Lipid metabolism. Evidence of a δ-oxidation pathway for saturated fatty acids

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Abstract
1. Specific radioactivities of milk triglyceride fatty acids and γ- and δ-hydroxy fatty acids were measured after the intramammary infusion of [1−14C]acetate, δ-hydroxy[1−14C]laurate and [1−14C]laurate as their sodium salts into fed lactating goats. 2. Net incorporations of the radioactive tracer into the total milk lipids were comparable, being 16, 17 and 21% of the label infused respectively. 3. The specific radioactivities of the C4–C8 fatty acids after [1−14C]acetate infusion were lower than those of the C10–C14 fatty acids. 4. After δ-hydroxy[1−14C]laurate administration the milk triglyceride fatty acids were labelled and their specific radioactivities were characterized by decreasing values with increasing chain length of the fatty acids, implicating C4 unit incorporation. 5. The γ- and δ-hydroxy fatty acids isolated after [1−14C]laurate infusion were highly labelled and the milk triglyceride fatty acids, other than laurate, exhibited a labelling pattern similar to that of the fatty acids derived from the radioactive δ-hydroxy fatty acid. 6. Evidence is presented for the existence of saturated fatty acid δ-oxidation in the mammary gland, in which the γ- and δ-hydroxy fatty acids are active intermediates.