OXIDATION OP CYTOCHROMES 177 



a reduced sample with CO and a reduced sample without CO has a 

 sharp peak and a shallow trough in the Soret region. (See Table I.) 

 This CO-binding pigment may be identical to Vernon and Kamen's 

 (5) Chromatium pseudohemoglobin. 



Although Chromatium is an obligate anaerobe, at least one of its 

 cytochromes becomes oxidized in the presence of oxygen. The spec- 

 trum of the difference between an anaerobic sample and an aerobic 

 sample is roughly similar to curve A in Fig. 2, but the respective 

 peaks have slightly different maxima as shown in Table I. Also the 

 change in optical density at 422 m^ upon aeration is only about 90% 

 of the total anaerobic Ught effect. However, irradiation of an aerobic 

 suspension gives an additional decrease in optical density. This 

 aerobic Ught effect is about 30% of the total anaerobic light effect. 

 These observations indicate that the cytochromes of Chromatium 

 are oxidized during irradiation whether the suspension is aerobic or 

 anaerobic. The fact that oxygen completely inhibits the growth of this 

 bacterium cannot be explained simply in terms of its reactions with 

 the cytochromes. The inhibition of growth may be caused by oxygen 

 "poisoning" or bj^ the reaction of the substrates with oxygen outside 

 the bacteria. 



The conclusions may be summarized briefly. Chromatium contains 

 at least three hemoproteins : a CO-binding pigment, a c-type cyto- 

 chrome, and a 6-type cytochrome. The c-type cytochrome has been 

 partially purified by Vernon and Kamen (5). The anaerobic light 

 effect clearly involves the c-type cytochrome and the 6-type cyto- 

 chrome. The CO-binding pigment is probably involved also, but its 

 appearance in the anaerobic light effect is masked. In the presence of 

 oxygen the c-type cytochrome is oxidized. This oxidation probably 

 requires the mediation of the CO-binding pigment, but the anaerobic 

 minus aerobic difference spectrum does not specifically indicate 

 changes in the CO-binding pigment or in the 6-type cytochrome upon 

 aeration. 



Van Niel's (6) mechanism for photosynthesis in purple bacteria is 

 given by the following equations : 



4(H20 + h.; > H + OH) (1) 



4H + CO2 > (CH2O) + H2O (2) 



2(2 OH + H,A > 2H2O + A) (3) 



