582 



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



were not on the main pathway of reduction of cytochrome a^, as suggested 

 for DPNH by Slater (1950), and for succinate by Chance (1952). 



The rapid reduction of cytochrome b by DPNH was in conflict with 

 Slater's (1950) observations with the microspectroscope. On re-examination 

 of this point, it was found to be a question of the concentration of DPNH, 

 The relatively low concentration of DPNH (sufficient to saturate the DPNH 

 oxidase system) used in the previous experiments was found to reduce the 



f, sec 

 Fig. 3. The rates of reduction of cytochromes b and a^, expressed as l/AA . dA/dt, 

 calculated from Fig. 2, and plotted as a function of time. O — O, Og, succinate; 

 A— A, b, succinate; D— D, Og, DPNH; •— •, b, DPNH. 



cytochrome b slowly and incompletely. The higher concentration (1-5 niM) 

 used in the experiment shown in Fig. 2 was seen in the microspectroscope 

 rapidly to reduce the cytochrome b (see Slater, 1958). Similar results have 

 been reported by Jackson and Lightbown (1958). 



In agreement with Chance's (1952) observations, it was found that the 

 reduction of cytochrome b by succinate in the presence of cyanide was much 

 slower than that calculated from the rate of the oxidation of succinate in the 

 absence of cyanide, on the assumption that cytochrome b were on the main 

 pathway. However, we think it possible that cyanide interferes with the 

 reduction of cytochrome b, and that this slow rate of reduction is not char- 

 acteristic of the uninhibited system (cf. Slater, 1958). In our view, the 

 evidence is consistent with the proposition that cytochrome b is in the main 

 pathway for succinate, and for high concentrations of DPNH. Mechanisms 

 incorporating this view will be suggested later in the paper. 



EFFECT OF ANTIMYCIN ON CYTOCHROME b 

 In agreement with the observations of Chance (1958), it was found that 

 when antimycin was added to a heart-muscle preparation already reduced by 



