382 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



the lack of carbon monoxide inhibition, and there is no sound theoretical 

 basis for the assumptions made by Ball in calculating tiie potential of the 

 oxidase in the presence of cyanide. A similar explanation is given by Ball 

 for the azide-resistant lespiration of resting frog muscle, while St&nna.rd{2608) 

 had assumed different enzymes at work in resting and stimulated muscle.* 

 Since we know very little about the affinity of azide for ferrous and ferric 

 hematin compounds, Ball's hypothesis is entirely speculative. It appears 

 more likely that the cyanide-resistant respiration is due to other cyanide 

 noninhibitable enzyme systems. Another possibility which must be con- 

 sidered, at least in some cases, is that cytochrome oxidase is present in so 

 large an excess over either cytochrome c, the substrate, or another inter- 

 mediate component of the reaction chain that one of these remains the limit- 

 ing factor even when a large part of the oxidase is inactivated by tlie inhibitor. 



Whether the cyanide-insensitive respiration is catalyzed by the 

 cytochrome oxidase or not, there is important evidence to show that 

 the enzyme is present in such cells, as is probably cytochrome c. 



Runnstrom {^.394) found that, in the presence of dimethyl-;>-phenylene- 

 diamine as substrate, the respiration of unfertilized sea urchin eggs was 

 inhibited by carbon monoxide; Runnstrom and Orstrom {2066, 2393, 239 Jf) 

 measured the affinity ratio of the enzyme for carbon monoxide and oxygen 

 under these conditions and found it to be of the same magnitude as that of 

 cytochrome oxidase. The enzyme system is therefore present in the cell, 

 but not "geared in." While Korr (1572) stresses the availability of cyto- 

 chrome c, Runnstrom considers it more likely that the availability of a suit- 

 able substrate is of greater importance {cf. 1'j76). In tliis connection, experi- 

 ments of Keilin and Hartree (i^^-i) are of interest. By damaging cytochrome 

 oxidase from heart by treatment with acids or pancreatin. they obtained 

 preparations which contained active oxidase and cytochrome c (to judge 

 from the catalytic effect on p-phenylenediamine) as well as succinic dehydro- 

 genase (methylene blue oxidation of succinate). Nevertheless such prepa- 

 rations did not oxidize succinic acid. As in sea urchin eggs, the addition of 

 dimethyl-/)-phenylenediamine to grasshopper egg brei in the diapause causes 

 the appearance of a cyanide-sensitive respiration. The cytochrome system 

 in the diapause {cf. below) appears thus to be present though normally not 

 activie. Again no cytochrome c has been observed spectroscopically, but it is 

 difficult to believe that cytochrome c present in the prediapause would 

 disappear afterward. For further discus.sion the reader is referred to Need- 

 ham's book {2017, p. J6G ff.). 



In plant tissues a carbon monoxide-sensitive respiration has often 

 been encountered, which is inhibited by cyanide or azide only at 

 high concentrations (lO^tolO^U) {5^8, l,i7hU01 ,1508,1871). The 

 fact that the carbon monoxide inhibition is light-sensitive indicates 

 that the cytochrome .system is nevertheless the catalyst. The respira- 

 * Cf. Horecker and Stannard {l.J.',7b). 



