensity and the destruction of tryptophanyl residue was found to be <tex-math>$10.3\pm 0.6\times 10^{-2}\ {\rm min}^{-1}$</tex-math> and <tex-math>$4.54\pm 0.45\times 10^{-2}\ {\rm min}^{-1}$</tex-math>, respectively. It was observed that the Km for dihydroxy-phenylalanine (dopa), catechol, and tyrosine was affected to varying degrees on irradiation. The <tex-math>$D_{37}$</tex-math> and kinetic data of native and irradiated tyrosinase support the existence of two active centers on the enzyme molecule. It is proposed that the loss in catalytic activity of tyrosinase may be due to conformational changes.

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