Lipid class and nonesterified fatty acid profiles in plasma of North Atlantic cod (Gadus morhua)

Abstract

Metabolic energy status is a critical metric for the evaluation of fish condition and health. Thus, we (i) conducted comprehensive and comparative measurements of nonesterified fatty acids (NEFAs) and other plasma lipids in fed and food-deprived (10 weeks) Atlantic cod (Gadus morhua) and (ii) compared three common methods for measuring total plasma NEFAs (Folch extraction/Iatroscan, Wako®enzymatic, and acetyl chloride extraction/GC). Plasma total lipid, phospholipid, triacylglycerol, and NEFA levels were 83%–95% lower in food-deprived fish. In contrast, the concentration of a previously unidentified lipid class (ethyl ketone) was only 60% lower and was in fact almost threefold higher when expressed as a percentage of total lipid. Considerable differences in the NEFA profile were also observed, for example, 22:1ω11 (dominant NEFA in fed fish) was not detected, 20:1ω9 was 97% lower, and monounsaturated fatty acids were selectively reduced. Importantly, the acetyl chloride/GC method resulted in an eightfold overestimation of NEFA in fed fish. These results (i) suggest that plasma lipids reflect the energetic/nutritional status of wild gadids and can be used to monitor their responses to changing environmental conditions and (ii) caution against using the acetyl chloride/GC method of NEFA measurement without prior separation of plasma lipids by solid-phase chromatography.