KINETICS 1611 



0-Phenanthroline. Warburg and Luttgens (1946) found a strong inhi- 

 bition of the photoreduction of quinone by o-phenanthroline, a compound 

 that, similarly to cyanide, forms complexes with heavy metals, and also 

 inhibits photosynthesis (c/. Vol. I, page 319) (50% inhibition by 4.3 X 

 10-« mole/liter, 100% inhibition by 8.5 X 10"^ mole/liter). Since Zn+2 

 ions form complexes with phenanthroline, Warburg and Luttgens suggested 

 that an enzyme containing zinc as active metal may be involved in the 

 Hill reaction, and thus probably also in photosynthesis. The phenanthro- 

 line inhibition could be cured by the addition of zinc sulfate. The effect 

 of phenanthroline on the quinone reaction was confirmed by Aronoff (1946) 

 and Arnon and Whatley (1949). The latter also confirmed (1949^) the 

 reversal of the phenanthroline inhibition by zinc ions, but showed that the 

 same effect could be produced by ferrous ions and copper ions (which are 

 required micro nutrients), as well as by nickel and cobalt ions (which are 

 not) . Added in 1:1 stoichiometric relation to phenanthroline, nickel and 

 cobalt produced a more effective reversal of inhibition than zinc ; the latter, 

 in turn, was more effective than copper or iron. The assumption that an 

 enzyme with a zinc ion in its prosthetic groups participates in photosynthe- 

 sis thus remains speculative. 



Wessels (1954) noted a difference in the effect of phenanthroline on the 

 Hill reaction with DCPI (almost complete inhibition by 6 X 10~^ mole/1.) 

 and with quinone (6-10 times more poison needed for the same effect). 

 He confirmed that the poisoning can be cured by Zn++, Co++ or Ni++ 

 ions, but not by Mn++, Ca++ or Mg++. The phenanthrohne effect seems 

 tc be due to its attachment to a specific spot in an enzyme (rather than to a 

 chelation of free zinc ions, as suggested by Warburg), because extracting 

 the chloroplasts with phenanthroline does not leave an impairment of 

 their activity after washing. Chelating agents, similar to phenanthroline 

 (a,Q:'-dipyridyl, 3,3'-dimethyl-2,2'-dipyridyl), do not inhibit the Hill reac- 

 tion, while the strong chelating agent "complexon" (ethylenediamine- 

 tetraacetic acid) even stimulate it. 



o-Phenanthroline inhibits the methemoglobin reduction by crude chloro- 

 plast suspensions (50% inhibition at about 5 X 10"* mole/1.). 



Carbon Monoxide. Warburg and Luttgens (1946) found that the qui- 

 none reaction is not inhibited by carbon monoxide (indicating the absence of 

 iron-porphyrin enzymes in the enzymatic apparatus of the Hill reaction) . 



Narcotics. Hill and Scarisbrick (1941) and Warburg and Luttgens 

 (1946) noted the inhibition of the reduction of ferric oxalate or quinone by 

 phenylurethan (50% inhibition by 0.1 mg. in 1 cc. according to Warburg 

 and Luttgens — approximately the same effect as on photosynthesis in 

 Chlorella, cf. page 323, Vol. I). Aronoff (1946) obtamed only 60% inhibi- 

 tion of the same reaction by a saturated phenylurethan solution; and 



