Specificity of two lipoxygenases from rice: unusual regiospecificity of a lipoxygenase isoenzyme

The regio- and stereospecificity of two lipoxygenases from rice were investigated using arachidonic acid as the substrate. Rice seed lipoxygenase-2 (RSL-2) catalyzed oxygenation of arachidonic acid into a mixture of 5(S)-hydroperoxy-6,8,11,14-eicosatetraenoic acid [5(S)-HPETE] and 15(S)-hydroperoxy-5,8,11,13-eicosatetraenoic acid [15(S)-HPETE]. In addition, two double dioxygenase products, 5(S), 15(S)-dihydroperoxy-6,8,11,13 -eicosatetraenoic acid and 8(S),15(S)-dihydroperoxy-5,9,11,13 -eicosatetraenoic acid, were obtained in a lower yield. The regiospecificity of the RSL-2-catalyzed oxygenation was pH-dependent. Thus, incubation at pH 6.7 led to the formation of 5(S)-HPETE and 15(S)-HPETE in a ratio of 52:48, and incubation at pH 9.8 strongly suppressed production 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] catalyzed oxygenation of arachidonic acid into a single hydroperoxide isomer of high optical purity, i.e., 15(S)-HPETE (99.5% S).