584 E. C. Slater and J. P. Colpa-Boonstra 



two, not three, cytochrome ^-hke pigments. One of these (hereinafter re- 

 ferred to as cytochrome b) is reduced by succinate alone, or by Na2S204. 

 The other (referred to as cytochrome b') is Hkewise reduced by NaaSgO^, but 

 is reduced by succinate only in the presence of antimycin. 



Chance (1958) provides further data on the effect of antimycin which are 

 very important for an understanding of the mechanism of its action. These 

 measurements show that the addition of antimycin increases 50-fold the 

 velocity of reduction of ferricytochrome b by succinate in the presence of 

 cyanide and decreases 20-fold the velocity of oxidation of ferrocytochrome b 

 by fumarate, also in the presence of cyanide. 



Chance (1952) originally attributed the additional absorption obtained 

 with antimycin to a combination of the latter with cytochrome b, or with a 

 factor operating between cytochromes b and c. In his most recent article, 

 Chance (1958) abandons this interpretation, because the intensified absorp- 

 tion was found with mitochondria, fly sarcosoraes and other preparations. 

 Instead, he suggests that the additional absorption resulting from the anti- 

 mycin treatment is due to an inactive form of cytochrome b, i.e. one that 

 can be activated only by the more rapid electron transfer to cytochrome b, 

 which occurs in the presence of antimycin. 



We agree that the increased absorption is due to reduction of a form of 

 cytochrome b which is inactive in the absence of antimycin. However, the 

 fact that antimycin both stimulates the reduction of active cytochrome b by 

 succinate in the presence of cyanide, and inhibits its oxidation by the respira- 

 tory chain, suggests to us rather strongly that antimycin reacts also with the 

 active cytochrome. We should like to suggest that antimycin combines with 

 both cytochrome b and cytochrome b', leading to an increased oxidation- 

 reduction potential and thereby facilitating the reduction of both forms of 

 cytochrome b by succinate. From the equilibrium constant determined by 

 Chance (1958) in the presence of antimycin, it is possible to calculate that, 

 under these circumstances, the Eq is about 100 mV. However, this probably 

 represents a value that hes somewhere between those for the antimycin com- 

 pounds of the two forms of cytochrome b. It is possible that the reason for 

 the failure of cytochrome b' to react with succinate is that, during prepara- 

 tion of the heart-muscle preparation, its structure is altered in such a way 

 that its oxidation-reduction potential is decreased to a value well below that 

 of succinate-fumarate. 



There are two possible explanations for the fact that the addition of 

 antimycin to a heart-muscle preparation reduced with succinate causes a 

 displacement of the a- and y-bands to a higher wavelength: (i) the bands of 

 cytochrome b' are at a higher wavelength than those of cytochrome b; 

 (ii) combination of both forms of cytochrome b with antimycin causes a 

 displacement of the band towards longer wavelength. 



Deul (1959) has made two observations which are of interest in connection 



