66 THE BACTERIAL PHOTOCHEMICAL APPARATUS 



in chromatophore preparations, even those from strict anaerobes. 

 Some qualitative observations indicate the catalase of Chromatium 

 to be a heme protein (17), Proto-heme, the usual prosthetic group, 

 can be extracted by acid- acetone (18), but this may arise wholly from 

 the 6-type cytochrome which is present, 



(e) "o-Type" cytochromes. The dark oxidase activity of R. rubrum 

 seems to require the presence of a particle-bound heme protein with 

 spectrochemical characteristics like those of a cytochromoid (see 

 below), in the visible range. No isolation of these proteins has been 

 reported. Researches, based on analysis of spheroplasts, obtained by 

 lysis of dark-grown cells, and other systems enriched in the aerobic 

 phosphorylation system, are needed. 



It may be remarked here that not a single bacterial oxidase, let 

 alone the "o-type" proteins, has been characterized as yet. 



(f) Cytochromoids (3e). This class illustrates best the surprises 

 which may be in store for biochemists, when they begin to take bac- 

 terial heme proteins more seriously. Again, we owe to Leo Vernon 

 the original observation which led to the discovery of these proteins. 

 He noted in 1953 that trichloracetic acid extracts of R. rubrum con- 

 tained a heme protein, other than a cytochrome of the "c" type, with 

 an absorption spectrum like that of myoglobin, or hemoglobin (2), 

 Since then, our laboratory has been engaged in continuous research 

 in an effort to elaborate the nature and function of what we now call 

 "cytochromoids," Parenthetically, I may add that the existence of cyto- 

 chromoids is a surprise only to those who persist in ignoring possibili- 

 ties of combinations such as those given in the classification scheme I 

 have described above. 



The official definition, as proposed by the Commission on Enzymes 

 of the International Union of Biochemistry (5), is "heme proteins with 

 a hemoglobin- like spectrum and a reactivity with ligands which do not 

 react with cytochrome c." Cytochromoids are essentially heme pro- 

 teins in which the normal heme prosthetic group retains the high- 

 spin or mixed-spin character of "open" type heme proteins (19), while 

 being bound covalently as in cytochrome c. The mixed- spin character 

 used in classification emerges only in the oxidized forms of cyto- 

 chromoids so far studied. There are just two specimens— one from 

 R. rubrum (I), the other from Chromatium (II). Another specimen, 

 recently isolated from Rps. palustris (2), shows the same spectral 

 characteristics and ligand behavior as I and II, but differs in that it 

 has a high oxidation potential (E^i 7 250 mv) and is not autoxidizable, 

 whereas I and n have low oxidation potentials (Ej^ 7 ~-8 to -5 mv) and 

 are rapidly autoxidizable (3e). Thus, with just three specimens ex- 

 amined thoroughly, the cytochromoids appear already to exhibit a wide 

 range of physicochemical character, just as do the "c" cytochromes. 



Both I and II have molecular weights close to 28,000, and contain 

 two heme groups per mole. Sequence analysis for the diheme- 



