Electron Transfer in Nitrate and Sulphate Respiration 



403 



The retardation of hydrogen absorption after 1 mole uptake was observed 

 in the presence of arscnite (Fig. 5). 

 The formation of sulphite was demonstrated with partially-purified enzyme 



Inhibition of APS reduction by arsenite. 

 without AsO^. 



preparation, which had activity to reduce APS but not sulphite or sulphate 

 in the presence of ATP (Table 13). 



Table 13. Reduction of adenosine-S'-phosphosulphate to sulphite 

 by partially-purified enzyme 



Reaction mixture contained 50 /mioles of phosphate buffer, pH 7-0, 1 //mole of methylviologen. 1 '0 ml 

 of partially-purifieJ enzyme preparation (10 mg organic substance) and 2-75 //moles of APS in total 

 volume of 2-0 ml. Centre wells contained 0-2 ml of 2 n NaOH. Gas phase: hydrogen. Temperature: 

 sec. Preparation of the enzyme was as follows. The crude extracts obtained by sonic disintegration 

 were treated with ammonium sulphate and precipitate between 33 % and 67 ?„ saturation was dissolved 

 in 005 M-phosphate buffer, pH 7-0. After dialysis, calcium phosphate gel adsorption was carried out. 

 This first eluate with 0125 M phosphate buffer, pH 70, could reduce APS, but hardly reduce sulphite 

 to sulphide, as shown below. 



These results agree with those of Peck (1959), which showed also the 

 intermediary formation of sulphite as well as the stoichiometry of related 

 phosphorus metabohsm, 



3'-Phospho-adenosine-5'-phosphosulphate (PAPS) which is known as 

 'active sulphate' involved in esterification of sulphate (Robbins and Lipmann, 

 1957) and as an intermediate in sulphate reduction in yeast extracts (Wilson 

 and Bandurski, 1958; Hilz and Kittler, 1958), was not reduced under similar 

 conditions and proved to have no effect on sulphate reduction in the presence 

 of ATP in the extracts (Table 14). The participation of PAPS was thus 

 excluded. 



