The incorporation of [1-14C]acetate into the methyl ketones that occur in steam-distillates of bovine milk fat

Abstract

The 14C-labelling of the fatty acids and the methyl ketones in steam-distillates of milk fat from a lactating cow that had been injected intravenously with [1-14C] acetate was determined. The labelling patterns of the Ce-Cie fatty acids and the corresponding methyl ketones with one fewer carbon atoms were similar, particularly so for the C5-C10 compounds at 9 and 22 hr. after the injection of [1-14C]acetate. The isolation of 14C-labelled methyl ketones in the range C3-C15 is evidence that the [beta]-oxo acid precursors, which are glyceride-bound in the milk fat, are synthesized in the mammary gland from acetate. The absence of heptadecan-2-one in steam-distillates and the extremely low specific radioactivity of stearic acid are further evidence for this biosynthetic pathway. The specific radioactivities of the C5-C15 methyl ketones were, higher (with the exception of Cg methyl ketone in the second milking) than the specific activities of the corresponding fatty acids with one more carbon atom. This is consistent with the methyl ketone precursors being formed during the biosynthesis of fatty acids rather than being products of [beta]-oxidation of fatty acids.