610 B. Chance 



On the basis of the lack of an effect of the oxidation-reduction state of the 

 carriers upon the ATPase reaction (Boyer, Falcone and Harrison, 1954; 

 Cohn and Drysdale, 1955), an additional component, X, was introduced 

 into the reaction mechanism (cf. Slater, 1958). Chance and WiUiams (1955c) 

 have used the same symbol for kinetic reasons and to avoid the fault of the 

 Lehninger mechanism (Lehninger et al., 1958) which shows an uncoupling 

 reaction on addition of phosphate. More recently. Slater (1959) has accepted 

 the possibihty of an effect of the oxidation-reduction state of the carriers upon 

 the dinitrophenol-stimulated ATPase activity of mitochondria at pH 7-4. 

 Although Slater states that these results weaken the basis on which he has 

 introduced X into the reaction mechanism, we had previously pointed out 

 that the increased binding of both X and I (as carrier'--'I and X '^ I) would 

 occur with carrier reduction and would thereby inhibit the ATPase and 

 exchange reactions (Chance and Hollunger, 1957b; but cf. Slater, 1958, 

 p. 250 (see Note 5)). 



Substances such as the hypothetical inhibitory substance, I, would provide 



a 2-electron intermediate in cytochrome reactions as has been pointed out by 



Chance and Williams (1956a), i.e. cytochrome could accept one electron and 



I the other. 



c+3.i_|.2e--c+2~l-i (11) 



This possibility has been taken up by Slater (1959) in a speculative review 

 and indeed has some merit since it neatly avoids the problem of the 2 to 1 

 electron transition between flavin and cytochrome. We feel that no experi- 

 mental data allow us to distinguish between this hypothesis and the dimeriza- 

 tion hypothesis proposed by Chance and Williams (1956a). 



2c+3.I + 2e--2c+2.i (12) 



The thermodynamic difficulties implicit in single electron transfer in the cyto- 

 chrome chain are no longer considered formidable in view of the ready inter- 

 change of energy conserved at one site with that at another as exemplified by 

 the succinate-linked pyridine nucleotide reduction (Chance and Hollunger, 

 1957a; 1960) (see below). 



REVERSAL OF ELECTRON TRANSFER 

 The reactions of cytochromes c and (a + a^) appear to be irreversible: for 

 example, the reduction of oxygen to water shows no measurable reversibility. 

 Moreover, cytochromes c and (a + ^3) are completely reduced in anaero- 

 biosis. However, cytochrome b of the Keilin and Hartree preparation can be 

 oxidized by fumarate, although kinetic studies show that this may not be an 

 equihbrium reaction, since the titration and kinetic data are in strong dis- 

 agreement (Chance, 1958a). Another reaction was observed by Slater (1950) 

 to occur rather slowly — the oxidation of DPNH by fumarate : 



H+ + fumarate -j- DPNH ^ succinate + DPN+ (13) 



