1912 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



able in time for inclusion in the pertinent discussions in chapters 32 and 

 35. 



(h) Oxygeri 



Inhibition hy Excess Oxygen. Tamiya and Huzisige (1949) again 

 studied the inhibiting effect of high oxygen tension on photosynthesis 

 (c/. chapter 13, section A). Using Chlorella ellipsoidea in white light of 

 25 klux, at 25° C, they found fig. 37D.9 for the rate as function of [O2], 

 with [CO2] as parameter. The curves could be represented by the quad- 

 ratic equation : 

 ,„P,^, P (inhibited) _ [0.,]' 



The constant K increased with [CO2], but appeared to be independent 

 of temperature (4-25° C). The oxygen inhibition was reversible. As 

 mentioned on p. 316, carbon monoxide has no effect on photosynthesis; 

 Tamiya and Huzisige therefore believed that the oxygen effect cannot be 

 attributed to reversible binding of oxygen by a carrier of the type of hemo- 

 globin. Oxygen- — in contrast to cyanide — had no effect on the catalase 

 activity of Chlorella. 



Using the method by which Matsuyama and Hirano (1944) had deter- 

 mined the reduction state of ascorbic acid in Chlorella, Tamiya and Hu- 

 zisige found that this state is not affected by changes in oxygen tension. 



Tamiya and Huzisige interpreted the oxygen effect by assuming a 

 reversible association of oxygen with the "Ruben factor" (as they called the 

 carboxylation substrate, through which carbon dioxide enters the photo- 

 synthetic mechanism). To account for the quadratic form of equation 

 (37D.3), the equilibrium was assumed to be: 

 (37D.4) 2O2 + A , A(02)2 



Tamiya and Huzisige found the oxygen inhibition to be "competitive" 

 with cyanide inhibition (c/. section (o) above). They assumed that cya- 

 nide does not affect the primary carboxylation step, but inhibits a follow-up 

 reaction, by which the primary carboxjdated product (designated by us as 

 RCOOH, or ACO2) is transformed into the substrate of photoreduction 

 (perhaps phosphoglyceric acid). They argued that the "competitive" be- 

 havior of the two inhibitors (KCN and O2) does not require that they act on 

 the same reaction, but can be understood also if they affect two different 

 links in a catenary reaction series. They derived equations for the separate 

 and combined action of the two inhibitors, which could reproduce satisfac- 

 torily the experimental results. (That the problem of the primary cyanide- 

 sensitive factor in photosynthesis is still open was stated above under (a).) 



From the point of view of Tamiya and Huzisige 's hypothesis, excess 



