SATURATED FATTY ACIDS 19 



Popjak and Tietz^^ reported that, in addition to glucose, pyruvate, 

 oxaloacetate, a-ketoglutarate, and succinate stimulate fatty acid synthesis 

 from acetate, but to a lesser extent than does glucose. Oxaloacetate had 

 the greatest effect, next to glucose, and, in addition, it inhibited the oxi- 

 dation of acetate. The other substrates either had no effect or stimulated 

 the oxidation of acetate. Homogenates of mammary tissue, like slices, 

 were found incapable of bringing about fatty acid synthesis when acetate 

 alone or with ATP was added as the only substrate. Glucose alone or with 

 ATP also failed to promote lipogenesis from acetate by mammary homoge- 

 nates. The system producing most fat from acetate under such condi- 

 tions was composed of acetate, oxaloacetate, and ATP, together with ho- 

 mogenates of the mammary gland. 



Although the evidence cited above for the lipogenic activity of manunary 

 tissue is largely indirect, since it is based upon physiologic indices such as 

 R.Q., there is considerable direct proof of this reaction. Thus, Cowie 

 et alP were able to demonstrate the synthesis of isotopically labeled fatty 

 acid in the perfused isolated bovine udder, while negative results were ob- 

 tained with sodium bicarbonate-C^*. In vivo tests on the pregnant rabbit 

 by Popjak and Beeckmans''^ likewise showed that glyceride fatty acids 

 are synthesized by the udder even before milk production begins. The 

 later work of Popjak and associates^ *~'^^ on intact animals furnishes further 

 direct evidence of the fat synthesis from acetate in the udder. Since, 

 after injection of radioacetate, the specific activities of the blood plasma 

 were of a much lower order than those of the volatile fractions of milk 

 fat, the synthesis of the fatty acids must occur in the udder. Independent 

 confirmation of this reaction was obtained by Kleiber and co-workers''^ 

 in the case of the cow, and by French and Popjak,^* who used rabbits. 

 McClymont,^^'^^ using cows and sheep, arrived at similar conclusions by 

 the use of a different technic. The disappearance of acetate from the 

 mammaiy gland was established by demonstrating arteriovenous differ- 

 ences in acetate concentration across the udder of 2 to 6 mg. %, depending 

 upon the acetate level in the arterial blood.''^-'^^ 



^1 G. Popjdk and A. Tietz, Biochem., J., 54, xxv (1953). 



" A. T. Cowie, W. G. Duncombe, S. J. Folley, T. H. French, R. F. Glascock, L. 

 Massart, G. J. Peeters, and G. Popjdk, Biochem. J., 49, 610-615 (1951). 



" G. Popjdk and M. L. Beeckmans, Biochem. J., 46, 547-558 (1950). 



'^ G. Popjak, S. J. Folley, and T. H. French, Arch. Biochem., 23, 508-510 (1949). 



" G. Popjdk, T. H. French, and S. J. Folley, Biochem. J., 46, xxviii-xxix (1950). 



7« G. Popjdk, T. H. French, and S. J. FoUey, Biochem. J., 48, 411-416 (1951). 



" M. Kleiber, A. H. Smith, and B. Tolbert, Abst. Commun. XVIIIth Physiol. Congr., 

 Copenhagen, Aug., 1950, p. 304. 



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



" G. L. McClymont, Australian J. Agr. Research, 2, 158-180 (1950). 



