In vitro conjugation of ethanolamine with fatty acids by rat liver subcellular fractions

Previous studies from our laboratory have shown the enzymic formation of fatty acid (FA) conjugates of xenobiotic alcohols and amines. In the present study, the formation of FA conjugates of a bifunctional compound, ethanolamine was investigated by incubating [1-14C]oleic acid (1 mM) with ethanolamine (25 mM) at 37 degrees C in the presence of various rat liver subcellular fractions. The resultant product (or products) was separated by thin-layer chromatography (TLC) and the radioactivity corresponding to the relative flow of fatty acid amide was determined. Under similar conditions, formation of ethanolamides of palmitic, stearic, linoleic, linolenic, and arachidonic acids were also examined. The formation of ethanolamine conjugate with oleic acid was found to be 16.3 nmol/h/mg protein as compared to 6.7, 6.2, 8.1, 8.3, and 7.6 nmol/h/mg protein for palmitic, stearic, linoleic, linolenic, and arachidonic acids, respectively. The formation of oleoyl ethanolamide was found to be 18.9, 40.1, 65.9, and 0.3 nmol/h/mg protein in postnuclear, mitochondrial, microsomal, and cytosolic fractions, respectively. Mass spectrometric and nuclear magnetic resonance spectroscopic data of the TLC-purified product confirm the formation of oleoyl ethanolamide, and amidation appeared to be a preferred reaction over esterification. The results of this study suggest that the enzyme responsible for the amidation of fatty acids resides mainly in the microsomal fraction of the liver, and that oleic acid is a better substrate than other fatty acids used in the present study.