Electron Transfer in Nitrate and Sulphate Respiration 



405 



Table 16. Effect of cytochrome Cg on the reaction rate of 



SULPHATE AND APS REDUCTION 



(a) The extracts obtained by sonic disintegration were centrifuged at 100,000 g for 1 hr and the 

 supernatant was passed tJirough the column of Amberlite-IRCSO (NHi+ type) to eliminate cyto- 

 chrome C3. The solution obtained was used as an enzyme preparation. Warburg manometers were 

 employed and hydrogen uptake was measured in a hydrogen atmosphere. Temperature: 30°C. The 

 reaction mixture contained the enzyme preparation (1 -61 mg N), hydrogenase preparation (0-70 mg N) 

 (Ishimoto et al., 1957). 50 /moles of Tris buffer, pH 7-2, 20 /<moles of NaF. 10 /<moles of KjSO,, and 

 5 /imoles of ATP. Total volume was 1-3 ml. (W APS reduction to sulphide. The conditions were 

 similar to (a) but reaction mixture contained the enzyme preparation (107 mg N), hydrogenase pre- 

 paration (0-34 mg N). 50 //moles of Tris buffer. pH 7-2, and 2-1 /^moles of APS in total volume of 

 0-8 ml. (c) APS reduction to SO3 . The reaction mixture contained the partially-purified enzyme 

 (the same sample in Table 13) (0-35 mg organic substance), hydrogenase preparation, 50 /tmolea of 

 phosphate buffer, pH 70, and 1-56 //moles of APS in total volume of 1-2 ml. 



Oxidation of Cytochrome Cg by APS in the Extracts 



In living cells of Desulphovibrio, it was found spectroscopically that cyto- 

 chrome C3 was reduced by the addition of hydrogen or formate and oxidized 

 by the addition of sulphate, sulphite or thiosulphate (Ishimoto et al, 1954). 

 Similar experiments have now been carried out with the cell-free extracts. 



The cytochrome Cg in the extracts was reduced with gaseous hydrogen in 

 Thunberg tubes. By this reduction the a-band at 553 m^ appeared, which 

 was observed by a microspectroscope. The tubes were then evacuated and 

 sulphate, sulphate plus ATP, APS, sulphite or thiosulphate was added from 

 the hollow stoppers. At room temperature, the absorption band was weakened 

 in 10 min in the tubes of sulphate plus ATP and APS, but only slightly in the 

 sulphite tube even in 2 hr and no change was observed in the others as well 

 as in the control tube without any addition. The results can be conceived to 

 indicate the participation of cytochrome Cg in the reduction of APS. The 

 little change when sulphite an(i thiosulphate were added may depend on the 

 considerably low potential of the systems compared with the potential of the 

 cytochrome Cg (£"0: -0-205 V (Postgate, 1956)). 



From the results of these experiments, the reductase in the extracts of 

 Desulphovibrio can be regarded to play the role of the terminal oxidase of 

 cytochrome Cg just like the role of cytochrome oxidase in the aerobic systems. 



