Energy Transfer and Conservation in the Respiratory Chain 603 



sensitive point (Pumphrey and Redfearn, 1959) (cf. Equation 5). Titration 

 studies recently conducted in this laboratory (Chance and Redfearn, in 

 preparation), give further support to this scheme and show cytochromes c 

 and a to be reduced completely at DPNH concentrations that cause little 

 reduction of ubiquinone. Thus quinone does not play a part in interactions on 

 the oxygen side of the antimycin-A sensitive point, and evidence in favour of 

 an inter-cytochrome shuttle via a quinone system does not exist at present. 



The possible function of ubiquinone remains obscure, although it could 

 mediate flavin-cytochrome b interaction in phosphorylating particles. How- 

 ever, the situation may be different in non-phosphorylating Keihn and 

 Hartree particles in which there is a relatively active ubiquinone reductase 

 activity and a sluggish response of cyctochrome b. Nevertheless quinone 

 function enjoys much speculation, particularly because various dehydro- 

 genases can act as quinone reductases (Martins, 1959), for example, vitamin 

 Kg reductase. Slater (1959) has most recently postulated a compound of 

 DPNH and ubiquinone in which the quinol is involved in a configuration 

 which he terms DPN '--' IH. It appears that this postulated intermediate 

 may be similar to the inhibited form of DPNH that Chance and Williams 

 write as DPNH '--' I (see Discussion of Dickens, this volume, p. 637). 



REACTION SEQUENCE OF THE CYTOCHROMES 



Purification and Fragmentation 



The hypothesis that purification and fragmentation of the particles does 

 not disturb electron transfer through the components has yet to be sub- 

 stantiated. Indeed, contrary evidence is currently accumulating. In phos- 

 phorylating mitochondria, the kinetics of cytochrome b follow those of 

 cytochrome c and flavoprotein with succinate or j5-hydroxybutyrate as 

 substrate (Chance and WiUiams, 1955a), whereas in non-phosphorylating 

 systems such as the Keilin and Hartree heart-muscle preparation (Keilin and 

 Hartree, 1939) cytochrome b clearly responds too sluggishly to participate 

 significantly in succinate (Chance, 1952) or DPNH oxidation (Slater, 1950). 



Some fragments of the respiratory chain (Green, 1959) lack cytochromes 

 b or q, and others have relatively greater amounts of some forms of cyto- 

 chrome b than are observed in intact beef heart mitochondria (Chance, 

 unpublished results). For these reasons, studies of the intact electron transfer 

 particle are highly desirable. 



A second difference is the locus of the inhibitory site for Amytal in pyridine 

 nucleotide oxidation and the comparative effectiveness of this compound as 

 an inhibitor. Previous studies with mitochondria carrying out oxidative 

 phosphorylation have shown (Chance, 1956) that Amytal inhibits the oxida- 

 tion of DPN linked substrates at the locus of reaction of reduced pyridine 

 nucleotide and oxidized flavoprotein, a concentration of 0-2 mM Amytal being 



