Monophenolase activity of avocado polyphenol oxidase |
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Authors: | Varda Kahn Seymour H Pomerantz |
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Institution: | Department of Biological Chemistry, University of Maryland, School of Medicine, Baltimore, MA 21201, U.S.A. |
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Abstract: | Polyphenol oxidase of avocado mesocarp catalyses (a) the orthohydroxylation of monophenols like l-tyrosine, d-tyrosine, tyramine and p-cresol, and (b) the oxidation of the corresponding o-dihydroxyphenols to quinones. The rate of step b is much greater than that of step a. The hydroxylation of monophenols occurs after a lag period. DOPA or ascorbate effectively eliminate the lag but not dl-6-methyltetrahydropteridine or tetrahydrofolic acid. At 1.66 × 10?4 M, α,α-dipyridyl has no effect, while diethyldithiocarbamate at this concentration inhibits the hydroxylation reaction by 90%. The tyrosinase activity of avocado polyphenol oxidase is inactivated in the course of the reaction; this inactivation occurs faster and is more pronounced in the presence of exogenously added DOPA. This inactivation is partially prevented by a large excess of ascorbate. The Km values indicate that tyramine, dopamine, p-cresol and 4-methyl catechol are better substrates for avocado polyphenol oxidase than tyrosine or DOPA. |
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Keywords: | Lauraceae avocado fruit polyphenoloxidase monophenolase DOPA: 3 4-dihydroxyphenylalanine dopamine: 3 4-dihydroxyphenyl ethylamine DETC: diethyldithiocarbamate |
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