Abstract
| - The inhibition of mushroom tyrosinase by cucumber extracts was evaluated. The inhibitory effectwas measured by both polarographic and spectrophotometric methods. The commercial aldehyde,trans,cis-2,6-nonadienal, described as a major volatile compound of cucumber, was characterizedas a noncompetitive inhibitor against 4-tert-butylcatechol oxidation by mushroom tyrosinase. The KIobtained was 3.4 mM. Polyphenol oxidase (PPO) activity was not detected in cucumber skin extracts.However, the presence of PPO was revealed by Western blot; a single band was found with a Mr of53 kDa. These results support the assumption that the enzyme PPO is present in the cucumberskin, but its activity is inhibited. Peroxidase (PO) was also found in cucumber skin extracts. Thisenzyme was detected in the soluble fraction but not in the membrane fraction. The kineticcharacterization of PO was carried out. Native isoelectric focusing revealed several acidic POisoenzymes with a pI in the range between 5 and 6, a basic isoenzyme, and one principal neutralisoenzyme of pI = 7.2. Keywords: Tyrosinase inhibitory activity; polyphenol oxidase; peroxidase; noncompetitive inhibition;Cucumis sativus L.; trans,cis-2,6-nonadienal
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