Daniel I. Anion 337 



a catalyst in the oxygen evolution. They analyzed for the inorganic 

 constituents of chloroplasts including iron and zinc and, on the basis 

 of the amount of zinc found and the amount of inhibitor added, con- 

 cluded that zinc was the metal concerned in the oxygen evolution re- 

 action of chloroplasts. 



The problem of o-phenanthroline inhibition of the oxygen-evolution 

 reaction has been independently investigated in our laboratory as part 

 of a general study of chloroplast reactions (12). In agreement with 

 Warburg and Liittgens, we found that o-phenanthroline inhibition was 

 fully reversed by an excess of zinc and much less effectively by iron. 

 Zinc was also very effective in reversing the inhibition even when added 

 not in a tenfold excess but in a stoichiometric amount in relation to 

 o-phenanthroline (Figure 4). However, when added in stoichiometric 

 amounts, two other metals not known to be essential for plant growth, 

 nickel and cobalt, were found to be even more effective in reversing 

 o-phenanthroline inhibition. In our experience these two metals not 

 only protected by binding the o-phenanthroline before it was mixed 

 with the chloroplasts as observed by Warburg and Liittgens (52), but 

 also reactivated the previously inhibited preparations in a manner simi- 

 lar to zinc (Figure 4). 



Reactivation was also obtained with copper, a micronutrient of estab- 

 lished status, known to occur in chloroplasts. The reactivating efficiency 

 of copper was, on the basis of stoichiometric amounts, intermediate be- 

 tween iron and zinc. Unlike iron,* however, an increase in the concen- 

 tration of copper gave total reactivation in a manner similar to zinc. 

 As seen in Figure 4, doubling the concentration of copper brought 

 about a marked increase in its effectiveness. 



It was found then that the reversal of o-phenanthroline inhibition 

 was accomplished by two elements, nickel and cobalt, not known to 

 be essential for plant life and by two, copper and zinc, recognized as 

 micronutrients. It is considered unwarranted, on the basis of evidence 

 now available, to associate any one metal with the photochemical evo- 

 lution of oxygen. Deductions on the basis of composition of plant tis- 

 sues appear to be, in the light of previous discussion, not wholly reliable. 



*After this manuscript was submitted for publication, new evidence on this 

 point became available. It will be published elsewhere. 



