THE ROLE OF MALIC ACID 1667 



However, subsequent experiments did not square with the assumption 

 that malic acid is an intermediate in the main reaction sequence of photo- 

 synthesis. 



Bassham, Benson and Calvin (1950) found that malonate inhibits, by 

 as much as 70-97%, the formation of tagged malic acid in light, without 

 inhibiting photosynthesis as a whole by more than 15-35%. Malonate 

 had no effect either on the amount of tagged phosphoglyceric acid or 

 on the distribution of tracer between the carboxyl and the a- and /3-carbon 

 atoms in this compound. These experiments indicate that malic acid is 

 not a link in the direct sequence of reactions leading from carbon dioxide 

 via phosphoglyceric acid to sugars. Since, however, it is one of the first 

 compounds to be labelled in light even when the labelling of fumaric acid 

 and succinic acid is negligible, as well as in darkness (where the latter acids 

 form the bulk of the tagged compounds), it appears that tagged malic acid 

 is produced not only by the respiratory but also by the photosynthetic 

 C*02 uptake mechanism. It was suggested that in the second case, malic 

 acid is not an intermediate in the carboxylation cycle, but a side product, 

 a "storage reservoir" of C*, isotopically equilibrated with a C4 product in 

 the main reaction sequence (such as oxalacetic acid, OAA). The early ap- 

 pearance of phosphopyruvic acid (PPA) suggested the scheme: 



_llQ +C*02 ^ main sequence to sugars 



(36.4A) PGA ^ PPA > OAA 



(enol ) 



4-2[H] 



: ^ malic acid 



-2[H] 



This equilibration could conceivably be interrupted by malonate, while the 

 main reaction sequence remains unaffected. 



Badin and Calvin (1950) chromatographed the products of C*02 

 fixation in Scenedesmus obtained by photosynthesis in very low light. The 

 most interesting finding was that in low light the first identifiable C*- 

 tagged product was malic acid (30% of total C* uptake, if extrapolated to 

 zero time). Organic phosphates (including phosphoglyceric acid) ac- 

 counted, in such low light, for less than 30%o of total C* (as against the 

 90% found in phosphoglyceric acid alone after 5 sec. photosynthesis in 

 strong light). 



From these experiments, Calvin et al. concluded that two primary C*02 

 uptake reactions occur in photosynthesis: one leading directly to the for- 

 mation of phosphoglyceric acid, the other, indirectly, to malic acid. The 

 separate formation of these two products was supported by the observation 

 that the percentages of total C* present in organic phosphates and malic 

 acid extrapolated to finite values at time zero. Calvin and co-workers sug- 

 gested that the primary uptake of C*02 in photosynthesis involves the two 

 reactions: 



