Lipoxin A. Stereochemistry and biosynthesis.

Lipoxin A (LXA) was prepared by incubation of either (15S)-15-hydroxy-5,8,11-cis-13-trans-eicosatetraenoic acid (15-HETE) or (15S)-15-hydroperoxy-5,8,11-cis-13-trans-eicosatetraenoic (15-HPETE) with human leukocytes stimulated by either the ionophore A23187 or the chemotactic peptide fMet-Leu-Phe. Comparison with four trihydroxyeicosatetraenoic acids prepared by total synthesis showed that biologically derived LXA is 5S,6R,15S)-5,6,15-trihydroxy-7,9,13-trans-11-cis-eicosatetraenoic acid. Three isomers of LXA were also identified in extracts of leukocytes utilizing an improved isolation procedure. These were (5S,6S,15S)-5,6,15-trihydroxy-7,9,13-trans-11-cis-eicosatetraenoic acid (6S-LXA), (5S,6R,15S)-5,6,15-trihydroxy-7,9,11,13-trans-eicosatetraenoic acid (11-trans-LXA), and (5S,6S,15S)-5,6,15-trihydroxy-7,9,11,13-trans-eicosatetraenoic acid (6S-11-trans-LXA). 18O2-labeling studies indicated that formation of LXA and its isomers occurred with incorporation of 18O at their C-5 but not C-6 positions. These results suggest that 15-hydroxy-5,6-epoxy-7,9,13-trans-11-cis-eicosatetraenoic acid or its equivalent may serve as one intermediate in the biosynthesis of LXA and 6S-LXA. When added to guinea pig lung strips LXA provoked contractions which were slow in onset and long lasting. In addition, dose response studies showed that biologically derived LXA and synthetic LXA were indistinguishable in this bioassay whereas synthetic 6S-LXA and biologically derived 6S-LXA did not share this activity. Taken together, these results suggest that activated leukocytes utilize exogenous 15-HETE to generate lipoxins which in turn can modulate cellular responses.