412 J. POSTGATE 



Reduction of Sulphite 



Oxidation of c^ by sulphite can be observed spectroscopically in intact 

 bacteria, but the enzymological evidence for participation of cytochrome Cg 

 in sulphite reduction is inconclusive. Some enzyme preparations show small 

 stimulations of sulphite reduction when extra cytochrome Cg is added. 

 Recent work by the author has shown that the sulphite reductase system is 

 complex, as indicated by the following results. 



Cells of DesuJphovibrio desuJphuricans (Hildenborough) grown in continu- 

 ous culture were disrupted by treatment with liquid Ng (Moses, 1955) and 

 treated with deoxyribonuclease (to keep the preparation liquid). Centri- 

 fugation yielded a red particulate fraction having most of the sulphite 

 reductase activity (together with much bound cytochrome Cg) and a super- 

 natant fraction without which the sulphite reductase activity of the particulate 

 preparation was negligible unless benzylviologen were added. The particles 

 then showed 1-5 to 3% of the sulphite reductase activity of the original cells; 

 the liquid fraction had only slight reductase activity though hydrogenase 

 alone was plentiful. The liquid 'co-sulphite reductase' could be further 

 fractionated into two heat-labile proteins (one soluble in 36% ammonium 

 sulphate, the other not) and a dialysable factor that could be recovered by 

 freeze drying the dialysate. Addition of cytochrome Cg to the complete 

 preparation (particles and soluble fraction) did not influence the rate of 

 reduction of sulphite at all (there is probably sufficient cytochrome Cg bound 

 with the particles) even if cytochrome Cg had previously been removed from 

 the soluble fraction by ion-exchange. Complete enzyme preparations were 

 stimulated further by benzylviologen but not by diphosphopyridine nucleo- 

 tide, triphosphopyridine nucleotide, coenzyme A, a-lipoic acid, hypoxanthine, 

 methyl-l:4-naphthoquinone, pyridoxal phosphate, adenosine-3'-phosphate, 

 pyruvate, succinate, cysteine, glutathione, a concentrate of desulphoviridin, 

 or by salts of the following metals : Mo, V, Mn, Co, Ni, Mg, Zn or Ca. 

 Modest stimulation was observed with adenosine triphosphate (ATP) and 

 acetyl phosphate and a slight effect occurred with ferrous ions ; these materials 

 did not, however, replace the dialysable component referred to earlier. 



Some quantitative data obtained with particulate sulphite reductase 

 preparations are given in Table 3. 



These observations show that the sulphite reductase system involves 

 several components : 



1. hydrogenase 



2. thermolabile protein A 



3. thermolabile protein B 



4. cytochrome Cg 



5. a dialysable co-factor 



6. a sulphite reductase. 



