NITRITE AND SULFITE METABOLISM 451 



(Butt and Lees, 1960). However, the effect of cyanate is markedly depen- 

 dent on the oxygen concentration (see following tabulation), so that at 

 low oxygen levels inhibition is reversed to stimulation. It was suggested 



that a membrane transport system brings nitrite into the cells, and at mod- 

 erate nitrite concentrations and normal oxygen pressures the rates of 

 transport and oxidation are comparable. At low oxygen concentrations 

 there is accumulation of nitrite in the cells with consequent inhibition of the 

 nitrite-oxidizing enzyme (since substrate inhibition occurs when nitrite is 

 above 1 vaM). Cyanate may interfere with the transport so that at normal 

 oxygen concentrations the accumulation of nitrite is suppressed and the 

 substrate inhibition released. It would be interesting to know what effect, 

 if any, chlorate has on the nitrite transport. 



The oxidation of sulfite by the liver is inhibited by thiosulfate and this 

 action is probably exerted on sulfite oxidase (Fridovich and Handler, 1954, 

 1956). Thiosulfate inhibits both aerobically and anaerobically (methylene 

 blue reduction). Although it was originally stated that the inhibition is 

 competitive, it has been found more recently (MacLeod et al., 1961) that 

 it is not, at least with respect to sulfite. The following reaction scheme was 

 suggested: 



-S 



-S + HSO3 



-S— H 



-S— H 



(+ H2O) 



-s— SO3- > h-s— H + SOr + H^ 



and it was proposed that thiosulfate competes with the active thiosulfonate 

 in the hydrolytic step of the sequence. Methanesulfonate, ethanesulfonate, 

 benzenesulfonate, and pyridine-3-sulfonate are noninhibitory. 



Wild-type Neurospora crassa can utilize sulfate as the sole source of sulfur 

 but certain mutants require more reduced forms for growth (Ragland and 

 Liverman, 1958). Some strains can use sulfite and others only thiosulfate. 

 Since sulfate inhibits competitively the utilization of thiosulfate but does 

 not interfere with the utilization of sulfite, and inasmuch as previously it 



