LIPOXYGENASE IS AN ABUNDANT PROTEIN IN CUCUMBER EXUDATES

The presence of lipoxygenase (LOX) has been reported in many plant org ans. High LOX activity (1-2 mukatal/mg protein) was detected in exudat es from cut cucumber (Cucumis sativus L.) stems and petioles. Exudate LOX had a pH optimum of 5.0, an estimated molecular weight of 95 kDa a nd cross-reacted on sodium-dodecyl-sulfate gels with anti-LOX antibodi es raised against soybean leaf LOX isoenzymes. Lipoxygenase activity w as detected on native gels stained with o-dianisidine using linoleic a cid as a substrate. Enzyme activity was similar with linoleic and lino lenic acid and 2 times greater with arachidonic acid as substrate. At pH 6.8, LOX metabolized linoleic acid into 13- and 9-hydroperoxides at a ratio of 1:2. Linolenic acid was preferentially oxidized at carbon 13. Lipoxygenase activity was inhibited by n-propyl gallate (IC50 300 nM) and nordihydroguaiaretic acid (IC50 25 nM), but not by nonsteroida l anti-inflammatory drugs. LOX activity was enhanced 4.5-fold by 300 m M Ca2+. Spermine at 1 mM, and putrescine and spermidine at 2 mM comple tely inhibited LOX activity, but at low concentrations spermine (100 m M) and spermidine (100-500 mM) significantly stimulated LOX activity: 8- and 4.5-fold, respectively. Tissue printing of stem, petiole and hy pocotyl sections with subsequent incubation with the antiserum raised against soybean leaf LOX revealed the presence of LOX in the internal and external phloem and in the sieve tubes.