460 J. A. BASSHAM, M. KIRK 



These results are summarized and compared with steady state Expt. 18 in Table VI. 

 Though not shown in the table, the maximum rate of appearance of ^*C in observable 

 compounds derived from the carboxylation reaction leading to PGA (the carbon 

 reduction cycle) was 70 to 90 % of the externally measured rate of i*C uptake. 



DISCUSSION 



When Calvin and Massini" reported the formation of PGA in an overall reaction 

 requiring ribulose diphosphate and CO 2 they proposed that the reaction in the light 

 gave one molecule of PGA and one of triose phosphate but in the dark gave two 

 molecules of PGA. Wilson^* discussed this possibility further after it was realized 

 that the carboxylation did not involve an intermediate splitting of the ribulose to 

 triose and diose. The dark reaction in whole plants'^ and the reaction in isolated 

 enzyme systems^*-!' was found to give rise to two PGA molecules. Also, it is clear 

 from previous kinetic studies^- ^^ of carbon fixation during photosynthesis that the ^^C 

 entering the carbon reduction cycle via the ribulose carboxylation passes through 

 the carboxyl group of PGA initially. This is consistent with the fact, established for 

 the isolated enzyme system by Horecker'*, that the CO 2 is bonded to the number 

 two carbon atom of ribulose diphosphate. More recently Park^' has shown by means 

 of inhibition studies in broken spinach chloroplasts that "C entering that system 

 must pass through PGA. That is, PGA is a biochemical intermediate compound — not 

 merely a compound formed by thermal breakdown after the plant is killed. 



We shall present here an argument, based on kinetic data, which indicates that 

 the carboxylation of RuDP in vivo during photosynthesis gives rise to only one 

 molecule of 3-PGA. 



If the i*C which has just entered PGA from "CO2 is subtracted from the total "C 

 in PGA, the i*C in the remaining carbon atoms of the PGA must all be derived 

 from ribulose diphosphate. 



Let us consider the two reactions : 



The position of the '*C which has just entered the cycle as "CO^ is indicated by the 

 asterisks. In reaction D, there are five remaining carbon atoms of PGA (numbers i 

 to 5) which must be derived from RuDP, while in reaction L there are two such 

 "residual" carbon atoms (numbers i and 2). The steady state concentration of PGA 

 in steady state Expt. 18 is 3.0 /xmoles of carbon/ml algae, hence the carboxyl carbon 

 concentration is i!o jumole of carbon. However, if reaction D is correct, only one-half 



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