641 



J. A. Bassham 



This ^^C labeling curve of sugar phosphates suggests that there is a mech- 

 anism which allows only a small amount of reoxidation of the newly photosyn- 

 thesized sugar phosphates and then blocks further oxidation of these sugar 

 phosphates by triose phosphate dehydrogenase type reactions. Prom the usual 

 ccncept of the PSCR cycle in which PGA is reduced by triose phosphate dehydro- 

 genase in the presence of ATP and NADPH, one might have predicted that once 

 these cofactors were exhausted in the dark, the triose phosphate dehydrogenase 

 reaction would be reversed and the newly formed sugar phosphates rapidly 

 oxidized. Thus the result obtained here suggests once again the possibility 

 that phosphoglyceraldehyde in the PSCR cycle exists only in an enzyme-bound 

 form which cannot readily be oxidized. This hypothesis would require a coro- 

 llary - the formation of the bound phosphoglyceraldehyde directly from an 

 intemediate in the carboxylation reaction rather than from reduction of PGA. 



Another interesting point seen in the preillumination study is the rapid 

 rate of labeling of sugar monophosphates between 5 and 10 seconds. This rate is 

 equal in magnitude to that of the labeling of PGA despite the fact that during 

 this period the PGA carboxyl group in all probability can be no more than 20 - 

 30% saturated. (2) r^^^ rapid labeling of sugar monophosphates at a time when 

 the PGA pool is only partly labeled appears to be rather direct evidence for 

 the reduction of seme bound form of newly incorporated 1^^002 which is not in 

 equilibrium with the PGA pool. 



I have mentioned several bits of evidence which suggest the possibility that 

 a carboxylation intermediate proceeding PGA might be converted directly to the 

 sugar phosphates without equilibration of label between the intennediate com- 

 pound and PGA. From the standpoint of chemical possibilities the most likely 

 reaction for the acconplishment of this direct conversion would sean to be a 

 reductive carboxylation reaction. However, the in vitro enzyme systen for car- 

 boxylation of ribulose diphosphate accomplishes only the non-reductive dismuta- 

 tion to give two molecules of PGA. There is to date no enzymic evidence what- 

 ever for a reductive carboxylation. One must suppose that If such a reaction 

 exists, it is mediated by some organized or multifunctional enzyme system 

 which is most difficult to isolate intact fran the living system. 



In looking for reasons why such a system might be difficult to isolate, one 

 could propose that the system is particulate or an enzyme of high molecular 

 weight, and that the system is easily disrupted into soluble enzymes of smaller 

 molecular weight which lack the necessary organization. A more plausible reason 

 would be that in the in vivo system there is some direct link between the photo- 

 chemical reactions which produce ATP and electrois, and the carbon reducing 

 system. This link might then be lost when the system is isolated. We know that 

 the green lamellar structures which carry out the photochemical reactions can 

 be rather easily separated from the soluble carbon fixing enzymes (18). 



When phosphopyridine nucleotide reductase (PPrJR), discovered by San Pietro 

 (19,20) J was shown by Tagawa and Amon (21) to be a non-heme iron protein of 

 the ferredoxin type (22,23) it became a good candidate as the link between the 

 photochemical apparatus and the carbon photosynthetic apparatus of photosyn- 

 thesis. It iias been generally supposed thiat chloroplast ferredoxin or FPl^l 

 mediates the transfer of electrons from the light reaction to NADPH, with the 

 latter cofactor then being used to bring about the reductions of the carbon 

 reduction cycle. However, ferredoxin is a stronger reducing agent tiian fJADPH, 

 being comparable to tl2. If it were used directly in the carbon reducing 

 reactions, only one molecule of ATP per two electrons of reducing agent would 

 be required ratiier than one and one half molecules of ATP per two electrons as 

 would be the case with IJADPH as sole reducing cofactor (5,2'i-;. i iiave proposed 



