BIOCHIMICA ET BIOPHYSICA ACTA 447 



DYNAMICS OF THE PHOTOSYNTHESIS OF CARBON COMPOUNDS 

 I. CARBOXYLATION REACTIONS 



J. A. BASSHAM and MARTHA KIRK 



Lawrence Radiation Laboratory, University of California, Berkeley, Calif. {U.S.A.) 



(Received January 30th, i960) 



SUMMARY 



Kinetic studies have been made of the rates of appearance of ^*C in individual com- 

 pounds formed by Chlorella pyrenoidosa during steady state photosynthesis with 

 "COjj. These rates have been compared with rates of COj and ^*C disappearance from 

 the gas phase during the same experiments. 

 The following results were obtained : 



1. After the first few seconds, the rate of appearance of ^*C in compounds stable 

 to drying on planchets at room temperature is 95 to 100 % of the rate of uptake of 

 carbon from the gas phase. 



2. After the first few seconds, the rate of appearance of carbon in compounds 

 isolable by usual methods of paper chromatography constitutes at least 73 to 88 % 

 of the rate of uptake of carbon from the gas phase. Compounds formed from the 

 carbon reduction cycle via the carboxylation of ribulose diphosphate account for a 

 least 70 to 85 % of the uptake, while carboxylation of phosphoenolpyruvic acid 

 appears to account for at least another 3 %. 



3. The induction period in the appearance of ^*C in stable compounds may be 

 due to a reservoir of intracellular COj and HCO3 or to some other volatile or unstable 

 compound. If so, this reservoir contains no more than 1.5 )umoles of carbon, corre- 

 sponding to about 7 sec carbon fixation in the experiment in which it was measured. 



4. No other carboxylation reactions, such as the carboxylation of y-aminobutyric 

 acid, could be observed. The rate of labeling of glutamic acid after 5 min of exposure 

 of the algae to i*CO, reached a maximum rate of about 5 % of the total uptake rate, 

 but this labehng appears to be due to conversion of labeled intermediates formed 

 from the carbon reduction cycle or phosphoenolpyruvic acid carboxylation. 



5. The in vivo carboxylation of ribulose diphosphate in the light appears to be 

 followed by conversion of the product to one molecule of phosphoglyceric acid, 

 containing the newly incorporated ^^COj and one molecule of some other (kinetically 

 distinguishable) three carbon compound. This reaction would be different from the 

 one reported for the isolated enzyme system and the in vivo reaction in the dark, 

 which produces two molecules of 3-phosphoglyceric acid. 



Abbreviations: PGA or 3-PGA, 3-phosphoglyceric acid; PEPA, phosphoenolpyruvic acid; 

 RuDP, ribulose 1,5-diphosphate; ATP, adenosine triphosphate; TPNH, reduced triphospho- 

 pyridine nucleotide. 



103 



