LIPIDS PRESENT IN MILK FAT 805 



ing upon the interval after feeding. 666 - 667 Thus, evidence is available not 

 only that acetate is formed in the rumen of cattle and goats, but also that 

 it is absorbed ; it can also be demonstrated as a component of the peripheral 

 blood. The final evidence of the incorporation of acetate into milk fat has 

 been derived from feeding experiments with C 14 -labeled acetate. Popjak 

 and associates 568 found that, when this compound was administered to a 

 lactating goat, 80% was oxidized, and 10% participated in the synthesis of 

 milk fat in the udder within six hours after its administration. The ace- 

 tate hypothesis also offers the simplest explanation for the presence of the 

 volatile fatty acids in milk fat. 



Early experiments, which attempted to restore the normal short-chain 

 fatty acid content to the milk fat in fasting cows by injecting acetate, 

 gave negative or inconclusive results, 569-571 irrespective of whether the ace- 

 tate was introduced intravenously or by infusion into the rumen. The 

 reasons for these negative results have not been explained. However, 

 experimental data obtained by means of various procedures have consist- 

 ently yielded positive results bearing on the question of whether or not 

 acetate is the mother substance of milk fats. The various proofs for this 

 finding are classified below. 



a'. Proof of the Synthesis of Milk Fat from Acetate Based upon the 

 Respiratory Quotient of Mammary Gland Slices: Folley and French 557 

 reported that surviving mammary gland slices from cow, sheep, and goat 

 were able to utilize acetate as the sole substrate with an R.Q. > 1. These 

 data are interpreted as indicating that the mammary tissue of the rumi- 

 nant is capable of utilizing acetate carbon as the sole source of carbon for 

 fatty acid synthesis, and thus of effecting a net synthesis of fat from acetate. 

 It is believed that the short-chain fatty acids represent intermediate stages 

 in long-chain acid formation which have been "trapped" and stabilized 

 by incorporation into triglycerides. 



On the other hand, slices of mammary tissue from non-ruminants cannot 

 utilize acetate alone for fat formation. In the presence of this substrate, 

 the R.Q. was less than 1.00. However, in the presence of glucose, acetate 

 is utilized for fat synthesis, and slices of the udder exhibit an R.Q. greater 

 than unity. It would thus appear that glucose is able to stimulate the 



666 R. L. Reid, Australian J. Agr. Research, 1, 338-354 (1950). 



667 G. L. McClymont, Australian J. Agr. Research, 2, 158-180 (1951). 



668 G. Popjak, T. H. French, and S. J. Folley, Biochem. J., 48, 411-416 (1951). 



569 F. H. Malpress, "Discussion on Digestion in the Ruminant," Proc. Roy. Soc. Med., 

 39, Sect. Comp. Med., 805 (1946). 



870 A. I. Mann and J. C. Shaw, J. Dairy Sci., 30, 183-196 (1947). 

 671 G. L. McClymont, Biochem. J., 45, i-ii (1949). 



