220 H. TAMIYA, S. MIYACHI, T. HIROKAWA 



algae, viz., (1) CO2, (^) oxidants such as quinone or oxygen, and (5) 

 cyanide. It is unlikely that these agents of entirely different nature 

 would act upon R by the same chemical mechanism. The paradoxical 

 facts can, however, receive a coherent explanation if we make the 

 following assumptions: 



a. R is a reducing agent which is formed photochemically from 

 its precursor P (an oxidized form of R) by a reaction accompanied by 

 a liberation of oxygen, such as: 



p _f. H2O _J!f!^ R (or PH2) + V2O2 (I) 



h. R reacts with oxygen in the manner : 



/e + O2 -^ p + H2O2 (II) 



followed by the catalytic decomposition of H2O2: 



catalase „ , „ /txx\ 



H2O2 > H2O + V2O2 (HI) 



and, in so far as catalase functions normally, the latter process oc- 

 curs much faster than the former, so that the overall reaction pro- 

 ceeds according to: 



R + V2O2 -> P + H2O (IV) 



c. R reacts also with other oxidizing agents such as quinone (Q) 

 and H2O2 in the manner: 



R+Q-*P + QB.t (V) 



R + H2O2 -*P + 2H2O (VI) 



d. In the presence of CO2, R is involved in the following sequence 

 of reactions which were postulated by Bassham, Benson et at. (2) :* 



CO2 + RDP -* 2PGA (VII) 



2PGA + 2/2 ^ 2TP + 2P (VIII) 



2TP > V5RDP (IX) 



V5RDP + V6CO2 -* VsPGA (VII) 



and among these reactions (VIII) is the rate-determining step,t 

 so that the overall reaction proceeds according to: 



R + VsCO. -^ P + VaPGA (X) 



* Abbreviations used: RDP for ribulose-diphosphate, PGA for phospho- 

 glyceric acid, and TP for triose-phosphate. 



t Relative rates of these reactions are assumed to be in the order: (VII) > 

 (IX) > (VIII). 



