DAI JD IF. KROGMANN 617 



Although the oxygen exc hange reaction is not dependent on photo- 

 phosphoiylation, it is ol sufficient magnitude to be stoichiometric 

 with the oxygen-dependent photophosphorylation. The rate of thirty 

 to fifty microatoms of oxygen consumed and evolved per milligram 

 of chlorophyll per hour is to be compared to a rate of sixty to one 

 hundred micromoles of phosphate esterified per milligram of chloro- 

 phyll per hour in a reaction shown to have a P/O ratio of two. 

 It would appear that phosphorylation is dependent upon an electron 

 transport process, but that the electron transport process is not 

 obligately coupled to phosphorylation. 



Hydrogen Peroxjde as an Intermediate in Oxygen Exchange 



An oxygen exchange reaction by illuminated chloroplasts is by 

 no means a new observation. Mehler has described a type of Hill 

 reaction in which the photochemically generated reducing power of 

 chloroplasts is used to reduce atmospheric oxygen to hydrogen per- 

 oxide (6) . If catalase is present, the hydrogen peroxide is decom- 

 posed to water and oxygen. Under these circumstances, oxygen pro- 

 duction equals oxygen consumption. We found sufficient catalase in 

 the spinach chloroplast preparations to decompose hydrogen peroxide 

 at a rate commensurate with the rate of oxygen exchange observed in 

 the mass spectrometer experiments. Thus it seemed reasonable that 

 the dye-catalyzed oxygen exchange and even the dye-catalyzed oxida- 

 tive phosphorylation might be attributable to a Mehler reaction. 



Mehler had devised an experiment in which the light-generated 

 hydrogen peroxide Avas trapped by the peroxidative activity of catalase 

 in the oxidation of ethanol to acetaldehyde. This reaction sequence 

 is outlined in Fig. 2. Under these circumstances, the reaction could 

 be observed by net oxygen consumption or by the appearance of 



Mehler Reaction 



2H2O > 2[h] + 2[0H] 



2 [oh] > HgO + 1/2 O2 



2[h] + 02 ^ H2O2 



H202+ethanol ^^^^'^^^ > acetaldehyde+H20 



1/2 02+ethanol— py-^ ocetaldehyde+H20 

 Fig 2. The Mehler reaction in the presence of catalase and ethanol. 



