Metabolic Pathways of Bacterial Nitrification 395 



18 ) . This has been ascribed to some modification in the cell mem- 

 brane whereby the ammonia oxidizing portion of the reaction se- 

 quence is more adversely affected than the hydroxylamine oxidiz- 

 ing system which remains intact (29). 



Nitrosoinonas sps. have always been regarded as strict chemo- 

 autotrophs metabolizing only the inorganic ammonium salts. 

 According to a recent report by Ruban (36), these bacteria also 

 possess specific deaminases which enable them to utilize the 

 amino groups of several purine derivatives such as guanine, inic 

 acid and allantoin. The amino acid alanine, however, was not 

 metabolized unless it was pretreated with Fseudomonas auto- 

 lyzates for its deamination. 



Thus far ammonia oxidation at the cell-free level has not 

 been accomplished in spite of innumerable efforts by various 

 workers (*, 24, 25). Although Imshenetskii and his school (24, 

 25) claim to have demonstrated the oxidation of ammonia and 

 hydroxylamine by Nitrosomouas autolysates, their data hardly 

 support a positive conclusion. They prepared autolysates by 

 shaking Nitrosomouas cells with glass powder for twenty-four 

 hours at 40° C, and performed their experiments at 37° C with 

 incubation periods ranging as high as five days under conditions 

 which favored the complete denaturation of the metabolizing 

 enzymes. No precautions were indicated to eliminate unbroken 

 cells or contamination by other microorganisms. The transforma- 

 tion of ammonia to nitrite taken as an average of fifteen experi- 

 ments, gave inconsistent results and insignificant amounts of ni- 

 trite (in the order of 0.54 ppm. NO-r-N per 5 days). According 

 to the recent studies of Anderson (9), cell-free Nitrosomorms 

 extracts failed to oxidize ammonia because its oxidation to hy- 

 droxylamine is an endergonic reaction (Af = 4.7 K.Cal.) which 

 presumably requires energy-rich phosphate for its activation. The 

 procedures for preparing cell-free extracts might have disrupted 

 the energy coupling mechanisms. It would be worthwhile to 

 determine whether ammonia oxidation is catalysed by cell-free 

 Nitrosomorms extracts in the presence of added or generated 

 nucleoside tri-phosphates. 



* unpublished data. 



