Ascorbic acid oxidase

Ascorbic acid oxidase is a copper-containing enzyme that mediates the oxidation of free ascorbic acid first to dehydroascorbic acid and next to diketogulonic acid, oxalic acid, and other oxidation products (see Chapter 6, Section 6.6.2.1).

Ascorbic acid oxidase occurs in many fruits and vegetables such as cucumbers, pumpkins, lettuce, cress, peaches, bananas, tomatoes, potatoes, carrots, and green beans. Its activity varies with the type of fruit or vegetable. The enzyme is active between pH 4 and 7. Its optimum temperature is about 38°C. When plant cells are disrupted the compartmen-talization of substrate and enzyme is removed. Therefore, if vegetables and fruits are cut, the vitamin C content decreases gradually. In fresh juices, 50% of the vitamin C content is lost in less then one hour. Being an enzyme, ascorbic acid oxidase can be inhibited effectively by blanching of fruits and vegetables.

Ascorbic acid can also be protected against ascorbic acid oxidase by substances of plant origin. Flavonoids, such as the flavonoles quercetin and kempferol, present in vegetables and fruits, strongly inhibit the enzyme. (As far as risk evaluation is concerned, it should be noted that quercetin has also been reported to induce adverse effects: see Part 1, Chapter 2, Section 2.2.1.1).

OH O

Kempferol (R1 = R2 = H) Quercetin (R1 = OH; R2 = H)

OH O

Kempferol (R1 = R2 = H) Quercetin (R1 = OH; R2 = H)

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