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

1. The (14)C-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-(14)C]acetate was determined. 2. The labelling patterns of the C(6)-C(16) fatty acids and the corresponding methyl ketones with one fewer carbon atoms were similar, particularly so for the C(5)-C(10) compounds at 9 and 22hr. after the injection of [1-(14)C]acetate. The isolation of (14)C-labelled methyl ketones in the range C(3)-C(15) 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. 3. The specific radioactivities of the C(5)-C(15) methyl ketones were higher (with the exception of C(9) 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.