12 II. BIOSYNTHESIS 



to demonstrate experimentally that sterols and fatty acids could be ac- 

 cumulated in yeast cells grown on ethanol or acetate as the sole source of 

 carbon, without the intermediate formation of carbohydrate. Steroid 

 synthesis was inhibited by sulfite, while that of fatty acids continued to 

 operate under such conditions. This finding could be interpreted to mean 

 that acetaldehyde might be the direct intermediate in the synthesis of 

 yeast steroids, while acetate was the precursor of the newly synthesized 

 fatty acids. Sonderhoff and Thomas^" reported that, when deuterio- 

 acetate was metabolized by yeast, approximately ten times the quantity of 

 isotopically labeled fatty acids was formed as compared with the amount 

 of tagged carbohydrate. This indicates that carbohydrate is not an inter- 

 mediate of acetate in the formation of fatty acids. 



In addition to yeast cells, bacteria are likewise able to use acetate as a 

 materia] for the synthesis of fatty acids. For example. Wood and asso- 

 ciates^^ reported that butyl alcohol was formed during the fermentation 

 carried out in the presence of CHsC^^OOH by Clostridium hutijlicum and 

 by CI. acetobutylicum. In view of the fact that these organisms can re- 

 duce butyric acid to the alcohol, the following series of reactions was sug- 

 gested : 



2 Acetate > Butyrate > Butanol 



This reaction is supported by the finding that C^^ appeared in the 1 and 3 

 positions in butanol. Another example of synth(\sis of butyric acid by fer- 

 mentation of acetate and of ethanol by bacteria is afforded by the results 

 of Barker et al.,^^ who employed CI. kluyverii. When CHsC^^OOH was 

 used, it was found that the butyrate formed contained approximately the 

 same proportion of C^^ on the carboxyl and jS-carbon atoms. It was be- 

 lieved that the C'^ was present on alternate carbon atoms in the caproate 

 formed by means of this organism, since the carboxyl C^^ accounted for 

 only one-third of the isotopic carbon. Further information concerning 

 the mechanism of synthesis is afforded by analysis of the caproic acid 

 formed when fermentation occurred with normal ethanol and CH3-CH2- 

 CII2 • C'^OOH. The hexanoic acid was found to contain an isotopic carbon 

 atom, but it was not located in the carboxyl group; this indicates that the 

 active C2 unit from ethanol must have condensed with the carboxyl of 

 butyric acid. 



Equally convincing proof that fatty acids originate from acetate has 



20 R. Sonderhoff and H. Thomas, Ann., 530, 195-213 (1937). 



21 H. G. Wood, R. W. Brown, and C. H. Werkman, Arch. Biochem., 6, 243-260 (1945). 

 " H. A. Barker, M. D. Kamen, and B. T. Bornstein, Proc. Nat. Acad. Sci., 31, 373-381 



(1945). 



