582 LIGHT AND LIFE 



tionally unable to accomplish oxygen evolution, and it seems that 

 the same general considerations discussed here will explain their re- 

 actions to the introduction of oxygen. 



An advantage that we have gained from these experiments is that 

 of an added criterion to distinguish a cyclic electron transport se- 

 quence (eq. 1 , 2, and ■/) from an oxygen exchange reaction (eq. 1 , 2, 

 3, and 5) . The cycle shotdd be almost entirely resistant to inhibition 

 by CMU, although, as we have seen, it may take highly anaerobic 

 conditions for the full extent of the resistance to appear. Of the 

 previous criteria, the oxygen exchange between water and O2 is 

 difficult to measure routinely. The oxygen dependency of the ex- 

 change sequence might not be infallible if the oxygen produced in 

 reaction 3 could be used very efficiently in reaction 5 [23]. 



We have been able to use these criteria for an analysis of the 

 "endogenous" phosphorylation, i.e., the small amount of light-depen- 

 dent ATP formation that occurs without any added redox cofactors 

 (manuscript in preparation) . We found that this endogenous phos- 

 phorylation is oxygen-dependent and inhibited by CMU, and there- 

 fore cannot be a cyclic sequence of reactions 1, 2, and 4. Actually, 

 in this case a net uptake of oxygen is observed (Table 2) and indi- 

 cates that the reaction sequence is of the type discovered by Mehler 

 (21) , in which reaction 5 is replaced by ^a: 



^H. + Oo -^ H.Oo + A (5rt) 



The full sequence here would be reactions /, 2, 3, and 5a. Since one 

 mole of oxygen is taken up in 5a, and only y% mole evolved in re- 

 action 3, a net consumption of oxygen is observable. 



It now becomes a matter of some interest that the full sequence, 

 involving both oxygen uptake and ATP formation, is very considera- 

 bly stimulated by the addition of photosynthetic pyridine nucleotide 

 reductase (PPNR) (Table 2) . This enzyme is thus seen to function 

 in the reduction of oxygen as well as TPN and heme proteins (26) . 



Adding relatively small amoiuits of ascorbic acid to the system with 

 PPNR, but none of the usual cofactors, stimulates ATP formation 

 and either has no effect or in many cases inhibits the oxygen uptake 

 (Table 3) . We shoidd emphasize that this effect can be found with 

 .0005 M ascorbate, and indeed the higher concentrations of ascorbate 

 used previously (28, 29) often tend to increase oxygen uptake. Since 

 the observed oxygen uptake does not increase upon the addition of 

 ascorbate, it seemed possible that a cyclic sequence {1 , 2, and -/) might 

 have been set up. Again, this could be ruled out because even with 



