376 RESPIRATORY METABOLISM 



are not so well known nor so clear-cut as, for the sake of clarity and 

 brevity, they have been made to appear in the above outline. Whenever 

 we conclude, on the basis of the action of certain reagents on respiration, 

 that one respiratory mechanism is very important and that another is 

 not, we should do so only with certain mental reservations, and the 

 conclusions should not be considered final, but merely indicative. 



2. EXPERIMENTS WHICH CONCERN THE CYTOCHROME-CYTOCHROME 

 OXIDASE SYSTEM OF HYDROGEN ACCEPTORS 



We have, through the action of HCN and CO on respiration, a tool 

 for determining how much of the respiratory activity of a given organism 

 is carried on by means of the cytochrome-respiratory enzyme system and 

 how much is not. Respiration which is not cyanide and CO sensitive 

 may be due to aerobic dehydrogenases, or to anaerobic dehydrogenases 

 plus an enzyme of the yellow-pigment type or perhaps to the action 

 of peroxidases. Such analyses have been made for several types of bio- 

 logical material. It has been determined that respiration of some cells 

 is extremely sensitive to HCN (e.g., yeast, B. col/, most bacteria, and 

 mammalian tissues), while that of others is quite resistant [Chlorella, 

 Paramecium, Sarcina, Pneumococcus, B. acidophilus. Streptococcus, 

 Staphylococcus); also, the same organism may differ in sensitivity at 

 different periods during its life history (grasshopper eggs, Robbie, Boell, 

 and Bodine, 1938). One technical precaution which should be ob- 

 served in cyanide experiments is the use of a KOH-KCN absorbing 

 fluid for COo (van Heyningen, 1935). By the selection of the proper 

 KOH-KCN mixture, the osmotic transfer of KCN through the air from 

 the experimental material to the KOH solution can be prevented. This 

 is apparently one possible source of error in all work on the effect of 

 cyanide on protozoan respiration — that none of the authors has used 

 balanced KOH-KCN solutions. 



Among the Protozoa the effect of cyanide has been studied on several 

 ciliates and flagellates. It is quite well established that the respiratory 

 mechanism of Para?necium is insensitive to cyanide (Lund, 1918b; Shoup 

 and Boykin, 1931; Gerard and Hyman, 1931), and the work of Shoup 

 and Boykin (1931) shows that the addition of iron salts does not 

 increase respiration and that very little or no iron is present in Para- 

 mecium. These results may be interpreted to mean that the cytochrome- 



