Effect of Saturated and Unsaturated Fatty Acids on the Desaturation in Vitro of Palmitic, Stearic, Oleic, Linoleic, and Linolenic Acids

Quantitative oxidative desaturation of labeled palmitic, stearic, oleic, linoleic, and linolenic acids to palmitoleic, oleic, octadeca-6,9-dienoic, γ-linolenic, and octadeca-6,9,12,15-tetraenoic acids, respectively, by liver microsomes of rats was studied by gas-liquid radiochromatography after incubation of the acids in a medium containing adenosine triphosphate, reduced nicotinamide adenine dinucleotide, reduced nicotinamide adenine dinucleotide phosphate, MgCl 2 , coenzyme A, glutathione, nicotinamide, NaF, and NaCN in oxygen at pH 7 and 35°. The rates of conversion of oleic into octadeca-6,9-dienoic, linoleic into γ-linolenic, and linolenic into octadeca-6,9,12, 15-tetraenoic acids were measured, and Lineweaver-Burk plots showed the existence of competitive reactions among the three acids. Saturated acids were ineffective in altering the percentage of desaturation of the acids, whereas polyunsaturated acids of 20 and 22 carbon atoms generally activated the conversion of the three acids. Docosahexaenoic acid decreased the percentage of desaturation of oleic, linoleic, and linolenic acids. The unsaturated acids of 18 carbons apparently competed for the same oxygenase, and the order of affinity was linolenic, linoleic, and oleic acids. Substrate fatty acids and products after the incubation were shown to be mainly esterified to choline and ethanolamine glycerophosphatides.