Book with abstracts from the COST Action 0905 meeting in ... - UMB

FROM ENZYMATIC BROWNING IN FRUITS AND VEGETABLES
TO MODIFICATION OF PROTEINS BY REACTION WITH
COPPER, ASCORBATE AND OXYGEN
Avi Golan-Goldhirsh
Ben-Gurion University of the Negev, Blaustein Institutes for Desert Research
French Associates Institute for Agriculture and Biotechnology of Drylands
Albert Katz Department of Dryland Biotechnologies
Midreshet Ben-Gurion 84990, Israel
Keywords: ascorbic acid, copper, enzymatic browning, histidine, polyphenol oxidase
Enzymatic browning is common in many fruits and vegetables. The problem is acute
especially in food processing. If not taken care off, it may lead to undesirable taste and
color and loss of nutritive value. The reaction is catalyzed by polyphenol oxidase, a
metallo-enzyme, containing copper as a prosthetic group. Prevention of enzymatic
browning by ascorbate is generally thought to involve reduction of the product, a
benzoquinone, to O-dihydroxyphenol, the substrate of the enzyme reaction, with the
oxidation of ascorbate to dehydroascorbate. However, incubation of polyphenol oxidase
with ascorbate alone, without the phenolic substrate, still led to the loss of enzymatic
activity, suggesting a non-enzyme catalyzed reaction. It was hypothesized that free copper
in solution in the presence of ascorbate catalyzed the reaction. Copper is an essential trace
element in most fruits and vegetables. It is necessary for a complete diet. Ascorbate is a
vitamin found in most of our plant derived foods. Ascorbate in the presence of added Cu 2+
was effective in inactivation of the enzyme significantly more than either ascorbate,
dehydroascorbate or copper, each alone. O 2 was required for the reaction. There were
changes in the amino acid composition of polyphenol oxidase following treatment with
ascorbate-Cu 2+ . The most distinctive change was a decrease in histidine from 4 to 1
mol/mol enzyme subunit. A similar modification, relatively specific to histidine, was
obtained by treatment of other proteins by ascorbate-Cu 2+ .
The site-directed specificity toward histidine residues of proteins can best be explained by
postulating that the Cu 2+ involved in the reaction is bound to the imidazole group of
histidine prior to reaction with ascorbate ion and O 2 . There is indirect evidence that the
reactive system leading to degradation of histidine residues in proteins is a quaternary
complex between the imidazole group, Cu 2+ , ascorbate and O 2 reacting to form reactive
species in proximity to the imidazole group leading to its degradation. The involvement of
heavy metals, especially copper, in modification of proteins in food, is it beneficial or
damaging?