332 



VITAMIN REQUIREMENTS 



We may assume that synthesis of the pyrimidine moiety follows that 

 of other pyrimidines (Chapter 8). The origin of thiazole in Neuro- 

 spora crassa is apparently related to amino acid metabolism (50). 



Two fungi, Phy corny ces blakesleeanus and Sclerotium rolfsii, which 

 can use the two moieties in place of thiamine, also destroy or convert 

 to an inactive form part of the thiazole, whether added as thiamine or 

 as free thiazole. Phytophthora cinnamomi, which requires intact thi- 

 amine, does not destroy free or combined thiazole. Thus the destruc- 

 tion of thiazole seems to be unrelated to the use of thiamine in the 

 growth processes of the fungi (120). The effect of temperature on the 

 enzyme which destroys thiazole may explain the observation that the 

 efficiency of thiamine for growth increases with decreasing temperature 



(212). 



The compound "pyrithiamine" is inhibitory to those fungi, e.g., 

 Phytophthora cinnamomi, which require intact thiamine, and does 

 not inhibit those, e.g., Neurospora crassa, which synthesize thiamine; 

 organisms requiring only one moiety of thiamine are intermediate in 

 susceptibility (312, 314). However, the problem needs further study, 

 in view of the fact that thiamineless mutants of Neurospora crassa do 

 not respond according to expectations (279). Further, it is believed 

 that "pyrithiamine" is a mixture of different compounds; neopyrithi- 

 amine has been synthesized (303) and has the structure, of a pyridine 

 analogue of thiamine, originally assigned to "pyrithiamine." Neo- 

 pyrithiamine competitively inhibits thiamine metabolism in animals 

 (162). "Pyrithiamine" can be used by pyrimidine-deficient fungi as a 

 source of the pyrimidine ring (207). 



Figure 2. The response of Stachy- 

 botrys sp. (SN 1) to biotin. From 

 data of Marsh and Bollenbacher 

 (161). 



0.004 0.04 0.4 



Biotin, rriMg per ml 



4.0 



