Inhibition of lysophospholipase D activity by fish egg extracts

Lysophospholipase D (lysoPLD) is known to convert lysophosphatidylcholine (LPC) to lysophosphatidic acid (LPA). In this study, we examined the inhibitory effect of fish egg extracts, containing lipids, on bovine lysoPLD activity. Fish eggs extracts were tested for the inhibition of lysoPLD activity, and the inhibitory action was expressed as 50% inhibitory concentration (IC50). Among fish egg extracts of 20 fish species, the most potent inhibition was expressed by Hairtail egg extract (IC50, 0.07 ± 0.01 mg egg weight/mL), followed by extract of Spanish mackerel egg extract (0.11 ± 0.02 mg egg weight/mL) and extract of Pacific saury egg (0.48 ± 0.03 mg egg weight/mL). In ESI/MS analysis, major lysoPLD-inhibitory lipid components in egg extracts were identified to be species of LPC, LPA and fatty acid. From these results, it is suggested that the strong inhibition of lysoPLD activity by fish egg extracts might be ascribed to the presence of lysophospholipids. In a separate study, enzymatic oxidation using lipoxygenase or non-enzymatic oxidations such as HOCl oxidation or Cu2+-catalyzed oxidation enhanced the inhibitory activity to some extent, suggesting that the oxidation of polyunsaturated lysophospholipids might contribute to the increase of lysoPLD-inhibitory action. Taken together, it is suggested that fish eggs may contain potent lipid inhibitors of lysoPLD, and that the inhibitory action of lipid inhibitors was enhanced by oxidative process.