588 



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

 fpH + ci+++ ^ fp + C++ + H+ 



Z>+^ 



I2 + C+++ + X ^ b+++ 

 .r-.^X^^^b+++ 



I2+ ~ X + C++ 

 I./ + X 



Sum DPNH + 2c+++ -> DPN+ + 2c++ + H 



(15) 



(11) 



(9) 



(16) 



The slow and incomplete reduction of cytochrome b often observed with 

 heart-muscle preparation and DPNH could be due to dismutation of b^++ — 

 I2 (back reaction (12)). Higher concentrations of DPNH might perhaps be 

 expected to increase the rate of formation of DPN '^ I^H to a point where 

 sufficient survives hydrolysis to bring about reduction of cytochrome b by 

 reaction (5) (see Note 3). 



Although the reduction of cytochrome b by succinate can be adequately 

 described by the sequence SHg -^ fp -> 2b++~^', where the fp is succinate 

 dehydrogenase, we should also hke to bring forth the possibihty that it 

 proceeds by a mechanism similar to that suggested above for the reduction 

 of cytochrome b by DPNH, with the exception that no labile energy-rich 

 intermediates are formed. Bringing the — SH group of succinate dehydro- 

 genase into the reaction, we could write 



COOH 



CH2 



1 

 CH2 



I 

 COOH 



-f fp + ESH ^ 



COOH 



I 

 HC— SE 



1 + 



CHo 



I 

 COOH 



fpH, 



(17) 



COOH 



I 

 HC— SE 



I + b+++ 



CH2 



COOH 



COOH 



I 

 C— SE 



I 

 CH2 



I 

 COOH 



-I- b++ -U + H+ 



(18) 



COOH 



I 

 C— SE 



I + b+++ 



CH2 



COOH 



COOH 



I 

 C— SE 



CH 



I 

 COOH 



+ b++ - I2 -I- H^ 



(19) 



