Effects of Individual Circulating FFAs on Plasma Oxylipin Levels: Implications for FFA Epoxide‐diol Ratios as Indices of Soluble Epoxide Hydrolase Activity
Authors: Youn Jang , Yang Jun , Hammock Bruce , Oh Youngtaek 
Published: May 2022

Oxylipins, oxidation products of essential free fatty acids (FFAs), are involved in various cellular signaling. Among them, FFA epoxides are associated with beneficial effects in metabolic and cardiovascular health. FFA epoxides are metabolized to usually biologically less active diols by soluble epoxide hydrolase (sEH). Inhibition of sEH, which increases FFA epoxides, improves glucose homeostasis and cardiovascular health. Plasma epoxide‐diol ratios have been used as indices of sEH activity.Objective and HypothesisThe objective of this study was to examine the effects of acute elevation of plasma FFAs on oxylipins, particularly, epoxides, diols, and their ratios. We tested if FFA epoxide‐diol ratios are altered by circulating FFA levels (i.e., substrate availability) independent of sEH activity.MethodsMale Wistar rats received a constant intravenous infusion of olive (70% oleic acid [OA]), safflower seed (72% linoleic acid [LA]), and fish (rich in w‐3 FFAs) oils (20% wt/vol emulsion, 0.5 ml/h; n = 6 for each infusion) for 2 hours to selectively raise OA, LA, and w‐3 FFAs (eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]), respectively. Blood samples were collected before and after the infusions and analyzed for oxylipins using LC‐MS and individual FFAs using GC‐MS. As LA, DHA, and EPA are metabolized to epoxides and diols, we evaluated their mass‐action effects to alter epoxides, diols, or their ratios. In contrast, OA is not metabolized to epoxides and diols detected in our assays, and the olive oil infusion served as control for any non‐mass‐action effects.ResultsAs expected, olive, safflower seed, and fish oil infusions selectively raised plasma OA (57%), LA (87%), and EPA (70%) and DHA (54%), respectively (P < 0.05 for all). Interestingly, olive oil infusion had significant effects to decrease FFA epoxides and diols with more dramatic effects on epoxides than diols (P < 0.01), resulting in large (up to 6‐fold) increases in diol‐to‐epoxide ratios. We found similar effects with safflower seed and fish oil infusions, although these infusions also showed mass action effects to increase epoxide and diol levels derived from LA (safflower seed oil) and DHA and EPA (fish oil). The decreases in FFA epoxides with acute elevation of circulating FFAs (i.e., increased substrate availability) were unexpected and appeared to be due to increased epoxide metabolism independent of sEH. We are currently examining the possibility that acute elevation of plasma FFAs stimulates glucuronidation of FFA epoxides to increase their removal via urinary excretion.ConclusionsRaising plasma FFAs has profound effects to decrease plasma FFA epoxides, significantly altering plasma epoxide‐diol ratios. These data suggest that caution should be taken in comparing epoxide‐diol ratios, as indices of sEH activity, between conditions with different circulating FFA levels. Our data also suggest that plasma FFAs regulate epoxide levels by stimulating a metabolic process independent of sEH, possibly glucuronidation followed by renal excretion.