254 E. KESSLER 



(19). A concentration of 2 X 10"^ molar DNP at pH 6.3 decreases 

 nitrite reduction in the dark by about 85%, whereas the reaction in 

 the light is inhibited b\- onlj' 159o. 



It is of interest in this connection that (1) glucose assimilation 

 in the light, a process dependent upon ATP formed by photosyn- 

 thetic phosphorylation, attains saturation at low light intensity (20) ; 

 (2) phosphorylation in the light seems to be less sensitive toward 

 DXP than is phosphorylation coupled to respiration (21): (3) ATP 

 formation in the light is only slightly decreased at low temperature 

 (22\ In these three properties photosjmthetic phosphorylation (23) 

 and nitrite reduction in the light show a striking similarity. 



Since the reduction of nitrite to ammonia, in contrast to the reac- 

 tion nitrate -* nitrite, seems to require ATP (19), the effect of light 

 on nitrite reduction might be due to a supply of energy-rich phos- 

 phate bonds by means of photos\Tithetic phosphon-'lation. This 

 possibihty has recently been discussed also by Stoy (2-4). 



Discussion 



Pirson: You have shown that it is possible to separate photos}-n thesis from 

 nitrite reduction by lower temperatures. Would nitrite reduction be a pure photo- 

 chemical process without any enzjTnes? 



Kessler: No, I think that possibility is completely excluded. If this were the 

 case, one should assmne that cells in the light with no CO2 should reduce very 

 large amounts of nitrite without any decrease in rate of reduction. 



Pirson : You must assume that the temperature coefficients of the photosjTi- 

 thetic enz\-mes and those Involved in nitrite reduction are completely different. 



M. B. Allen : I would like to ask Dr. Kessler if he has any idea how the carbo- 

 hydrate might affect nitrate reduction. 



Kessler : It might be that it acts as a hydrogen donor, that the reduction of 

 nitrate is somehow connected with the o.xidation of carbohydrate. This is prob- 

 ably the simplest explanation. 



M. B. Allen: I find this result rather difficult to reconcile with the experiment 

 of van Niel and myself and the demonstration of Evans and Nason that one can 

 reduce nucleotides with chloroplasts, add these to nitrate-reducing enzjTnes, and 

 get nitrate reduction. 



Kessler: The results of Evans and Nason on photochemical nitrate reduction 

 are, according to my opinion, not conclusive. They added to the chloroplasts 

 nitrate reductase and TPN. However, it is not necessarily so that these substances 

 must also be involved in light nitrate reduction in the living cell. And the fact 

 remains that nitrate is not reduced by our algae in the absence of CO2, whereas 

 nitrite is. If one assumes this Evans-Nason type of reduction, then one should 

 expect that nitrate will be readily reduced by the cell.-, but this does not occur. 

 Only nitrite will be reduced, which is not reduced by the Evans-Nason system. 



