268 INTERMEDIATES IN REDUCTION OF CO2 CHAP. 10 



caused Wetzel and Ruhland (1932) to apply to the latter process the 

 alternative resoiration cycle, 10.11, originally suggested by Toenissen 

 and Brinckman (1931) to explain the failure of attempts to induce 

 certain tissues to use acetate (instead of pyruvate) for the formation 

 of succinate — a substitution which should be possible according to 

 scheme 9. II. 



NET REACTION: CHjCOCOOH+aHjO— ►COg* 2HC00H* 6{h} 



Pyruvic acid 

 I 



-► 2 (Pyruvic acid) 

 2CH--CO-COOH 



I- 



(Pyruvic acid) Oikatoodipic acid ^l^/ 



CH,-CO-COOH 



COa-f- 



CH^-CO-COOH 



(Oxalacefic acid) 



H»h 



+2H2O 



(Malic acid) Succinic acid 2( Formic acid) 



(CH^COOH)^ 2H-COOH 



+H2O 



Fumaric acid 2{h} 



J 



Further as in Schema 9.11 



Scheme 10.11. — The respiration cycle after Toenissen and Brinckman. 



Scheme 10.11 avoids the formation of carbon dioxide between pyruvic 

 and malic acid; but it calls instead for the formation of two molecules of 

 formic acid, which has not been observed in succulents, and whose fate 

 must be explained before the scheme can be considered as plausible. 



Schemes 9. II and 10.11 do not include citric acid. However, in the 

 study of animal respiration, citric acid has also been found to play an 

 important part. Krebs and Johnson (1937), Martins and Knoop (1937), 

 and Martins (1937, 1938) have attempted to account for this part by 

 a new cycle, which starts with one molecule each of oxalacetic acid and 

 pyruvic acid, and ends with the restoration of oxalacetic acid and the 

 decomposition of pyruvic acid. This cycle includes citric, aconitic, 

 isocitric, oxalosuccinic, a-ketoglutaric and succinic acids as intermediates; 

 the further transformation of succinic acid follows scheme 9. II. Since 

 many details of this cycle are controversial, we do not reproduce it here. 

 The essential point is that, according to it, both citric and malic acid are 

 intermediates of respiration, with citric acid preceding malic acid in the 

 cycle. It is thus tempting to apply this cycle to the formation of malic 

 and citric acid in plants. Pucher, Clark, and Vickery (1937) noticed that 

 the sum of the malic and citric acid in rhubarb is approximately constant 

 throughout the leaf, but the proportion of citric acid increases and that 

 of malic acid decreases from stem to tip, thus indicating an interconver- 

 sion of the two acids. However, it seems that, in leaves, malic acid is 



