CYTOCHROMES 169 



oxidation was due primarily to development of cytochrome c. 

 Nevertheless the Qo^ of the tissue increased little during this period 

 and at all times was over 90 per cent sensitive to 0.001 M. cyanide. 

 Thus there is an apparent anomaly in that cyanide, the inhibitoiy 

 action of which is generally considered to be on the cytochrome 

 oxidase system, is inhibiting a respiratory system which does not 

 even contain a complete cytochrome system. This is not the first 

 instance in which the "single point of attack theory" of cyanide has 

 been questioned. Parallel experiments with cyanide and azide (see 

 Stannard, 83) on intact tissues have also suggested that cyanide 

 probably combines with one or more other enzymes involved in 

 respiration. The question arises, therefore, what is the nature of the 

 "oxygen-activating" system in the absence of cytochrome c? If 

 cyanide can also block an enzyme systeoi near the "dehydrogenase 

 end" of respiration, then flavin enzymes as well as cytochrome b 

 (which is relatively cyanide-stable aij^cX autoxidizable) become pos- 

 sibilities for this role, despite the fact that their oxidation is gener- 

 ally considered to be through the oxidase system. 



A somewhat similar situation arises in Stannard's experiments 

 (83) on the "resting" and "activity" oxygen consumption of frog 

 muscle. It was found that the resting metabolism was insensitive to 

 azide, although sensitive to cyanide. On the other hand, the activity 

 metabolism (caffeine or electrical stimulation) was greatly inhibited 

 by azide. When the azide concentration in the stimulated prepara- 

 tion was increased, the inhibition *came to an abrupt stop, leaving a 

 respiration equal to that of the resting state. The latter finding 

 excludes the possibility that the azide stability of the resting respira- 

 tion can be explained by failure of the inhibitor to penetrate or its 

 absence in a form capable of exerting its typical inhibition on the 

 oxidase system. Stannard (84, 85), who has studied the anaerobic 

 glycolysis of frog muscle, does not believe that the azide insensi- 

 tivity of resting muscle can be explained by the lack of substrate for 

 "saturation" of the oxidizing enzymes. He has concluded that the 

 oxygen transfer for the resting metabolism and for the extra metab- 

 olism resulting from stimulation are due to different systems. The 

 extra metabolism due to stimulation was apparently by way of the 

 cytochrome system, but the nature of the "oxygen-activating" en- 

 zymes of the resting metabolism remain unknown. 



Recently Korr (30) has carried out experiments on the metabolism 

 of slices of mammalian tissues during rest and during a stimulated 

 phase. Salivary glands stimulated by acetylcholine or adrenalin, 



