Lipid Mediator Mechanisms in Fish

Abstract

Salmonids synthesize platelet-activating factor (PAF: 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), and much of the interest in the potential use of lipids to mediate physiological events in fish metabolism is focused on this compound. In sharp contrast to mammalian cells, salmonid cells acylate lysoPAF with a high degree of specificity for omega-3 fatty acids. Polyunsaturated fatty acids in the lipids in tissues of rainbow trout Oncorhynchus mykiss are compartmentalized differently than in mammalian tissues. On extracellular challenge by PAF, trout leukocytes exhibit both chemotaxis and respiratory burst responses. During the metabolism of PAF in rainbow trout leukocytes, acylation of lysoPAF appears highly selective for docosahexaenoic acid (C22:6 n-3) and eicosapentaenoic acid (C20:5 n-3) and is localized in the endoplasmic reticulum. The acyl moiety in the sn-2 position of the acylation product 1-O-alkyl-2-acyl-glycero-3-phosphocholine may be removed by hydrolysis to produce lysoPAF in the endoplasmic reticulum, or the acylation product may be transported to the plasma membrane where lysoPAF may be generated. LysoPAF is acetylated at the plasma membrane by an acetyltransferase to produce PAR This PAF is degraded by a cytosolic acetylhydrolase to produce lysoPAF, the substrate for the transacylase. Thus, rainbow trout offer an opportunity to investigate omega-3 fatty acids as lipid mediators in a system less complex than mammalian systems. The results of such studies are certain to affect future practices in aquaculture as attempts to increase output intensify and stress to cultural animals becomes an inherent problem.