CLASSIFICATION AND STRUCTURE OF FATTY ACIDS 43 



Although it is not certain that the results obtained in the in vitro test 

 with B. leprae will apply in all cases to human leprosy, they are most im- 

 portant as pilot experiments. The confirmation of the results by clinical 

 studies will require a prolonged period and will in all probability be less de- 

 cisive than the in vitro tests. 



(.9) Dicarboxylic Acids 



Although the dicarboxylic acids do not occur in appreciable amounts as 

 components of lipids, some of them are important metabolic products of 

 these substances, inasmuch as they originate from them by w-oxidation. 

 Moreover, the lower members of the series which can be synthesized de 

 novo by the animal are of great importance in the so-called tricarboxylic 

 acid cycle. This series of reactions is responsible for the oxidation not 

 only of carbohydrate but also of a considerable portion of the fatty acid in- 

 termediates. These acids will therefore be briefly described. 



The saturated dicarboxylic acids (also known as the oxalic acid series) 

 have the type formula (CH2)„(COOH)2. Since they have two carboxyl 

 groups, they may form two series of esters, salts, or other derivatives, de- 

 pending upon whether one or both carboxyl groups are in combination. 

 They are considerably more soluble in water and less soluble in the fat sol- 

 vents than are the members of the monocarboxylic fatty acids. In general 

 they occur free, as salts, as amides, or as esters with short-chain alcohols. 

 Combinations of the dicarboxylic acids with glycerol do not occur in nature. 



The acids in this group are almost exclusively referred to in the litera- 

 ture by their common names. The systematic name, which has the ad- 

 vantage of indicating the hydrocarbon from which they may be considered 

 to be derived, is made up by adding dioic to the name of the hydrocarbon 

 which has the corresponding total number of carbon atoms. Thus, the C2 

 acid, oxalic, is called ethanedioic, the Ce acid, adipic, is referred to as 

 hexanedioic. Some confusion exists in the older literature, as the names of 

 the acids were derived from the number of methylene groups present. 

 Thus, adipic acid in this system is tetramethylene-l,4-dicarboxylic acid. 



The common and systematic names of the dicarboxylic acids of biologi- 

 cal interest, together with data on their discovery, are included in Table 6. 



The lower dicarboxylic acids occur chiefly as salts in vegetables. Thus, 

 oxalic acid is present as the potassium salt in rhubarb, malonic acid occurs 

 as the calcium salt in beet roots, while succinic acid is distributed in both 

 plant and animal sources such as grapes, plant resins, amber, and in muscle 

 tissue. The German terminology for succinic acid, Bernsteinsaure, is 

 based upon the fact that it is the acid found in amber. Glutaric acid occurs 



