Synthesis of milk fat from acetic acid (CH314COOH) by the perfused isolated bovine udder

Abstract

Two separated halves of a lactating cow udder were respectively perfused with blood containing carboxyl-labelled acetate (1 mc. CH3C14OONa; "acetate" half) and labelled bicarbonate (1 mc. NaHC14O3; "bicarbonate" half) by a technic which permits of separate perfusion with no mixing of blood in the 2 circulations. Heavy water (D2O) was added to the blood containing NaHC14O3. The specific activity of the blood CO2 in the "acetate" udder half was maximal at the outset and thereafter fell to a fairly constant level throughout the rest of the experiment. Because of this and the fact that about 40% of the added C14 was found in the udder fat at the end of the expt., it would seem that the initial high specific activity of the blood CO2 was due to oxidation of the free acetate, which was probably taken up rapidly by the udder tissue, the subsequent lower values corresponding to the oxidation of udder constituents, mainly fatty acids, into which the CH3C14OONa had been incorporated. C14 was incorporated into fatty acid fractions isolated from the milk and udder glycerides of the "acetate" half, the mean specific activity fell with increasing mean chain length. The conclusion from previous work on a living lactating goat, that the udder utilizes acetate as a substrate for the synthesis of milk fatty acids, is confirmed. The specific activities of the udder fat fractions were higher than those of corresponding fractions from the milk. This could be explained by dilution of newly synthesized fat by fat already existing in the udder tissue and by a rapid turnover of fat in the epithelial cells. The specific activities of pure fatty acids isolated from the udder fat fell progressively with increasing chain length up to stearic and oleic acids. The short-chain acids could, therefore, not have been formed directly by degeneration of long-chain acids. The mean specific activities of fatty acid fractions isolated from udder fat from the "bicarbonate" half were very low, indicating negligible incorporation of C14O2- The D2 content of these fractions was also low, probably due to dilution of newly formed fat. Cholesterol isolated from milk and udder fat from the "acetate" half was radioactive; the specific activity of the udder cholesterol was higher than that from the milk, confirming previous work indicating that cholesterol is synthesized in the udder. Lactose isolated from milk from the "acetate" half was appreciably radioactive, while milk lactose from the "bicarbonate" half was not. The udder tissue must therefore incorporate acetate C into lactose by some mechanism other than CO2 fixation. The application of the udder perfusion technic to biochemical studies of milk formation is discussed in the light of the results of this expt. Perfusion conditions must be carefully controlled if valid comparisons are to be made with the lactating udder in vivo.