Novel oxylipins formed from docosahexaenoic acid by potato lipoxygenase-10(S)-hydroxydocosahexaenoic acid and 10,20- dihydroxydocosahexaenoic acid

Potato tuber lipoxygenase (ptLOX) has been shown to catalyze the aerobic formation of at least four major oxygenated derivatives of DHA. Two of the products-7,17(S)- and 10,17(S)-dihydro(pero)xy-DHA [7,17-and 10,17-diH(P)DHA]-were formed from soybean 15-LOX-derived 17(S)-hydro(pero)xy-DHA [17(S)-H(P)DHA], whereas two novel oxylipin compounds-10(S)-hydro(pero)xy-DHA and 10,20-dihydro(pero)xy-DHA [10(S)-H(P)DHA and 10,20-diH(P)DHA, respectively]-were the major direct products of DHA oxidation by ptLOX. The reactions proceeded relatively slowly but could be stimulated by catalytic amounts of SDS. Micromolar concentrations of 10(S)-HPDHA effectively abolished the kinetic lag period of ptLOX activation. Enzymatic activity with DHA or 17(S)-HPDHA as substrate was about 8% of that with linoleic acid-a standard natural ptLOX substrate-whereas 17(S)-HDHA was converted at a rate of ∼1%. The enzyme was relatively unstable and quickly inactivated during the reaction with DHA on with 17(S)-HPDHA (first-order kinetic constant of inactivation k_in = 1.5 ± 0.3 min^-1), but not with 17(S)-HDHA. Both 7,17- and 10,20-diH(P)DHA were clearly products of double oxygenation catalyzed by soybean 15-LOX and/or ptLOX. Our observation that ptLOX could convert 17-HDHA to 10,17-diH(P)DHA indicates that this dihydroxylated derivative of DHA also can be formed via a double lipoxygenation mechanism.