494 W. D. Bonner, jr. 



short, higher plant tissues possess a common unity of cytochromes 

 components. 



It becomes increasingly clear that there is no unity of cytochrome com- 

 ponents throughout the biological world ; such a unity exists only within each 

 major biological group (cf. Estabrook, this volume, p. 454). 



At the present time there is insufficient evidence to assign a localization or a 

 specific name to many of the plant cytochromes. However, one can consider 

 what we know about cytochromes involved in electron transport to oxygen 

 in the cyanide-sensitive portion of the respiratory chain of plant mitochondria. 



It would appear that the reaction with oxygen is mediated by cytochrome 

 oxidase and, on the basis of spectral evidence and inhibitor studies, cyto- 

 chromes a and a^ in plants are very nearly identical to those of animal 

 tissues. This conclusion is by no means a new or surprising one. 



The substrate of cytochrome oxidase, cytochrome c, is likewise present in 

 plant tissues and in good concentration. Again, the demonstration of cyto- 

 chrome c in plant tissues is by no means new. It is interesting that the plant 

 cytochrome c, as delineated most clearly by its reduction upon addition of 

 substrate to mitochondria in the presence of cyanide or azide, has an a-band 

 absorption maximum (— 190°C) which is consistently 2 m/^ toward the red as 

 compared to animal preparations and pure animal or wheat germ cytochrome 

 c solutions, all measured under the same conditions. Such a modest optical 

 variation is not at all surprising. The important thing is that the plant c 

 responds very similarly to its animal counterpart in steady state studies. A 

 component corresponding to cytochrome q, as defined for animal tissues, 

 has not been found in any plant tissues so far examined. 



On the basis of the above discussion we formulate the electron transport 

 system in cyanide-sensitive plant mitochondria as follows : 



DPNH->? 

 Substrate -> -* c^- a-^ a^-^O.^ 



Succinate -> ? 



It is the question marks that are difficult to discuss at our present state of 

 knowledge. The question marks, of course, refer to the baffling nature of 

 the cytochrome b components. This problem is further comphcated by the 

 fact that possible mitochondrial contamination by plastid fragments, and 

 consequently plastid cytochromes, has not been eliminated. 



The spectral data that have been presented in this study show no cyto- 

 chrome component that can be clearly designated as 'cytochrome b\ Keilin 

 and Hartree heart-muscle preparations and yeast suspensions show on 

 substrate reduction in the presence of Antimycin A or HOQNO, one sharp 

 absorption band with a maximum (at — 190°C) of 560 m/f. The plant 

 preparations do not reveal a 560 m/^ component when similarly treated. 

 Chance and Hackett (1959) also found no evidence for cytochrome b in their 

 study of skunk cabbage mitochondria (on reduction by substrate). 



