Arachidonate 5-lipoxygenase (ALOX5) (Ueda et al. 1987) converts 15S-hydroperoxy-eicosatetraenoic acid (15S-HpETE) into lipoxin A4 (LXA4) and B4 (LXB4) (Serhan et al. 1984A, Serhan et al. 1984B). One of the reaction intermediates of this process might be 5S,6S-epoxy-15S-hydroxy-7E,9E,11Z,13E-eicosatetraenoic acid (5,6-Ep-15S-HETE) (Puustinen et al. 1986). However, its generation from LTA4 is unclear but it can be hydrolysed to form the lipoxins.