540 PHOTOCHEMISTRY OF PIGMENTS IN VIVO CHAP. 19 



oxygen by photosynthesis. This explanation was suggested by Warburg 

 and Negelein, who thought that the acceleration of reaction (19.1) may 

 be caused by an increase in the permeability of the cells to HNO3 mole- 

 cules in Hght; one could, however, also think of (19.1) as a genuine 

 photochemical reaction — a chlorophyll-sensitized oxidation-reduction re- 

 action between an organic reductant and nitrate, analogous to the 

 "photoreduction" of carbon dioxide by organic hydrogen donors in 

 adapted algae and purple Athiorhodaceae. 



Another hypothesis, which leads to particularly interesting specula- 

 tions, is that the photochemical reaction is the sensitized reduction of 

 nitrate by water (and not by organic hydrogen donors), that is, a photo- 

 synthesis with nitrate substituted for carbon dioxide as reductant: 



light 



(19.2) (HN03)aci. + H2O > (NH3)aq. + 2 O2 - 98 kcal 



According to this hypothesis, oxygen is produced directly by "nitrate 

 photosynthesis," and not indirectly, by a superposition of ordinary 

 photosynthesis on light-stimulated "nitrate respiration." Of course, 

 reactions (19.1) and (19.2) may run concurrently, as two competitive 

 processes — similar to the photoreduction of carbon dioxide by hydrogen 

 and glucose (c/. Chapter 6, page 141). 



In the presence of phenylurethan, the rate of nitrate reduction in 

 light remains undiminished, but pure carbon dioxide is liberated (as in 

 the dark) instead of oxygen. One could suggest that this is an argument 

 in support of the two-step mechanism of nitrate reduction. The first 

 step, the Hght-stimulated nitrate respiration, may be as insensitive to 

 urethan as the corresponding dark reaction (c/. above) ; while the second 

 step is the urethan-inhibited ordinary photosynthesis. However, the 

 effect of urethan can also be explained on the basis of direct "nitrate pho- 

 tosynthesis" by assuming that urethan inhibits the last, oxygen-liberating 

 stage of reaction (19.2), and thus directs the process into an alternative 

 channel in which the primary photochemical oxidation product (desig- 

 nated by {OH} or Z in chapter 7) is reduced by available organic hydrogen 

 donors, instead of liberating oxygen from water. In other words, 

 urethan could convert "nitrate photosynthesis" into "nitrate photoxi- 

 dation" in the same manner in which it converts ordinary photosynthesis 

 into ordinary photoxidation (c/. Noack's experiments described on page 



528). 



More detailed experiments, with specific inhibitors of the type of 

 hydroxylamine, could help to analyze the mechanism of photochemical 

 nitrate reduction and establish its relation to ordinary photosynthesis. 

 Unfortunately, this subject has not received further attention since 1920, 

 although it is certainly worth renewed study. It is not clear whether 



