12-Lipoxygenase from rat basophilic leukemia cells: separation from 5-lipoxygenase and temperature-dependent inactivation by hydroperoxy fatty acid

12-Lipoxygenase in Lewis lung carcinoma cells: molecular identity, intracellular distribution of activity and protein, and Ca(2+)-dependent translocation from cytosol to membranes

Recently we demonstrated that Lewis lung (3LL) tumor cells express 12-lipoxygenase (12-LOX) mRNA and protein, respectively. In this study we partially sequenced the 12-LOX cDNA after reverse-transcription polymerase chain reaction amplification of 12-LOX mRNA from cultured 3LL cells. Comparison with platelet and leukocyte 12-LOX indicates that 3LL 12-LOX is identical with the platelet-type enzyme at least within the sequenced region. Further, we investigated the intracellular distribution of both 12-LOX enzyme protein and its activity which are prerequisites for understanding 12-LOX regulation. 12-LOX activity was monitored via the production of 12-hyroxyeicosatetraenoic acid from 3LL cells and their subcellular fractions using reverse-phase high performance liquid chromatography. 12-LOX protein was measured by direct slot blot and by Western Blotting. In 3LL cells, both 12-LOX activity and 12-LOX protein were predominantly localized in the cytosol. This 12-LOX activity was optimal at 37 degrees C. However at 24 degrees C and 10 degrees C, it showed 87% and 61% of this activity, respectively, thus differing distinctly from 12-LOX in platelets or rat basophilic leukemia cells. Incubation of 3LL cell homogenates with 0-100 microM free Ca2+ and subsequent separate analyses of cytosol and membrane fractions indicated that, as in platelets, an increase in intracellular free Ca2+ caused a loss of cytosolic 12-LOX activity. However, no significant Ca(2+)-induced increase in membrane-associated 12-LOX activity was observed under these conditions in 3LL cells. In contrast, at the 12-LOX protein level we observed a Ca(2+)-dependent loss in the cytosol and a concomitant increase in the membrane fraction. Thus, we suggest that 12-LOX in 3LL cells undergoes rapid translocation from cytosol to membrane in a Ca(2+)-dependent manner, but is no longer active or becomes inactivated at the membrane site.

12-Lipoxygenase from rat basophilic leukemia cells, an oxygenase with leukotriene A4-synthase activity

Rat basophilic leukemia cells exhibit 12-lipoxygenase activity only upon cell disruption. 12-Lipoxygenase may also possess 15-lipoxygenase activity, as is indicated by the formation of low amounts of 15(S)-HETE, in addition to the predominant product 12(S)-HETE, upon incubation of partially purified 12-lipoxygenase with arachidonic acid. With 5(S)-HPETE as substrate not only 5(S), 12(S)-diHETE and 5(S), 15(S)-diHETE are formed, but also LTA4, as was indicated by the presence of LTA4-derived LTB4-isomers. 12-Lipoxygenase from rat basophilic leukemia cells has many features in common with 12-lipoxygenase from bovine leukocytes. As was suggested for the latter enzyme, 12-lipoxygenase from rat basophilic leukemia cells may represent the remaining LTA4-synthase activity of 5-lipoxygenase, of which the 5-dioxygenase activity has disappeared upon cell disruption. Such a possible shift from 5-lipoxygenase activity to 12-lipoxygenase activity could not simply be induced by interaction of cytosolic 5-lipoxygenase with a membrane fraction after cell disruption, but may involve release of membrane-associated 5-lipoxygenase upon disruption of activated rat basophilic leukemia cells.

Improved purification of 12-lipoxygenase from rat basophilic leukemia cells and conditions for optimal enzyme activity

12-Lipoxygenase from rat basophilic leukemia cells was purified about 300-fold by protein-HPLC in a single run. Maximal 12-lipoxygenase activity was observed at pH 7.5, while the enzyme became almost inactive at pH 6 and 9. Although Ca2+ was not essential for 12-lipoxygenase activity, the partially purified enzyme was stimulated approx. 2-fold in the presence of 0.1-5.0 mM Ca2+. Contrary to 5-lipoxygenase from RBL-1 cells, 12-lipoxygenase was not inactivated by preincubation with Ca2+ for 1-10 min, nor was it stimulated by 0.1-10 mM ATP.