Properties of Cytochrome b.. 



513 



of 80 moles of lactate/hr/mole of haem (Appleby and Morton, 1959b). With 

 prolonged exposure to air, small amounts of HoOo may be formed {vide supra). 



With 0-5 M lactate and niM EDTA, cytochrome b^ is only slowly oxidized 

 by air. Under these conditions, the oxygen probably reacts slowly with the 

 haem group rather than with the flavin group, so that HgOa is not formed 

 (see Fig. 1). 



However, by contrast, Yamashita et al. (1958) have found that the flavin- 

 free haemoprotein derivative of cytochrome b^ 'has a remarkable aut- 

 oxidizability'. Other typical cytochromes of group B, such as cytochrome b^, 

 also are fairly rapidly autoxidizable (Strittmatter and Velick, 1956; Morton, 

 1958). If the crystalline haemoprotein derived from cytochrome b2 by the 

 procedure of Yamashita et al. (1957) is not seriously denatured, it would 

 appear that the presence of the flavin group modifies the reactivity of oxygen 

 with the haem group of intact cytochrome b^. This is further evidence in 

 support of the close juxtaposition of the two prosthetic groups on the one 

 apoprotein in the intact cytochrome b.^. 



As compared with the rates with oxygen, the cytochrome /72-catalysed 

 reduction of ferricyanide, cytochrome c and dyes is very much greater. The 

 optimum pH values for reduction of these acceptors vary somewhat accord- 

 ing to the purity of the enzyme (Appleby and Morton, 1959b). For crystafline 

 cytochrome b2, the optimum pH values are: ferricyanide, 8-0; cytochrome 

 c, 7-0; methylene blue, 6-8. 



The relative rates of reduction of ferricyanide, heart-muscle cytochrome c 

 and methylene blue are substantially the same for crystalline cytoclii-ome 62 

 as for an extract of dried yeast (Table 6). This shows that the reactivity of 



Table 6. Comparative lactate dehydrogenase actfvities of a crude extract 

 OF bakers' yeast and of crystalline cytochrome 60 WITH different acceptors 



For the crude extract, dried yeast was extracted with 015 m NaCl at 38'C for 1 hr and then centrifuged 

 at 10* X g for 1 hr at 5^C. The clear supernatant, containing 2-6 mg of protein N/ml, was used. 

 Activities were determined under comparable conditions: these are sub-optimal in respect of cyto- 

 chrome c concentration and pH (see Appleby and Morton, 1959a). Activities are expressed as units/ 

 mg Cumoles of acceptor rcduced/hr/mg of protein at 18-20°C). Results of Appleby and Morton 

 (1959a). 



