HUMAN 5-LIPOXYGENASE ASSOCIATES WITH PHOSPHATIDYLCHOLINE LIPOSOMES AND MODULATES LTA(4) SYNTHETASE-ACTIVITY

A Ca2+ and a phosphatidylcholine (PC) as stimulatory factors to human 5-lipoxygenase (5-LO) were assessed to examine aspects of the regulatory mechanism of 5-LO. In the presence of Ca2+ (1 mu M or less), PC liposomes distinctly stimulated the dual activities of 5-LO for the production of 5-HPETE from arachidonate and for its subsequent conversion to LTA(4). At the same concentration of Ca2+ 5-LO was found to bind to PC liposomes. As with 5-LO activities, the binding was dependent on the range of Ca2+ concentration. The conversion ratios of 5-HPETE to LTA, were dependent on PC liposome concentration and reached a maximum of 50% conversion. Among the four cell membrane Lipids examined, PC liposomes demonstrated the highest conversion ratio of 5-HPETE to LTA(4) by 5-LO. Most of the arachidonate added to the reaction mixture localized in PC liposomes. These results confirm that the intracellular increase of Ca2+ concentration causes 5-LO to associate with the cell membrane and perform an interfacial reaction. They also suggest that this binding of 5-LO to the cell membrane enhances the subsequent conversion from 5-HPETE to LTA(4).