At what stage in metazoan evolution did leukotriene generation first appear?--key insights from cartilaginous fish

Leukotriene (LT) B4 is a key player in inflammatory responses in mammals. During the generation of this derivative of arachidonic acid, the unstable product of 5-lipoxygenase, termed LTA4, is converted to LTB4 by LTA4 hydrolase. Invertebrates do not generate LTs yet all vertebrates from bony fish onwards synthesize this compound. As cartilaginous fish are the most primitive living jawed vertebrates, we investigated if the leukocytes from such a fish, the Thornback ray (Raja clavata) could generate LTB4. Supernatants from ionophore-challenged leukocytes generated the 5-lipoxygenase products, 6-trans-LTB4 and 6-trans-12-epi-LTB4 but were unable to synthesize LTB4. To determine if these cells contained an active LTA4 hydrolase, LTA4 was incubated with lysates from ray leukocytes. Such preparations did not contain any demonstrable LTA4 hydrolase activity. Our findings imply at the stage of cartilaginous fish evolution over 350 million years ago that the evolution of an active LTA4 hydrolase had yet to occur.