Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.10.3.1 (tyrosinase)
 9,065 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A kinetic study of the inhibition of mushroom tyrosinase by tropolone has been made. Three tyrosinase isoforms were used: two commercial tyrosinases from Fluka and Sigma (isoelectric points of 4. 3 and 4.1, respectively) and one purified isoform from mushroom strain U1 (isoelectric point of 4.5). Tropolone is a slow-binding inhibitor of these mushroom tyrosinase isoforms. Increasing tropolone concentrations provoked a progressive decrease in both the initial velocity and the final (inhibited) steady-state rate in the progress curves of product accumulation. A rapid formation of an enzyme-inhibitor complex, which further undergoes a slow reversible reaction, could take place since the inhibition of the different isoforms was partially reversed by the addition of CuSO(4). The kinetic parameters that described the inhibition by tropolone were evaluated by nonlinear regression fits. Incubation experiments of the different isoforms with tropolone demonstrated that this inhibitor only could bind to the "oxy" form of tyrosinase which justifies a mechanism previously proposed to explain the inhibition of tyrosinase by slow-binding inhibitors.
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PMID:Slow-binding inhibition of mushroom (Agaricus bisporus) tyrosinase isoforms by tropolone. 1055 38

The polyphenol oxidase from field bean (Dolichos lablab) seeds has been purified to apparent homogeneity by a combination of ammonium sulfate precipitation, DEAE-Sephacel chromatography, phenyl agarose chromatography, and Sephadex G-200 gel filtration. The purified enzyme has a molecular weight of 120 +/- 3 kDa and is a tetramer of 30 +/- 1.5 kDa. Native polyacrylamide gel electrophoresis of the purified enzyme revealed the presence of a single isoform with an observed pH optimum of 4.0. 4-Methyl catechol is the best substrate, followed by catechol, and L-3,4-dihydroxyphenylalanine, all of which exhibited a phenomenon of inhibition by excess substrate. No activity was detected toward chlorogenic acid, catechin, caffeic acid, gallic acid, and monophenols. Tropolone, both a substrate analogue and metal chelator, proved to be the most effective competitive inhibitor with an apparent K(i) of 5.8 x 10(-)(7) M. Ascorbic acid, metabisulfite, and cysteine were also competitive inhibitors.
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PMID:Purification and characterization of a polyphenol oxidase from the seeds of field bean (Dolichos lablab). 1099 79

This study evaluated the effects of inhibitors on polyphenol oxidase (PPO) activity, the effect of the PPO inhibitor tropolone on noodle darkening, and the correlation of PPO activity with darkening of alkaline noodles. The PPO inhibitors tropolone and salicylhydroxamic acid (each at 1 microM) reduced kernel PPO activity by approximately 50% in three hexaploid wheat cultivars but did not inhibit PPO activity in the two very low PPO cultivars, durum Langdon, and the synthetic hexaploid-derived ID580. Tropolone (100 microg/g flour) inhibited alkaline noodle darkening (deltaL*) by 13-25% in the low PPO wheat cultivar, ID377s, and by 39-54% in the high PPO wheat cultivar, Klasic. Alkaline noodle darkening among 502 wheat samples was correlated with kernel PPO activity (r = 0.64). Results substantiate the hypothesis that PPO plays a major role in darkening of alkaline noodles. However, results also indicate that substantial darkening would occur even at zero PPO activity, as measured in the kernel PPO assay. Therefore, darkening of alkaline noodles is probably due to the cultivar-specific level of PPO activity and the presence of at least one additional darkening mechanism. Further investigation is required to identify the phenolic discoloration agent(s) and to determine the potential roles of non-PPO discoloration mechanisms, both enzymatic and nonenzymatic, in wheat products.
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PMID:Delineating the role of polyphenol oxidase in the darkening of alkaline wheat noodles. 1653 22