380 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



In other tissues or under other conditions, this was not found, how- 

 ever, and the number of exceptions has gradually increased. By 

 some workers this has been accepted as proof for the existence of 

 alternative pathways of respiration, e.g., through the autoxidation of 

 flavoproteins (c/., e.g.. Commoner, ^69). Such systems are assumed 

 particularly to catalyze the oxidation of protoplasmic protein, in the 

 absence of suitable substrates, or of fats and proteins. While there 

 can be little doubt that such alternative pathways exist in some 

 organisms and tissues, insensitivity of the respiration to inhibitors of 

 the cytochrome system cannot be accepted as reliable evidence. 



Inhibition by carbon monoxide. Carbon monoxide inhibition of the 

 respiration of the whole organism can, of course, only be studied in 

 animals in which no oxygen carrier reacting with carbon monoxide is 

 present, otherwise tissue slices must be investigated. In the first 

 way the carbon monoxide sensitivity of the respiration not only of 

 microorganisms, such as yeast, but also of insects {1097,3118) could 

 be established. The respiration of plant cells {51^8,1371^,1508,1871) is 

 also inhibited by carbon monoxide and the afhnity ratio of the res- 

 piratory ferment for oxygen and carbon monoxide is of the same 

 order as that of the respiratory ferment in yeast or of the cytochrome 

 oxidase of heart muscle. 



No inhibition of the respiration was found, however, in rat retina, 

 allantois, chorion, and liver (1654) and in liver, skin, and spleen of 

 frogs (74^5); in frog muscle even an apparent stimulation of the res- 

 piration by carbon monoxide was observed {^03,745,24^51,2610). 

 95% carbon monoxide inhibits the respiration of all bacteria which 

 contain cytochromes, with the exception of Pseudomonas aeruginosa 

 {pyocanea), Serratia marcescens, and Corynehacterium diphtheriae 

 {962), but Keilin {14^9) found little or no effect of light on the carbon 

 monoxide inhibition of the respiration of Escherichia coli and Bacillus 

 subtilis. 



Lack of carbon monoxide inhibition does not prove, however, that 

 the respiratory enzyme is absent in such tissues, or even that it does 

 not catalyze their respiration. The apparent stimulation of the 

 respiration of frog muscle by carbon monoxide is explained by 

 Stannard {2610,2611) as due to an .oxidation of carbon monoxide to 

 carbon dioxide by the tissue, superimposed upon a carbon monoxide 

 inhibition of the cytochrome system. A catalytic oxidation of carbon 

 monoxide by certain chlorophyll iron and chlorophyll porphyrin iron 

 compounds had been observed by Negelein {2020). Fenn and Cobb 



