Expression of Lipoxygenase and Hydroperoxide Lyase Activities in Tomato Fruits

The distribution (or locarization) o f lipoxygenase (LOX) and hydroperoxide lyase (HPO lyase) activities in ripening and ripe tomato fruits was investigated. The highest LOX activity existed between skin and outer flesh o f tomato fruits. HPO lyase showed no tissue specificity. LOX specifically formed linoleic acid 9-£',Z-hydroperoxide (9-.E,Z-HPO) from linoleic acid (LA), whereas HPO lyase specifically cleaved 13-Z.^-HPO. Although a low level (0.36 ± 0.069 nmol/g fr. wt.) o f hexanal was detected in the intact tomato fruit, HPOs were not detected. When a tomato fruit was injured by cutting it into 8 fragments and incubated at 25 °C, hexanal increased to 1.642 nmol/g fr.wt. by 30 min. By homogenizing at pH 6.3, hexanal increased to 21.1 nmol/g fr.wt. during a 30 min incubation. UV irradiation o f tomato fruits also increased the formation o f hexanal. From these results, LOX and HPO lyase are considered to exist as latent forms and to begin the expression o f the activity upon injury.