SPECIFICITY OF 2 LIPOXYGENASES FROM RICE - UNUSUAL REGIOSPECIFICITY OF A LIPOXYGENASE ISOENZYME

The regio- and stereospecificity of two lipoxygenases from rice were i nvestigated using arachidonic acid as the substrate. Rice seed lipoxyg enase-2 (RSL-2) catalyzed oxygenation of arachidonic acid into a mixtu re of 5(S)-hydroperoxy- 6,8,11,14-eicosatetraenoic acid [5(S)-HPETE] a nd 15(S)-hydroperoxy-5,8,11,13-eicosatetraenoic acid [15(S)-HPETE]. In addition, two double dioxygenase products, (S),15(S)-dihydroperoxy-6, 8,11,13-eicosatetraenoic acid and (S),15(S)-dihydroperoxy-5,9,11,13-ei cosatetraenoic acid, were obtained in a lower yield. The regiospecific ity of the RSL-2-catalyzed oxygenation was pH-dependent. Thus, incubat ion at pH 6.7 led to the formation of 5(5)-HPETE and 15(S)-HPETE in a ratio of 52:48, and incubation at PH 9.8 strongly suppressed productio n of 5(S)-HPETE and led to formation of 5(S)-HPETE and 15(S)-HPETE in a ratio of 3:97. A PH dependent orientation of arachidonic acid at the active site is proposed to explain these findings. Rice leaf pathogen -inducible lipoxygenase [Peng, Y.-L., Shirano, Y., Ohta, H., Hibino, T ., Tanaka, K., and Shibata, D. (1994) J. Biol. Chem. 269, 3755-3761] c atalyzed oxygenation of arachidonic acid into a single hydroperoxide i somer of high optical purity, i.e., 15(S)-HPETE (99.5%, S.).