374 RESPIRATORY METABOLISM 



( 2 ) 2 reduced cytochrome -f- O, ==^ 2 oxidized cytochrome -j- HgOg 



oxidase 



(3) HA^=^ H,0 + l/2 0, 



catalase 



The substrate may be activated by "anaerobic" dehydrogenase, and 

 it is then oxidized by cytochrome, the cytochrome itself being reduced 

 in the process (equation 1). Cytochrome is, in turn, oxidized by an 

 oxidase system which may be identical with Warburg's respiratory en- 

 zyme (equation 2). During this process HoOo is formed and is then 

 broken down to water and molecular O2 by catalase (equation 3). 



The oxidase and catalase systems are inhibited by the presence of 

 HCN and HoS, and the oxidase system is also inhibited by CO. In the 

 presence of any of these reagents, reaction ( 1 ) can proceed but not 

 reactions ( 2 ) or ( 3 ) . Therefore all of the cytochrome becomes reduced, 

 and respiration by means of this mechanism is stopped. The dehydrogen- 

 ase systems are inhibited by narcotics (e.g., the urethanes), by warm- 

 ing and cooling, and these agents leave all of the cytochrome in the 

 oxidized state. These two general methods of treatment, therefore, may 

 be used as tools in studying the above respiratory mechanisms. There are 

 also aerobic dehydrogenases which, in addition to activating the sub- 

 strate, can react directly with molecular oxygen without the mediation 

 of cytochrome and oxidase. Respiration which is brought about by this 

 type of system is not supposed to be affected by HCN. 



It is possible to demonstrate that in some systems certain reversible 

 oxidation-reduction indicators (e.g., methylene blue) can replace the 

 cytochrome-cytochrome oxidase system, and that in this capacity the 

 action of these indicators may or may not be affected by HCN and CO 

 (e.g., grasshopper embryos, Bodine and Boell, 1937; Escherichia coli, 

 Broh-Kahn and Mirsky, 1938). If HCN and methylene blue are added 

 to such a respiratory system and inhibition does not occur, the ensuing 

 reactions might be visualized as follows: 



(4) substrate -j- methylene blue ^ — oxidized substrate 



dehydrogenase 



-j- leuco-methylene blue 



( 5 ) leuco-methylene blue -j- oxygen ^ methylene blue -{- H^Og 



Since catalase is inactivated by HCN, the hydrogen peroxide presumably 



