ones unless there are unknown pathways utilizing the same 

 intermediate compounds. 



A kinetic analysis of the appearance of C^'' in PGA 

 and RuDP in this experiment indicated that the carboxy- 

 lation reaction results in the formation of only one free mole- 

 cule of PGA per molecule of CO2 entering the cycle. The 

 kinetic analysis cannot say what the other three-carbon 

 fragment would be. It might be merely a molecule of PGA 

 bound in some way so that its labeling remains distinct from 

 that of the PGA from the other half of the six-carbon addi- 

 tion product. The only other compounds that seem to satisfy 

 the kinetic requirements and that could readily result from 

 the splitting of the six-carbon addition product are the triose 

 phosphates. The formation of a molecule of triose phosphate 

 in this way would require a reductive split of the addition 

 product, as indicated by the dashed line in Figure 2. 



That such a pathway differing from the in vitro reaction 

 may exist seems entirely reasonable, since the enzymes of the 

 carbon reduction cycle appear to be closely associated with 

 the molecular structures in which the TPNH is formed in 

 the chloroplast (21). In the intact plant the carboxylation 

 enzyme, as well as the enzyme responsible for the splitting of 

 the product and the enzyme that brings about the reduction 

 of TPN+ to TPNH, might be part of a structurally organ- 

 ized system. In fact, if a reductive scission does occur, the 

 reducing agent could be a substance formed from the oxida- 

 tion of water and preceding TPNH in the electron transport 

 chain. This substance might never be available in sufficient 

 concentration to be a factor in in vitro systems in which 

 carboxydismutase is coupled with isolated or broken chloro- 

 plasts. Such an explanation of the experimentally observed 

 kinetic result is purely hypothetical. We mention it to focus 

 attention on the possibility that a given biosynthetic pathway 

 may follow a different course in an intact cell than that which 

 would be predicted on the basis of studies with fragmented 

 cells or enzymes alone. 



23 



