FOLIC acid; strepogenin 105 



CI. acetobutylicum. Its metabolic function is still not clear. 

 It lias been shown to form part of the structure of folic acid, a 

 growth factor for several Lactobacteriaceae, but it may have 

 functions other than those involved in the production of folic 

 acid. FoHc acid itself is presumably a prosthetic group but 

 no enzymes requiring it have yet been discovered. The 

 deficient-culture technique does not seem to have yielded 

 results, but the discovery that folic acid can be replaced in 

 growth media by mixtures of thymine and other purines and 

 pyrimidines has led to the suggestion that the fohc-activated 

 enzymes are concerned with purine and pyrimidine synthesis. 

 Similar evidence suggests that it may also be concerned in 

 methionine synthesis while ^-amino-benzoic acid, apart from 

 folic acid, may be involved in the synthesis of other amino- 

 acids, such as lysine. 



A growth factor of unknown constitution and function is 

 Strepogenin. This is a substance necessary for the growth 

 of some Lactobacteriaceae and has the properties of a 

 peptide. It can be obtained from partial hydrolysates or 

 enzymatic digests of crystalline proteins such as insulin and 

 so must represent a part of the structure of these proteins. 

 It is thought that it is a specific peptide structure synthesised 

 by a specific enzyme which can be lost by mutation. 



Haemophilus influenzae is exacting towards h.aematin, but 

 can in some cases dispense with this if growth takes place 

 under anaerobic conditions. It would appear that haematin 

 is necessary for some oxidative process, and since catalase is 

 a haematin-enzyme, it has been suggested that the haematin 

 is required for the synthesis of catalase which protects the 

 organism from hydrogen peroxide formed during aerobic 

 existence. This suggestion is supported by the fact that 

 haematin can be replaced as a growth factor by cysteine, 

 which decomposes HgOg by reduction. 



In the same way that it is possible to select mutants of 

 Eherthella typhosa which can synthesise their own tryptophan, 

 it is also possible in some cases to select from cultures of 

 organisms exacting towards growth factors, mutants which 



