HEMATIN COMPOUNDS IN PHOTOMETABOLISM 153 



(38). It can act as a substrate for either the bacterial or mammalian 

 cytochrome c reductase (25). Since it is rapidly autoxidizable and 

 forms a CO compound in the reduced state it could function as a 

 terminal respiratory oxidase. Another interesting property of this 

 compound is its ability to reduce the bacterial cytochrome c non- 

 enzymatically. In this respect it behaves in a manner reminiscent of 

 mammalian cytochrome c with the bacterial cytochrome c or cyto- 

 chrome /. In other words, it provides a sort of oxidase for the cyto- 

 chrome c of the bacteria as well as for the low potential electron do- 

 nors present in the oxidative system. 



There is no satisfactory name for this compound at present. 

 It has been called variously "pseudohemoglobin," "yellow pigment," 

 and "CO-binding pigment." The difficulty of naming and classifying 

 it can be resolved only by precise determination of its functions in 

 bacterial metabolism. 



HEMATIN COMPOUNDS OF STRICTLY ANAEROBIC 

 PHOTOSYNTHETIG BACTERIA 



The green sulfur bacteria and the purple sulfur bacteria make up 

 two divisions of the group of photosynthetic bacteria (34) . They are 

 marked off from the other division, the nonsulfur purple bacteria, be- 

 cause they can grow only as photoanaerobes. In the absence of light, 

 they cannot use energy derived aerobically or anaerobically for 

 growth. Thus, they provide a unique opportunity for the study of the 

 function of hematin compounds in systems uncomplicated by path- 

 ways for utilization of energy other than that available in photo- 

 sjmthesis. 



In the green bacteria, only one genus — Chlorohium — is recognized 

 (22). The few species known are differentiated from each other by 

 characteristic substrate requirements for growth. Three hematin 

 compounds are known. The first to be isolated in cell-free extracts is an 

 iron porphyrin protein from C. limicola (17). Two others have been 

 detected in C. thiosulfaticum (13). The properties of C. cytochrome- 

 554 (1) and the C. limicola pigment are summarized in Table III. 

 It is seen that these pigments while resembling a cytochrome of the 

 "c" type show redox potentials more characteristic of cytochromes of 

 the "6" type. 



One hematin compound, isolated and purified from the purple sul- 

 fur bacterium, Chromatium, strain D, has been studied (26). Purifi- 



