518 



R. K. Morton, J. McD. Armstrong and C. A. Appleby 



Here, 



+ [A] "^ [S] 



V^ = 



1 + 



K 



and K,„ = 



K,. 



1 + 



K 



[AL [A], 



In this case, K.^JV^ is constant, and plots of 1/K^ or \jK„^^ against 1/[A]^ are 

 linear, with intercepts on the vertical axis of IjV or 1/A^„j, and on the hori- 

 zontal axis of IjK. 



From the results presented by Slater (1955), it appears that most flavo- 

 proteins conform to either mechanism / or //. The few exceptions (such as 



1/[S] 



'/[A], 



Fig. 5. Lactate-ferricyanide reductase activity of about 2 x 10^* m crystalline 

 cytochrome b^__ in 0-03 M sodium pyrophosphate-HCl buffer at pH 80 and 30°C. 



Units are as follows: v and K,., Af'joom/f/min ; [S], L(+)-lactate, mmoles/1.; 

 [AJa;, potassium ferricyanide, mmoles/1. 



Plot I. Curves for 0255 mw ferricyanide ( — O — ) and 0-720 mM ferricyanide 

 ( — □ — ). Compare Fig. 4, Plots A and B. 



Plot II. Curves for \IK,„^ (— O— ) and l/K, (— □— ) against 1/[A]:,. (The lowest 

 value oillVy. was not considered in fitting the curve, as the rate was clearly inhibited 

 at high ferricyanide concentrations). Compare Fig. 4, Plots C and D. 



notatin and DPNH-cytochrome c-reductase of heart muscle) may be ex- 

 plained in terms of the relative magnitudes of the rate constants involved in 

 V.^ and K^ so that the plot of K^ against Vy. passes through the origin (see 

 Appendix). Figure 4 shows how the three mechanisms may be differen- 

 tiated. 



As shown in Fig. 5, results obtained for the lactate-ferricyanide reductase 

 activity of cytochrome h.,^ appear to conform to mechanism / or //, although 

 the latter seems more probable. However, as shown by Fig. 6, the lactate- 

 cytochrome c reductase activity clearly conforms with mechanism ///. 



From the considerations presented above, we interpret the results in the 

 following way. 



