1666 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



j3 carbon). This indicates that at some point in the regenerative cycle a 

 symmetric intermediate must be formed (e. g., oxaHc acid, succinic acid, 

 or glycol) — a compound in which two equivalent groups have equal chance 

 to contribute a or j3 atoms for the formation of phosphoglyceric acid. 



Calvin (1949) confirmed the previously mentioned observation that, 

 while tagged C5 and Ce acids are absent among the products of brief photo- 

 synthesis, they appear within minutes after photosynthesis is stopped. 

 In these experiments, cells were allowed to photosynthesize in C*02, and 

 the vessel was then swept by helium for 90 sec. before killing the cells. If, 

 during the sweep, the cells were left in light, no tagged citric, isocitric or 

 glutamic acid was found. These three acids appeared, however, if during 

 the sweep the cells were left in darkness. It thus seemed that the inter- 

 mediate products of photosynthesis were drawn, immediately after the 

 cessation of illumination (but not before), into the respiration process, 

 yielding C5 and Ce acids of the Krebs cycle (and related amino acids) (com- 

 pare fig. 36.23). 



From observations of this type, and also from kinetic studies with 

 C(14) by Weigl, Warrington and Calvin (1950), it was concluded that nor- 

 mal respiration of the cells is largely inhibited during photosynthesis. 

 However, this generalization is not supported by other experiments, e. g., 

 those with isotopic oxygen. An alternative explanation of the tagging ex- 

 periments by Steward and Thompson (1950) will be mentioned in Chapter 

 37D, (section 3), where the whole question of the relations between photo- 

 synthesis and respiration will be reviewed. 



Another interesting result of the study by Weigl et at. — the strong iso- 

 topic discrimination in photosynthesis between C(12) and C(14)^ — also 

 will be discussed in chapter 37D, 



6. The Role of Malic Acid : One or Two Carboxylations in Photosynthesis? 



Malic acid has been found, in all experiments of Calvin and co-workers 

 (as well as those of Stutz, and Gibbs), to be a major early tagged product 

 (c/. fig. 36.14); but its true role in photosynthesis is still uncertain. 



At first it appeared likely that malic acid is another "first product" of 

 photosynthesis, possibly arising by reaction (36.1), followed by reduction of 

 oxalacetic acid to malic acid ; or by direct reductive carboxylation of pyru- 

 vic acid, as observed in chloroplast preparations by Vishniac and Ochoa, 

 Tolmach, and Arnon {cf. chapter 35, equation 35.34), catalyzed by the so- 

 called "malic enzyme." One could imagine, for example, that the forma- 

 tion of PGA (by carboxylation of an unknown C2 acceptor) is followed by 

 dehydration to pyruvic acid, and reductive carboxylation of the latter to 

 malic acid. 



