398 Marine Microbiology 



are present. The accumulation of the end product, NO3", does not 

 interfere with the nitrification process (2). Nitrite oxidation, 

 however, is markedly inhibited at alkaline pH by the initial sub- 

 strate of nitrification, namely ammonia, presumably because of 

 the shift in the ammonium-ammonia equilibrium towards am- 

 monia (2). These studies suggest the reason for nitrite accumu- 

 lation in soils of alkahne reaction following heavy applications 

 of ammonium fertilizers ( 3 ) . In striking contrast to extreme sensi- 

 tivity of intact Nitrobacter cells to low levels of ammonia, is the 

 fact that the cell-free nitrite oxidizing system is unaffected under 

 similar conditions (2, * * ) . 



The recent studies of Aleem and Alexander ( 1 ) , and Aleem and 

 Nason ( 4, 5, 6, * * ) have yielded considerable inf onnation about 

 the biochemistry of nitrite oxidation. Aleem and Alexander (1) 

 initially obtained an active cell-free nitrite oxidizing system 

 by sonic disintegration of Nitrobacter cells. The nitrite oxidizing 

 activity, which resides solely in the red particulate fraction, was 

 responsible for the stoichiometric conversion of nitrite to nitrate 

 as measured by nitrite disappearance, nitrate formation and O2- 

 uptake (Figs. 1 and 2). The enzyme possesses a relatively high 

 Michaelis constant for nitrite (3.3 X 10~^ M) and is uninhibited 

 even by concentrations as high as 8xlO"^M nitrite. The relatively 

 high substrate concentration necessary for optimal activity of the 

 cell-free nitrite oxidizing system is in striking contrast to the low 

 substi-ate-optimum (8 x IQ-' M) for the intact cells (1, 2, 28). 

 Indophenol, ferricyanide and menadione are enzymatically re- 

 duced accepting electrons either from NOi;" or DPNH* in the 

 presence of purified nitrite oxidase particles.** 



According to Aleem and Alexander (1), either ferric or fer- 

 rous ion stimulated nitrite oxidation by the cell-free extracts while 

 copper was found to be inhibitory. Subsequently a specific iron 

 requirement for nitrite oxidation was shown (4) by the unique 



* The following abbreviations are used: DPN and DPNH, oxidized and reduced 

 diphosphopyridinc nucleotide respectively; TPN TPNH, oxidized and reduced 

 triphosphopyridine nucleotide, respectively; FMN and FAD, flavin mono- 

 nucleotide and flavin adenine dinucleotide respectively. 



** unpublished data. 



