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antibrowning activities and to seek new tyrosinase inhibitors for applications as antibrowning and
depigmenting agents in the food and cosmetic industries. The isolated flavonoids were identified as
liquiritin, licuraside, isoliquiritin, liquiritigenin (from Glycyrrhiza uralensis Fisch.), and
licochalcone A (from Glycyrrhiza inflate Bat.) by UV, MS, (1)H NMR, and (13)C NMR analyses.
The inhibitory potencies and capacities of these flavonoids toward monophenolase activity of
mushroom tyrosinase were investigated. The IC(50) values of licuraside, isoliquiritin, and
licochalcone A for monophenolase activity were 0.072, 0.038, and 0.0258 mM, respectively. A
study of the mechanisms of monophenolase inhibition by these flavonoids indicated that they are
all competitive inhibitors. Different from the above flavonoids, no inhibitory activity was observed
for liquiritin, whereas liquiritigenin activated the monophenolase activity as a cofactor. The
inhibitory effect of licuraside, isoliquiritin, and licochalcone A on diphenolase activity with l-
DOPA as the substrate was much lower than those with l-tyrosine. Results suggest that licuraside,
isoliquiritin, and licochalcone A have the high potential to be further developed into effective
antibrowning and depigmenting agents.
- Gandia-Herrero F, Escribano J, Garcia-Carmona F.
Characterization of the activity of tyrosinase on betaxanthins derived from (R)-amino acids.
J Agric Food Chem. 53(23):9207-12, 2005.
The activity of tyrosinase (EC 1.14.18.1) on selected (R)-betaxanthins is characterized in depth,
demonstrating that the activity of the enzyme is not restricted to betaxanthins derived from (S)amino
acids. Conversion of (R)-tyrosine-betaxanthin [(R)-portulacaxanthin II] to the pigment (R)dopaxanthin
and its further oxidation to a series of products is described. Compound identity was
studied by high performance liquid chromatography and electrospray ionization-mass
spectrometry. The reaction rate on the (R)-isomer of dopaxanthin is 1.9-fold lower than that
obtained for the (S)-isomer in previous studies. Tyrosinase showed stereospecificity in its affinity
toward betaxanthins. The characterization of the activity of tyrosinase on (R)-betaxanthins
reinforces the role of the enzyme in the biosynthetic scheme of betalains.
- Gandia-Herrero F, Jimenez-Atienzar M, Cabanes J, Garcia-Carmona F, Escribano J.
Differential activation of a latent polyphenol oxidase mediated by sodium dodecyl sulfate. J
Agric Food Chem. 53(17):6825-30, 2005.
A kinetic study of the activity of soluble and membrane-bound latent polyphenol oxidase (PPO)
extracted from beet root (Beta vulgaris) was carried out. For the first time, two types of behavior
(hyperbolic and sigmoid) are reported in the same enzyme for PPO activation by the surfactant
sodium dodecyl sulfate (SDS), depending on substrate nature. A kinetic model based on
cooperative systems is developed to describe the activation effect of SDS, enabling the
determination of the number of surfactant molecules binding to the enzyme in the activation
process. The results indicate that the active site of the enzyme is not affected by SDS and that a
stepwise conformational change favors the access of hydrophobic substrates compared to
hydrophilic ones. Differential activation of PPO mediated by SDS may be of relevance in the
control of PPO activity since the enzyme is able to express activity toward a specific substrate
while remaining latent to others.
- Garcia-Molina F, Penalver MJ, Rodriguez-Lopez JN, Garcia-Canovas F, Tudela J.
Enzymatic method with polyphenol oxidase for the determination of cysteine and Nacetylcysteine.
J Agric Food Chem. 53(16):6183-9, 2005.
Thiols, such as cysteine and N-acetylcysteine, are included in many pharmaceutical products for
their mucolytic properties. The method described here uses mushroom polyphenol oxidase (PPO)
to determine two thiols and consists of measuring the lag period in the formation of the product
generated as PPO acts on o-diphenol in the presence of a thiol. In the experimental conditions, oquinone
is formed enzymatically and then reacts stoichiometrically with the thiol, originating the
corresponding thiol-diphenol adduct, which does not absorb visible light. Once the thiol has been
used up, the o-quinone can be observed in the medium. It must be borne in mind that the inhibition
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