42 



NITROGEN METABOLISM 



stutzeri) or appreciable quantities of NgO {B. subtilis). Ex- 

 tracts of both organisms convert nitrite to nitric oxide and 

 nitric oxide to Ng [aifl]. Further advances in the elucidation 

 of the pathways of the biological reduction of nitrate await 

 the collection of more information concerning the chemical 

 properties of nitroxyl (HNO), hyponitrous acid (HgNgOg), 

 nitramide and other similar compounds of nitrogen, and 

 the development of unequivocal methods for their identifi- 

 cation and estimation. The scheme given below is based on 

 those proposed by Kluyver [cf. 30] and by Allen and van 

 Niel: it will be realized that there is no direct evidence that 

 nitroxyl or hyponitrous acid or nitramide is a natural 

 intermediate. 



(R.NHo) 



(NO2.NH2) 

 t 

 N2O 



HNOa 



HNO, 



N, 



>NH3 



(HNO) 



(H2N2O2) 



-^N^O 



Hypothetical Pathways of Nitrate Reduction in Micro-organisms 



Routes 'a' and *6' may be operative in Ps. denitrificans, 

 and 'c' in the aerobic spore-forming bacilli [30]. Allen and 

 van Niel postulate that in Ps. stutzeri^ NO"^ enters into 

 organic combination and is reduced to an amino compound 

 (R.NH2) which then reacts with another substance, perhaps 

 NO"^ itself, to yield nitramide (route 'd'). 



The presence of oxygen tends to suppress the reduction 

 of no's, the degree of inhibition being determined both by 

 the partial pressure of Og and the organism concerned, e.g. 

 Esch. coli [29] is more sensitive than Ps. denitrificans [26]. 

 It is interesting to note that restricted aeration of cultures 

 of the denitrifying bacilli resulted in the production of large 

 amounts of NH3 , even by those strains which in anaerobic 



