Hacinoprotcin of Purple Pholosynthetic Bacteria 429 



as a hydrogen donor (Kanien and Vernon, 1954). R. nihruiu RHP itself, 

 in the reduced state, spontaneously reduces the corresponding R. ruhrum 

 cytochrome c, as well as mammalian cytochrome c (Kamen and Vernon, 1 954). 

 This behaviour is reminiscent of that shown by the mammalian microsomal 

 cytochrome b^ when incubated with cytochronie c (Velick and Strittmatter, 

 1956). The specific DPNH-linked cytochrome h^^ reductase of liver micro- 

 somes (Strittmatter and Velick, 1956) fails to catalyse the reduction of RHP 

 with DPNH. The Chromatium preparation, in its purified form, has not 

 been tested using Chromatium extracts rich in the DPNH-linked cytochrome 

 reductase. Early efforts to establish the enzymic character of Chromatium 

 RHP in cytochrome c were negative but inconclusive, because the haem 

 complex used was a mixture of the two pigments in an undefined state of 

 purity and at least partly denatured (Newton and Kamen, 1956). 



The possibility that RHP might be a peroxidase has been eliminated by 

 the demonstration that all peroxidatic activity in extracts of R. rubrum or 

 Chromatium concentrated in fractions other than that containing RHP. 



Is RHP an Artifact? 



We have discussed elsewhere the possibility that RHP is an artifact 

 (Bartsch and Kamen, 1958). Briefly, the reasons for dismissing this possibility 

 are at least as cogent as those for considering other haemoproteins, such as 

 cytochrome c, as real constituents of cells rather than artifacts produced by 

 isolation procedures. In one respect only is evidence lacking, and that is the 

 enzymic basis for the existence of this novel haemoprotein. 



The reasons for considering RHP as a bona fide haematin component of 

 living cells may be recapitulated here. 



1. It can be seen as a spectroscopic entity in living cells (Bartsch and 

 Kamen, 1958). 



2. It is obtained by mild extraction procedures in yields equal to or better 

 than those found using more drastic methods involving relative extremes of 

 heat and acidity (Bartsch and Kamen, 1958). 



3. It is not formed from other cellular haem components by the extraction 

 and isolation procedures used (Vernon and Kamen, 1954; Bartsch and 

 Kamen, 1958). 



4. Difference spectra of actively metabolizing cells, determined under a 

 wide variety of different metabolic conditions, can be correlated with known 

 spectroscopic constants and chemical behaviour of isolated and purified 

 RHP (Chance and Smith, 1955; Olson and Chance, 1958; Smith and 

 Ramirez, 1959). 



5. It is one of the most abundant proteins in the bacteria in which it is 

 found. As an example, it is present in R. rubrum in amounts as high as 

 0-6%' of all cellular protein. Although this figure is minimal, it is at least as 

 high as that of the bacterial cytochrome c content, wliich is one of the 



