SATURATED FATTY ACIDS 13 



been adduced from animal experiments. Thus, Rittenberg and Bloch^^-^^ 

 found that the fatty acids isolated from mice, which had been fed CD3- 

 C'^OOH, Avere labeled both with deuterium and with C'^, indicating that 

 both carbons of the acetate had been used in the synthesis of fat. It is 

 evident that the new^ly synthesized fat w-as formed by the condensation of 

 a number of molecules of acetic acid. The whole acetate was used for 

 this synthesis, as Avas indicated by the fact that only approximately 50 

 per cent of the carbon of the newly formed fatty acids contained C^' 

 atoms. 



There are two possible methods by w'hich condensation of the acetate 

 fragments might proceed. If the acetate having labeled carbon in the 

 carl)ox3d group is added to an unlabeled acid by condensation with the 

 carboxyl group of the latter, then only the new carboxyl group would con- 

 sist of C'^ and the rest of the molecule would be isotope-free. On the 

 other hand, if the tagged acetate adds to the co-carbon of the new acid, 

 the carboxyl will be unlabeled, and the C^^-atom will occur in the remainder 

 of the new molecule. 



In the case of long-chain acids, a number of investigators have proved 

 that elongation occurs by the addition of an acetate molecule. The inter- 

 conversion of palmitic and stearic acid by the addition or removal of C2 

 units was demonstrated both by Schoenheimer and Rittenberg^^ and by 

 Stetten and Schoenheimer,^* using deuterium-labeled fatty acids. Klem,^^ 

 and Stevens and Chaikoff^^ also proved that lauric (C12) and myristic 

 (C14) acids could be elongated to palmitic and stearic acids; how^ever, in 

 none of these studies was it evident at which end of the fatty acid the 

 extension occurred. Zabin-^ observed in both in vitro and in vivo 

 that the lengthening of palmitic to stearic acid involves the addition of 

 the methylene group of the acetate to the carboxyl group of palmitic acid. 

 This result offers concrete evidence for the first of the two possible methods 

 of addition of the acetate fragment in the synthesis of fatty acids. 



Although most workers agree that acetate is the essential component in 

 the sjaithesis of fatty acids, the fact remains that acetate as such is inef- 

 fective, and that some derivative of acetate possessing a greater reactive 

 ability must be the so-called "active acetate." According to Bloch,^ 



" D. Rittenberg and K. Bloch, /. Biol. Chem., 154, 311-312 (1944). 

 " D. Rittenberg and K. Bloch, /. Biol. Chem., 160, 417-424 (1945). 



25 R. Schoenheimer and D. Rittenberg, J. Biol. Chem., 120, 155-165 (1937). 



26 De W. Stetten, Jr., and R. Schoenheimer, J. Biol. Chem., 133, 329-345 (1940). 



=" A. Klem, Norske Videnskaps-Akad. Odo., Hvalradets Skrifter, No. 27, 1-26 (1943). 

 =» B. P. Stevens and I. L. Chaikoff, J. Biol Chem., 193, 465-472 (1951). 

 " I. Zabin, J. Biol. Chem., 189, 355-359 (1951). 



