142 PLANT RESPIRATION 



sugar. For example the carbon framework of citric acid is 

 identical with that of isoleucin : 



/CHo— COOH /CH(NHo)— COOH 

 COOH— COH<; CH3— CH< 



^CH2— COOH ^CHo— CH3. 



Citric acid Isoleucin 



It is e\ddent that citric acid might be formed from isoleucin and 

 some other cleavage products of protein substances without 

 the participation of synthetic processes. However, the greater 

 part of the citric acid is formed from sugar, not as a result of 

 respiratory processes but as a by-product of the formation of 

 amino acids and purine derivatives. 



The so-called forms of Citromyces produce considerable 

 quantities of citric acid according to Wehmer.^ According to 

 recent studies the capacity to form citric acid is peculiar to 

 various molds under anomalous conditions of nutrition,"' so 

 the genus Citromyces should perhaps be dropped. The detailed 

 studies by Maze and Terrier^ have established beyond doubt 

 that the formation of citric acid by molds represents an 

 extremely complicated process. In addition to this Buchner 

 and Wiistenfeld'* have sought in vain to isolate an enzyme of 

 citric acid fermentation^ from Citromyces. Moreover, Butke- 

 witsch*^ has shown that molds form citric acid only in the pres- 

 ence of considerable amounts of sugar. 



The general conclusion may therefore be drawn that the 

 plant acids, such as malic, tartaric, citric, etc., which are formed 

 in large quantities are not at all normal intermediate products 

 of oxygen respiration. These acids arise either from amino 



1 Wehmer, C. Ber. d. bot. Ges. 11 : iM. 1893; Zentralbl. f. Bakt., Parasitenk. u. Infek- 

 tionskrankh. (11) 15: 427. 1906. 



2 Currie, N. Jour. Biol. Chem. 31: 15. 191"; Elfving, F. Ofvers. af Fin. Vetenskaps- 

 Soc. Forh. A. Mathem. o. Naturw. 61 : I. 1920; Molliard. Compt. rend. l68: 360. 1919; 

 174: 881. 1922; 178: 41. 1924; Butkewitsch. Biochem. Z. 129: 445. 455, 4^4- 1922, 



2 Maze et Perrier. Ann. de I'inst. Pasteur 18: 553- 1905; 23: 830. 1909; c/. also 

 Elfving. loc. cil. 



< Buchner, H. und Wiistenfeld. Biochem. Z. 17: 39S. 1909. 



5 It is scarcely valid to designate the processes of acid formation as "oxalic acid fermenta- 

 tion," "citric acid fermentation," etc. According to the modern conception, true fermenta- 

 tions are independent systems of coupled reactions which may be separated from the living 

 protoplasm and which serve as sources of energy. 



6 Butkewitsch, W. loc. cit. 



