434 



Discussion 



At about the same time, Kamen and Vernon (/. bioL C/teni. 211, 643, 1954) began 

 the study of RHP-CO in purple bacteria. This compound shows, as does cytochrome 

 o, a CO-myoglobin type spectrum. Table 1 further compares the properties of the two 



Table 1, Comparison of the absorption characteristics of 

 cytochrome o-co with those of rhp-co 



* Kamen, personal communication. 



compounds There is fairly close agreement between the peaks of their absorption 

 bands, but the photochemical data give a larger y/a ratio for cytochrome o. The 



+0.01 



-0.02 



400 



A(m>j) 



7\{m}i) 



Fig. 2. Photodissociation difference spectra comparing cytochrome o with 



cytochromes a^ and 03 (left) and with RHP-CO (replotted from Kamen 



and Bartsch, this volume, p. 426). 



higher extinction coefficient of RHP-CO may strengthen the possibility that this haem 

 is a polymer, perhaps a dimer. (This has recently been found true for both R. rtibrum 

 and Chroinatium RHP (Kamen and Horio, personal communication).) 



A comparison of the difference spectra of the two compounds is afforded by Fig. 2. 

 On the left, cytochrome o is compared with and clearly distingushed from cytochromes 

 Oi and a-i. On the right, cytochrome o is compared with RHP-CO and a considerable 

 similarity is observed. 



The speed with whicli cytochrome o is oxidized can at present be determined in the 

 rapid flow apparatus by following the a band of cytochrome of type c (Fig. 3), a 

 method that indicates only the minimum value of the reaction velocity. The reaction 

 is slow for M. pyogenes var. alhus, but is somewhat faster for A. suhoxydans (right). 



A comparison of such kinetics with those of RHP in R. nibriiin is not possible 



