1694 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



a mechanism had to be invented by which oxalacetic acid could be con- 

 verted into C2 fragments without being first reduced to malic acid. Glycolic 

 or glyoxylic acid, for example, could be formed by hydrolysis of oxalacetic 

 acid, either directly or via tartaric acid: 



(36.12a) COOHCH2COCOOH + H2O ( > COOHCHOHCHOHCOOH) 



oxalacetic acid tartaric acid 



> COOHCH2OH + COOHCHO 



glycolic acid glyoxylic acid 



However, no labelled tartaric acid has been observed in radiograms of 

 products of short-time photosynthesis. 



Intermediate formation of dihydroxymaleic acid (to be split into two 

 molecules of glyoxylic acid) or of diketo succinic acid (to be split into oxalic 

 acid and glyoxylic acid) also were mentioned as possibilities by Calvin and 

 co-workers. However, the formation of these C4 acids would mean oxida- 

 tion of oxalacetic acid, while the cycle as a whole must be reductive. 



Another alternative discussed by Calvin et al. (1950) was the reduction 

 of the dicarboxylic C4 acid to dialdehyde level before its splitting into C2 

 compounds. The cleavage of (diphospho) tartaric dialdehyde into (phos- 

 pho)glycolaldehyde and (phospho)glyoxal is a plausible reaction, bearing 

 resemblance to the splitting of fructose diphosphate by aldolase. 



The assumption of two primary carboxylations — (36.10) and (36.11) — 

 was retained by Calvin and co-workers at that time for the reasons already 

 explained in sections 6 and 9. The tagging of malic acid immediately at 

 the beginning of the exposure to C*02 in light seemed to call for a more 

 direct mechanism of its equilibration with C*02 in the medium than could 

 be provided by secondary transformations of PGA. Reaction (36.10) of- 

 fered itself as a possibility, even if the reduction of OOA to MA had now to 

 be treated as a side reaction rather than as a step in the main reaction se- 

 quence of photosynthesis. The experiments of Badin and Calvin (1950), 

 mentioned in section 6, which indicated that in weak light, tagged malic 

 acid appears even earlier than tagged PGA, were considered by Calvin 

 et al. as supporting the hypothesis of a "second CO2 acceptor" — a C3 com- 

 pound, as contrasted to the "first CO2 acceptor," which had to be assigned 

 a C2 structure. 



It was suggested that in strong light the photochemically produced "C2" 

 acceptor is the more abundant of the two ; while in darkness, this acceptor 

 may disappear altogether. The "C3 acceptor," on the other hand, was as- 

 sumed to be regenerated by glycolysis. At the beginning of illumination, 

 C2 acceptor must first be built up to a steady level, which is higher the 

 stronger the illumination. In weak light, the C*02 taken up by the C3 

 acceptor will be largely stored as tagged malic acid. In the steady state, 



