64 FATTY ACID METABOLISM IN MICROORGANISMS 



mental basis for a plausible mechanism of anaerobic unsatu- 

 rated fatty acid biosynthesis (6). 



Of importance for appreciation of this mechanism is the 

 fact that C. hutyricum contains two pairs of homologous 

 unsaturated fatty acids, namely, 7,8-hexadecenoic-oleic and 

 9,10-hexadecenoic-cw-vaccenic acids. The acids in each 

 pair differ from each other by one "C2" unit at the carboxyl 

 end but the distance between the methyl group and double 

 bond is identical, i.e., seven methylene groups in the oleic 

 and five methylene groups in the c^Waccenic pair of acids. 



The hexadecenoic and octadecenoic acid fractions from 

 the octanoate and decanoate grown organisms, respectively, 

 were oxidized by the Lemieux procedure (7) and the ensuing 

 dicarboxylic acids were separated by gas chromatography 

 as the methyl esters. With octanoate-1-C^* as the precursor, 

 only the C9 dicarboxylic acid from the hexadecanoate and 

 the Cii dicarboxylic acid from the octadecanoate are radio- 

 active. In experiments with decanoate-1-C^^ the label is lo- 

 cated predominantly in the C7 dicarboxylic acid from the 

 hexadecenoate fraction and the C9 dicarboxylic acid from 

 the octadecenoic acids. These results show that the short- 

 chain acids are indeed converted into the long-chain un- 

 saturated acids by midtiple addition of "C2" units as 

 predicted from our growth studies (2) and that direct 

 double-bond interconversions do not occur. Based on the 

 distribution of label in the unsaturated fatty acids, Scheuer- 

 brandt et al. (6) suggested a scheme (Fig. 3.5) for the bio- 

 synthesis of unsaturated fatty acids in anaerobes, which ap- 

 pears to fit presently available experimental evidence. 



Octanoate (upper left) is postulated to undergo two 

 different types of reaction with a "Co" unit to form either 

 decanoate or via a hypothetical 3-hydroxydecanoic acid 

 3,4-decenoic acid. This latter acid, through multiple addi- 



