384 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



potential develops. The functional changes occurring with the onset 

 of secretion are correlated with the selective development of the 

 oxidase system in the epithelium. If the oxidase is inhibited by 

 cyanide, the potential difference and the selective transport of anionic 

 or cationic dyes across the plexus is abolished. Similar observations 

 were made with the ciliary body of the eye {953). 



In the developing kidney of the fetal pig a correlation between the 

 ability to perform thermodynamic work in secretion on the one hand, 

 and the higher oxidation-reduction potential and the concentration 

 of the cytochrome oxidase system on the other, was shown to exist 

 with regard to the development in time and space. The secretory 

 portion of the nephron had a much higher concentration of oxidase 

 and oxidation-reduction potential than Bowman's capsule and collect- 

 ing tubules, but this difference developed only with the onset of 

 secretory activity. 



A parallel in the plant kingdom is the correlation between the 

 cyanide-inhibitable oxidase system and accumulation of salts in 

 plants in the so-called salt (anion) respiration {1788,2286). 



The cytochrome system has been claimed to play a more specific 

 role in the formation of thyroxine, particularly in the incorporation 

 of inorganic iodine in the tyrosine molecule. This was studied by 

 Schachner and collaborators {2Jj.31) with surviving thyroid tissue and 

 radioactive iodine, P^^ Oxygen was found necessary, though 0.6% 

 oxygen in nitrogen sufficed. The incorporation of iodine was inhibited 

 by cyanide, azide, sulfide, and carbon monoxide, and with the last- 

 mentioned substance the inhibition was reversed by light. Several 

 workers {1720,2111,2431) have claimed inhibition of the cytochrome 

 system by thiourea or thiouracil, substances which are known to 

 inhibit thyroxine formation in the thyroid, but these results have not 

 been confirmed {1012,1845). Nor do sulfonamides which have a 

 similar though less intense action on the thyroid, inhibit cytochrome 

 oxidase {1865). 



6. THEORY OF MODE OF ACTION OF THE 

 RESPIRATORY FERMENT 



6.1. Autoxidation 



On first view one may be inclined to believe that the mode of action 

 of the cytochrome system can be perfectly understood on the basis 

 of the facts discussed in the preceding sections of this chapter. An 



