452 IX. HEMATIN ENZYMES, II 



and (in low concentrations) do not inhibit photosynthesis, prevent 

 the adaptation of the algae to the "reduced state" (972). 



The simplest explanation would be that a hematin enzyme, acting as 

 photocatalase, acts as hydrogenase when reduced to the ferrous state; when 

 the photocatalase is inactivated, the hydrogenase becomes active, and con- 

 versely. Inhibitors which inhibit photocatalase also inhibit adaptation; both 

 photocatalase and hydrogenase are inhibited by o-phenanthroline. The 

 photocatalase of higher plants, however, is not reduced to a hydrogenase nor 

 is the hydrogenase of purple bacteria oxidizable to a photocatalase. Other 

 observations {cf. 9^0,972-975,2198) also indicate that the photocatalase and 

 hydrogenase of the algae are independent catalysts, not only differing in the 

 valency of their iron. According to Gaffron {97Jf.,972) and Rabinowitch 

 {2198), the photocatalase is transformed by anaerobiosis into an enzyme 

 which, acting as peroxidase or oxidase, plays a role in the oxidation of hydro- 

 genase to the inactive ferri-state (deadaptation). 



The discussion on the mode of action of catalase in Section 4. makes it 

 indeed likely that the reduction of a hydrogen donor group in the protein 

 part of photocatalase might convert this enzyme into a peroxidase, or, if 

 combined with reduction of its iron to the ferrous state, into an oxidase. It 

 may well be that the dehydrogenation of this group is one of the series of 

 dehydrogenation reactions which are now postulated in the photosynthetic 

 process (cf. Rabinowitch, 2198). This would also explain the fact observed 

 by Weller and Franck {9Ifl,3029) that hydroxylamine inhibits photosynthesis 

 even in weak light, when the dark reaction is not normally the limiting 

 factor. Activation of the photocatalase by illumination was assumed by 

 these workers as a possible explanation of the phenomenon; the conversion 

 of the XHj into an X group by the photosynthetic chain of reactions may be 

 the way in which this occurs. 



