Incorporation of sodium [1−14C]hexanoate and sodium hydrogen [14C]carbonate into milk constituents by the perfused cow's udder

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Abstract
Perfusions of 2 halves of lactating cow udders were performed for 2 hr. with blood containing C14-bicarbonate and 1-C14-hexanoate. Both halves received inactive acetate added continuously throughout the experiments, and 150 and 290 ml of milk respectively were collected after perfusion. A slight incorporation of C14O2 into milk casein occurred, largely in aspartic acid and glutamic acid. Citric acid isolated from the milk of the same half-udder showed a higher activity. In the hexanoate experiment the specific activity of CO2 was low at the beginning but increased gradually; approximately 4% of the C14 was recovered as C14O2. The specific activities of the milk constituents in the hexanoate experiment decreased in the following order: citric acid, casein, lower fatty acids, higher fatty acids and lactose. Cholesterol, glycerol and phospholipids did not show any detectable activity in milk. Fatty acid fractions isolated from udder tissue were approximately 40 times as active as the corresponding milk fatty acids. The specific activity of the lower fatty acids from udder and milk increased with increasing chain length to reach a maximum at C8 and then fell progressively with further increase of chain length. In the hexanoate experiment 14% of the added C14 was recovered in fat at the end of the experiment. There was no evidence of direct esterification of 1-C14-hexanoate. Of the activity of milk casein obtained from the 1-C14-hexanoate experiment 97% was due to glutamic acid and aspartic acid, the activity of the former being 3 times that of the latter. These results are consistent with the assumption that 1-C14-hexanoate is broken down to C2 components with high acetylating capacity. These components are utilized for the synthesis of fatty acids and are metabolized by way of the Krebs cycle, which is demonatrated by the isolation of citric acid, succinic acid and fumaric acid with high specific activity. Hexanoate did not behave in a glycogenic manner in the perfused udder. The carbonate pool is involved only to a negligible extent in the incorporation of C14 from hexanoate into milk constituents by the perfused-udder preparation. The specific activities of the different fractions may be a reflexion of the secretory processes of the udder cells. The fat formed in the cells is secreted in the alveoli at a later time than in the other milk constituents.