430 M. D. Kamen and R. G. Bartsch 



highest known. The relative amounts of RHP and cytochrome c are in 

 accord with those estimated visually in chromatophores after acetone treat- 

 ment to remove the photoactive pigments. 



Reliable methods for accurate estimation of total haem content in C/iro- 

 matium and R. rubrum are not available so that it is not possible to say what 

 fraction of the total haematin content is accounted for as RHP, but it is very 

 likely that this fraction is higher than 10%. 



CLASSIFICATION OF RHP 



Morton (1958) has proposed that RHP be considered the prototype for 

 a new group of cytochromes which is to be labelled 'D\ This group includes 

 the 'new respiratory enzyme' (carbon monoxide-binding pigment) of a num- 

 ber of facultative bacteria described by Chance and co-workers (Smith, 1954; 

 Castor and Chance, 1959), and the RHP-like pigment of the halotolerant 

 nitrate-reducing coccus described by Taniguchi et al. (1958). There is little 

 question that the anomalous character of RHP, as isolated, requires a 

 revision of the nomenclature accepted at present, but sufficient evidence 

 based on actual isolation of the various pigments, hitherto detected as spectro- 

 scopic entities only, is not at hand to strengthen the rather tenuous assump- 

 tion that all of these haematin compounds are closely identical with RHP 

 chemically. 



In the one cytochrome with an RHP character described as occurring in a 

 non-photosynthetic system, that of Taniguchi et al. (1958), the preparation 

 shows the characteristic heterogeneous Soret peak in the reduced form, as 

 well as the haematin band in the oxidized form. There is also the proper 

 ratio of intensities between the haematin band, the a- and Soret bands of 

 the reduced form, as well as the same location of absorption maxima. The 

 a-band of the reduced form shows the characteristic heterogeneity exhibited 

 by RHP. However, there is also a marked /?-band in the reduced form, 

 located at approximately 521 mji, which is not characteristic of RHP. It is 

 probable that this preparation contains some cytochrome c admixed with 

 the presumed RHP. The hydroxylamine reductase activity of RHP, as 

 obtained from R. rubrum or Chromatium has not been investigated, so it 

 cannot be said whether RHP is like the haematin compound of the halo- 

 tolerant coccus in this respect. However, it remains to be demonstrated that 

 there is a correlation between this activity and the haematin content of the 

 coccal preparation, inasmuch as the preparation, while electrophoretically 

 homogeneous, has a very high molecular weight, a very low iron content 

 (approximately 0-03%), and shows spectroscopic evidence of heterogeneity. 

 Further purification may resolve these dilficulties and confirm the coccal 

 pigment as a haematin compound of the RHP type. 



In view of the present uncertainties both with regard to distribution and 

 function, it may be premature to adopt RHP as the basis for a new group of 



