Ascorbate oxidase is a copper-containing enzyme found in melons, seeds, cereals, and fruits and vegetables. It can oxidize ascorbic acid to produce water and dehydroascorbic acid. Compared with non-enzymatic oxidation, ascorbic acid oxidase acts to generate water, while the former generates hydrogen peroxide.

Figure 1. Ascorbate oxidase dimer.

Introductions

Ascorbate oxidase is a copper-containing enzyme that is located in the cytoplasm or combined with the cell wall and is coupled with other redox reactions to play the role of terminal oxidase. It can catalyze the oxidation of ascorbic acid and has anti-aging effects. Has an important role in the metabolism of substances. Under the catalysis of this enzyme, molecular oxygen can oxidize ascorbic acid to dehydroascorbic acid, which exists in the cytosol and cell wall, and the enzyme contains copper. Pyruvate, isocitrate, a. Ketoglutarate, malate, glucose, 6. Phosphoric acid, 6. Phosphoric acid can remove H protons under the action of dehydrogenase, transfer H protons to coenzyme, and then pass H protons to ascorbic acid via glutathione. Ascorbic acid is oxidized and deactivated by the action of ascorbate oxidase H, H combines with oxygen to form water.

Factors Affecting Ascorbate Oxidase Activity

1. Temperature

Studies have shown that ascorbate oxidase has two peaks of activity at temperatures below 45 ° C, and one peak at temperatures above 50 ° C. When the temperature is below 50 ° C, the enzyme activity lasts for a long time, reaching 40 minutes, and as the temperature rises, High, the activity peak will be advanced. When the temperature is above 55 ° C, the duration of enzyme activity is very short, only 20min at 60 ° C.

2. PH value

The activity of ascorbate oxidase is greatly affected by pH. Studies have shown that ascorbate oxidase exhibits higher activity in the pH range of 5-7, and the activity is the strongest at pH 5, which can reach 1.06 active units, As the pH value increases, the activity decreases and the larger the pH value, the greater the decrease. When the pH is 9, the activity is only 0.35 active units. It can be seen that the optimum pH value for the ascorbate oxidase effect is 5.

3. Light intensity and nitrogen form

Under different light and nitrogen treatments for tomato growth, it was found that the growth of plants supplying ammonium nitrogen was inhibited at higher light intensities, the chlorophyll content of fresh leaf unit weight was significantly reduced, and ascorbate oxidase was significantly higher than that of nitrate Nitrogen plants, but under low light conditions, there was no significant difference between nitrogen forms. In the process of tobacco growth, similar conclusions were obtained in different treatments of nitrate nitrogen and ammonium nitrogen, that is, as the amount of ammonium nitrogen increased, the ascorbic acid content increased, indicating that the ascorbate oxidase activity was low.

4. Low temperature, ozone and decompression

Low temperature can keep the enzyme activity low, reduce the respiration rate of fruit, and O3 inhibits the ascorbate oxidase activity. Tests show that the greater the O3 concentration between 0 ~ 500mg / m³, the stronger the inhibitory effect. O3 has the effect of maintaining the ascorbic acid content. The overall trend between 0 and 500 mg / m³ is that the higher the O3 concentration, the higher the ascorbic acid content. Decompression has significant effects on inhibiting ascorbate oxidase activity, maintaining fruit firmness, and ascorbic acid content. The reason may be that decompression makes this enzyme in a swelled state, which hinders binding to the substrate. Wet and cold storage of winter jujube were treated in two ways: cold and cold storage. Wet and cold storage could inhibit the vitamin C content and ascorbate oxidase activity of winter jujube, and improve the storage effect of winter jujube.

Reference

1. Boyer PD.; et al. The Enzymes. 8 (2nd ed.). New York: Academic Press. 1963, 297-311.