CHEMICAL KINETICS OF THE HILL REACTION '.]S7 



rate-limiting step of the dark sequence of steps under the ordinary- 

 conditions used for Hill reaction studies. Thus, deuterium oxide 

 seems to act by irreversibl}^ removing or denaturing some unknown 

 hidden participant. A similar situation may exist for Chlorella photo- 

 synthesis, where no influence of deuterium oxide on the activation 

 energy was found at high light intensities (10). The comparison here 

 is interesting, since photosynthesis frequently appears to be limited at 

 high light intensities by different slow steps from those in^'ol^'ed in the 

 Hill reaction. We must, therefore, conclude that there is little evidence 

 at present which allows one to fix any elementary reaction of the over- 

 all chain as the one at which water enters. 



EFFECTS OF FLASHING LIGHT ON THE HILL REACTION 



The kinetic studies in this laboratory by Gilmour on the behavior 

 of the Hill reaction of isolated chloroplasts in flashing Hght have been 

 briefly reported elsewhere (13). This work demonstrated a more com- 

 plex situation than was observed by Emerson and Arnold (14) for 

 Chlorella photosynthesis and in so doing established the method as one 

 of considerable power for studying the Hill reaction. The results re- 

 sembled quahtativel}^, though not quantitatively, those of Tamiya 

 and Chiba (15). A quantitative reconsideration of Gilmour's observa- 

 tions shows that they do not unequivocally support the interpreta- 

 tions previously made (13). Unfortunately, the experimental condi- 

 tions under which our data were obtained did not permit a determina- 

 tion of several important facts, such as the dark time required for the 

 ultimate maximum yield per flash. In many ways, and in fact wher- 

 ever a comparison has been possible, Gilmour's data support the 

 kinetic interpretation Kok has made of his flashing-light studies on 

 Chlorella (see B. Kok's paper in this volume). The comparison is so 

 close as to suggest that only quantitative differences due to different 

 values of the rate constants of the two systems are involved. It must 

 be noted, however, that both the steady-state velocity and the limit- 

 ing step at high light intensity in Kok's experiments were sensitive to 

 cyanide in low concentration, whereas we have not yet found any 

 steps in the Hill reactions which are highly sensitive to cyanide (16). 



INHIBITORS 



Inliibiliou studies of photosynthesis have not yet reached the 

 usefulness manifested in enzyme chemistry — a conseciuence of the 



