Incorporation of dl-[1−14C]leucine and [1−14C]isovaleric acid into milk constituents by the perfused cow's udder

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Abstract
Two separated halves of lactating cow udders were perfused with blood containing respectively carboxyl-labeled leucine (0.95 mc of DL-[1-C14]-leucine) and carboxyl-labeled isovalerate (0.5 mc of [l-C14]isovalerate) for 2 hr. Both halves (the "leucine half" and the "isovalerate half") had inactive acetate added continuously throughout the perfusion. The total quantity of C14 recovered as CO2 was 1% in the "leucine half" and 6% in the "isovalerate half". [1-C14] Leucine was incorporated to a large extent into milk protein. In the "isovalerate half", C14 was incorporated into fatty acids isolated from the milk and udder glycerides. The specific activity of the lower fatty acids increased with increasing chain length, to a maximum at C10 but fell progressively with further increase in chain length. No specific incorporation of C14 into branched-chain fatty acids could be demonstrated. Of the added C14, 20% was found in the fat. In the "isovalerate half", casein was the most active of the milk constituents isolated. Of the C14 activity in this fraction, 98% was due to glutamic acid and aspartic acid. The isolated lactose was slightly radioactive. These results are consistent with the assumption that [1-C14]isovaleric acid is broken down between C2 and C3, yielding a C14O2H-labelled C2 component with high acetylating capacity. This component is metabolized by way of the Krebs cycle, as shown by the isolation of very highly labelled citric acid, malic acid and succinic acid from the udder tissue. In all the perfusion experiments carried out in presence of C14-labelled precursors, the specific activities of udder-fat fractions were higher than those of the corresponding milk fractions by a factor varying between 10 and 100. The difference in specific activity of citric acid in udder and in milk was relatively very small.