Tyrosinase inhibitory activity of cucumber compounds: enzymes responsible for browning in cucumber

The inhibition of mushroom tyrosinase by cucumber extracts was evaluated. The inhibitory effect was measured by both polarographic and spectrophotometric methods. The commercial aldehyde, trans,cis-2,6-nonadienal, described as a major volatile compound of cucumber, was characterized as a noncompetitive inhibitor against 4-tert-butylcatechol oxidation by mushroom tyrosinase. The K(I) obtained 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 M(r) of 53 kDa. These results support the assumption that the enzyme PPO is present in the cucumber skin, but its activity is inhibited. Peroxidase (PO) was also found in cucumber skin extracts. This enzyme was detected in the soluble fraction but not in the membrane fraction. The kinetic characterization of PO was carried out. Native isoelectric focusing revealed several acidic PO isoenzymes with a pI in the range between 5 and 6, a basic isoenzyme, and one principal neutral isoenzyme of pI = 7.2.