PLANT METABOLISM 231 



animals. Such a scheme woulil accoiuu Um ihe minor (juauliLies of 

 other iatty acids normall) lound with even numbers ot carbon atoms. 

 It does not explain why the other known acids of this series are not 

 always found also, nor why pahnitic or some other member is 

 especially predominant. However, labeling experiments with either 

 carbon atom one or two marked in separate studies lead to palmitic 

 acid labeled only in the odd or even position in the two respective 

 cases. This specificity of positioning strongly indicates an acetyl 

 coenzyme A mechanism. 



There is still difference of opinion about the biosynthesis of the 

 luisaturated fatty acids. Some tracer experiments with acetate have 

 led to radioactive oleic, linoleic, and linolenic acids in flax, but the 

 radioactive atoms were not located to exclude other types of reactions. 

 Studies with other plants failed to detect radioactivity in unsaturated 

 fatty acids when labeled acetate was supplied. Thus another mecha- 

 nism has been assumed and some have been sketched, but the problem 

 is still quite open. 



Traces of free fatty acids appear to occur with the fats themselves. 

 Therefore, most investigators feel that the fatty acids are formed first 

 and then are esterified with glycerol to complete the process. However, 

 if the acyl coenzyme A route participates in fatty acid synthesis, the 

 high free energy inherent in the fatty acid coenzyme A complex could 

 be most effective in the esterification step. In this event the free acids 

 would not participate as intermediates in fat synthesis but would 

 play other roles. 



Be that as it may, plants show some degree of specificity in the fats 

 formed. In other words, the esterification step is not a hit or miss 

 proposition, and the fats tend to be of certain specific kinds. When 

 more than one fatty acid occurs in the fat in quantity, the triglycerides 

 seem to be mixed rather than solely of the tripalmitin or triolein type. 

 Furthermore, there is frequently a distinct specificity in the isomer 

 produced. Cacao butter seems to contain /3-oleodistearin, |8-palmito- 

 oleostearin, and ^-palmitodiolein to the exclusion of the corresponding 

 possible isomers. The last of these substances is cjuitc widespread and 

 occurs in quantity in olive and cottonseed oils, again only in the (3 

 form. However, other plants seem to contain both isomers of some 

 triglycerides. Palm oil contains roughly equal quantities of the a- and 

 /?-oleodipalmitins, but at least 90 per cent of the palmitodiolein is the 

 unsymmetrical isomer. 



This rather marked selectivity, though certainly not universal, 

 obviously requires some sort of biosynthetic control, imposing another 



