easily be converted back to PEPA. Rather, it must either go 

 to malic acid via reductive carboxylation or be oxidized to 

 acetyl CoA and CO2. 



The light-dark transient effect in C^^02 uptake during 

 photosynthesis has often been observed (16,20). When the 

 light is turned off, following a period of photosynthesis of 

 algae with C^^02, labeled glutamic acid and citric acid ac- 

 cumulate. One explanation of this effect has been given, 

 based on the proposed formation of acetyl CoA by pyruvic 

 acid oxidation. Lipoic acid in its oxidized form is required 

 to accept the electrons in this oxidation. It was suggested 

 that while the light is on this cofactor is kept mostly in its 

 reduced state, dihydrolipoic acid. The reduced cofactor could 

 not promote pyruvic acid oxidation. When the light is turned 

 off and reducing power is no longer generated, the oxidized 

 form of lipoic acid would be made, and the oxidation lead- 

 ing to acetyl CoA would occur. Subsequent reactions, via the 

 glyoxylate cycle, would then produce citric and glutamic 

 acids. 



However, if acetyl phosphate is formed by phosphoke- 

 tolase during photosynthesis, a different explanation can be 

 made. If we suppose that acetyl phosphate is still formed via 

 phosphoketolase just after turning off the light, it will tend to 

 accumulate. No reducing power or ATP is available for syn- 

 thesis of fatty acids in the dark inside the chloroplasts. There- 

 fore, acetyl phosphate will break down to free acetate, which 

 will diffuse out of the chloroplast into the cytoplasm. There 

 it will be used, via the glyoxylate cycle, in the synthesis of 

 glutamic acid (62). 



48 



