CAEBOXYLATION AND THE {CO2} COMPLEX 203 



10% of the absorbed activity. The uptake is an enzymatic process; 

 it is brought almost to a standstill (more exactly, to 0.3% of its normal 

 rate) by the presence of IQ-^ mole per liter of potassium cyanide. 



We referred to the saturation in figure 21 as "apparent" because of 

 the observation that the maximum uptake can be further increased by 

 alternative evacuation of the cell suspension and its exposure to radio- 

 active carbon dioxide. The maximum absorption obtained in this way 

 is about 0.03 mole per liter of cell volume. Ruben and coworkers 

 assumed that figure 21 represents the rate of association of labeled 

 carbon dioxide with the acceptor which was "denuded" of carbon 

 dioxide by evacuation. If the stabihty of the {CO2} complex is such 

 that it is only partially decomposed (decarboxylated) by evacuation, 

 then rapid carboxylation with labeled carbon dioxide will extend only 

 to the decarboxylated fraction. After this fraction has been recar- 

 boxylated, further entrance of radioactive C*02 can occur only by 

 exchange with the ordinary CO2 already present in the complex, and this 

 may be a much slower process. A second evacuation will decompose 

 another batch of {CO2} complexes and leave them bare for reoccupation 

 by labeled C*02, and so forth, until a uniform distribution of C* between 

 the gaseous phase and the {CO2} complex is reached. (However, this 

 explanation requires that the same total absorption of C*02 could also be 

 obtained by waiting under a stationary C*02 atmosphere for the same 

 period of time which was employed in the evacuations.) 



If this interpretation is correct, it means that the complex {CO2} has 

 a finite dissociation pressure — a conclusion which was mentioned once 

 before (page 201). 



One remarkable feature of figure 21 is the slowness of the C*02 up- 

 take. The "pickup" observations, to be described later in this chapter, 

 show that when, after a period of photosynthesis, the carbon dioxide 

 acceptor finds itself "denuded" of CO2, the regeneration of the complex 

 {CO2} is completed within 10 or 20 seconds; while the uptake of C*02 

 requires a whole hour. We will encounter a similar situation later when 

 speaking of the slow rate of reabsorption of the carbon dioxide hberated 

 in the "gush" of this gas which was observed by Emerson and Lewis 

 during the induction period of photosynthesis in Chlorella. If we insist 

 in attributing the dark C*02 uptake, the "pickup," and the CO2 absorp- 

 tion after the "gush" to one and the same chemical reaction — formation 

 of the complex {CO2}— we must assume that, in the first and last case, 

 this process is slowed down, either by the inactivation of the carboxylating 

 catalyst, Ea, in the dark, or by the inactivation of the acceptor— and 

 that only in the second case does the carboxylation proceed at its full 

 rate (meaning by this the rate which must be maintained in the steady 

 state of photosynthesis in intense light). 



