Detection of uronic oxidase activity in ripening peaches

Uronic acid oxidase activity was found in an extract from harvested peaches that was incubated with citrus pectin at pH 8.5. The product of this reaction was identified by GC-MS analysis to be galactaric acid. The reaction was linear at 37 degrees C for up to 20 h, and the pH optimum was 8.5. The activity found in firm peaches one day after harvest did not change as the peaches softened over 5 days to eating softness. The incubation conditions were those suitable for monitoring the activity of pectate lyase, but instead of finding an increase in galacturonosyl residue reducing groups due to generation of pectin-derived oligosaccharides, uronic acid oxidase catalyzed the oxidation of the aldehyde reducing functions to carboxyl groups.