LIPIDS PRESENT IN MILK FAT 803 



c. Synthesis of Milk Fat in the Udder from Small Molecules, (a) Car- 

 bohydrate as a Source of Milk Fats. It is well known that fats can 

 be readily synthesized from carbohydrates. Although it was formerly 

 believed that the liver was the main if not the only site of this synthesis, 

 more recent information has indicated that extrahepatic tissues, also, have 

 the ability to bring it about. Considerable evidence has been adduced to 

 prove that fat synthesis from carbohydrate proceeds readily in adipose 

 tissue. 550 In view of these facts, it would seem logical to postulate that fat 

 may also be synthesized from carbohydrate in the mammary tissue. 



Several experimental findings indicate that fat can be formed from 

 carbohydrate in the udder. In the first place, it has been proved by a 

 number of investigators 551-553 that the lactating ruminant udder in vivo 

 has a respiratory quotient (R.Q.) greater than 1.00. A metabolism of this 

 nature has usually been interpreted as indicative of the carbohydrate 

 -*• fat synthesis. The fact that, during fasting, the R.Q. drops promptly 

 below unity, affords a satisfactory control test to increase the validity of 

 the experiments on normal unfasted cows. 552 Smith and Dastur 531 showed 

 previously that the short-chain fatty acids disappear from the milk fat 

 during fasting, with a concomitant rise in oleic acid content. It has been 

 suggested that the synthesis of this short-chain fatty acid fraction occurs 

 in the udder. 552 This finding can be as readily interpreted to indicate that 

 short-chain acids are intermediates in the synthesis of oleic acid as that they 

 are formed from oleic acid. 



Another proof of the synthesis of fat from carbohydrate in the mam- 

 mary gland is obtained from in vitro tests. When slices of lactating mam- 

 mary gland, obtained from rat, rabbit, mouse, or guinea pig, are incubated 

 in glucose, they can utilize it as the sole substrate, with a resultant R.Q. 

 in excess of one. 554 Although the mammary tissues of ruminants (sheep, 

 goat, cow) are inert to glucose, they can utilize acetate with an elevated 

 j^q 555,556 F n e y an d French 557 are of the opinion that carbohydrate can 

 be used for fat synthesis if acetate is also present. 



A third approach in determining whether or not mammary tissue can 



660 E. Wertheimer and B. Shapiro, Physiol. Revs., 28, 451-464 (1948). 



651 W. R. Graham, Jr., O. B. Houchin, V. E. Peterson, and C. W. Turner, Am. J. 

 Physiol, 122, 150-153 (1938). 



552 E. P. Reineke, W. D. Stonecipher, and C. W. Turner, Am. J. Physiol, 132, 535-541 

 (1941). 



663 J. C. Shaw, /. Dairy Sci., 29, 183-197 (1946). 



664 S. J. Folley and T. H. French, Biochem. J., 45, 117-125 (1949). 



665 S. J. Folley and T. H. French, Biochem. J., 43, lv (1948). 



556 S. J. Folley and T. H. French, Nature, 163, 174-175 (1949). 



557 S. J. Folley and T. H. French, Biochem. J., 46, 465-473 (1950). 



