716 



THE BIOCHEMISTRY OF B VITAMINS 



Iron Porphyrins (Hemes) As Growth Factors and Inhibitors 

 The same porphyrin nucleus which is present in heme, the nonprotein 

 component of hemoglobin, occurs in the prosthetic group of various im- 



H 3 C- 



HC- 



-CH=CH 2 



-CH 



H 3 C- 

 HOOCCH 2 CH 2 - 



HC- 



N Fe N 



N 



v v 



/ 



HOOCCH 2 CH 2 — 



=CH 



-CH 3 



iron protoporphyrin IX 

 (heme or protoheme) 



-CH 3 



-CH=CH 2 



portant enzymes, including catalase, peroxidase and the cytochromes. 

 Compounds whose structures are identical with or closely related to that 

 of iron protoporphyrin IX have been found to be present in all anima 1 

 and plant cells that have been examined with the exception of some 

 anaerobic bacteria. 



Among the organisms which have been found unable to synthesize 

 protoporphyrin IX, the precursor of heme, are bacteria (Hemophilus 

 influenzae), protozoa (certain trypanosomidae), and one insect species 

 (Triatoma infestans). Consequently, protoporphyrin IX is an essential 

 growth factor for these organisms. Although it has never been classified 

 as a B vitamin, its mode of action, its role in respiratory enzyme systems, 

 and its probable (but still unknown) relationships to some of the B vita- 

 mins may justify its inclusion here. 



The role, mechanism of action and chemical structure of the iron por- 

 phyrins have been discussed in two excellent review papers by Granick 

 and Gilder no and by Lwoff . ni 



It was discovered as early as 1892 that Hemophilus influenzae did not 

 grow unless a small amount of blood or hemoglobin was added to the 

 culture medium. 112 It was shown by Davis 113 and by Thjotta and 

 Avery 114 that this organism required two growth factors, a heat-stable 

 substance X which is found in hemoglobin and a heat-labile factor V 

 which is present in yeast and in fresh animal and vegetable tissues. 



Lwoff and Lwoff 115 found that the V factor could be replaced by coen- 



