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Biochemical Journal
Andrawis, A., Department of Food Technology, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Kahn, V., Department of Food Technology, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Methimazole (1-methyl-2-mercaptoimidazole) inhibits both the mono- and the o-dihydroxy-phenolase activities of mushroom tyrosinase when assayed spectrophotometrically. With DL-3,4-dihydroxyphenylalanine as substrate, the inhibition was found to be a mixed-type one with K(i) 4.6 x 10-6 M. We found that methimazole can interact with the oxidation products of o-dihydroxyphenols, probably with o-quinones, to form a conjugate. The conjugate formed between methimazole and o-benzoquinone was separated by chromatography on Sephadex G-10 and was characterized by an absorption maximum at 248-260 nm. Our data suggest that methimazole inhibits mushroom tyrosinase activity in two ways: (1) by conjugating with o-quinones, thereby causing an apparent inhibition in pigmented product formation as judged by the spectrophotometric assay; and (ii) by chelating copper at the active site of the enzyme, as judged by assaying the release of 3HHO from L-[3,5-3H]tyrosine.
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Effect of methimazole on the activity of mushroom tyrosinase
235
Andrawis, A., Department of Food Technology, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Kahn, V., Department of Food Technology, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Effect of methimazole on the activity of mushroom tyrosinase
Methimazole (1-methyl-2-mercaptoimidazole) inhibits both the mono- and the o-dihydroxy-phenolase activities of mushroom tyrosinase when assayed spectrophotometrically. With DL-3,4-dihydroxyphenylalanine as substrate, the inhibition was found to be a mixed-type one with K(i) 4.6 x 10-6 M. We found that methimazole can interact with the oxidation products of o-dihydroxyphenols, probably with o-quinones, to form a conjugate. The conjugate formed between methimazole and o-benzoquinone was separated by chromatography on Sephadex G-10 and was characterized by an absorption maximum at 248-260 nm. Our data suggest that methimazole inhibits mushroom tyrosinase activity in two ways: (1) by conjugating with o-quinones, thereby causing an apparent inhibition in pigmented product formation as judged by the spectrophotometric assay; and (ii) by chelating copper at the active site of the enzyme, as judged by assaying the release of 3HHO from L-[3,5-3H]tyrosine.
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