340 



CELLS, TISSUES, AND ORGANISMS 



betahydroxethylthiazole. These two portions are synthesized and then 

 joined together by most microbial organisms. Various mutants or spe- 

 cial strains need the complete molecule or one or both of the two por- 

 tions. Animals are unable to couple the pyrimidine and the thiazole 

 to form thiamine. The coupling in microorganisms appears to take 

 place by phosphorylation of the hydroxymethyl group of the pyrimi- 

 dine compound. This phosphate ester then couples with the thiazole 

 group to form thiamine, as shown in Figure 4, and inorganic phosphate 

 is liberated ( Harris, 1957 ) . A second possibility is that the end product 

 may be thiamine monophosphate (Camiener and Brown, 1959). Phos- 

 phorylation of thiamine to form the coenzyme thiamine pyrophosphate 

 is readily carried out by living organisms. 



Riboflavin. Riboflavin is produced in large quantities by two yeast- 

 like organisms— Ere mothecium ashbyii and Ashbija gossypii. These 

 organisms have provided us with our best information on riboflavin 

 biosynthesis from various investigations, especially those of Plant and 

 McNutt, who have followed the incorporation of labeled precursors 

 into the riboflavin molecule. 



The B and C rings of riboflavin (the two right-hand rings of the 

 structure shown in Figure 5) form a group which is similar to a pteri- 

 dine or a purine. Forrest and McNutt ( 1958 ) found that adenine could 

 serve as a precursor of this group, with the loss of the 8-carbon atom 

 of adenine in the process. An intermediate product, 6, 7-dimethyl-8- 

 ribitvl lumazine ("compound G"), is then formed. Then ring A, the 



NHg 



CHs^,^ 



CH^OH 



CH, 

 I ^ 

 C=C-CH2CH20H 



N 

 CH-S 



ATP 



CH, 

 C=C-CH2CH20H 



CH-S 



ITniamine 



Figure 4. Condensation of the pyrimidine and thiazole portions of the 

 thiamine molecule. 



