CHEMOTHERAPEUTIC AGENTS I49 



provided pyridoxin, PAB, CO 2 and glycine were available. 

 Further evidence that pyridoxin as well as PAB partici- 

 pated in the synthesis of serine from glycine was obtained by 

 using cells grown in the absence of these two growth factors 

 in a medium containing all the other known grov^h fac- 

 tors together with amino-acids, purines and pyrimidines. 

 Washed suspensions of these cells would synthesize serine 

 only if they were provided with glycine, formate, glucose, 

 pyridoxin and PAB (or folinic acid); synthesis was com- 

 pletely inhibited by sulphanilamide. Similar experiments 

 have been done with Strep. faecaliSy Lb. hifidus and Sac. 

 cerevisiae. It appears that both PAB and vitamin B^g play 

 a role in the synthesis of methionine, the most active amino- 

 acid antagonist of the sulphonamides. A vitamin B^2 auxo- 

 troph of Esch. coli grew in the absence of B^g if the medium 

 contained methionine [4], while cells of a mutant requiring 

 PAB and grown in the absence of this factor (i.e. in a 

 medium containing amino-acids, purines and pyrimidines) 

 only synthesized methionine from homocysteine in the 

 presence of PAB and glucose. The simultaneous addition of 

 B12 stimulated synthesis threefold, and recent work suggests 

 that the methyl group used in the methylation is derived 

 from serine. There is some evidence that the role of PAB 

 in methionine synthesis cannot be explained solely on the 

 basis that it is required for the synthesis of B12 and 

 serine [29]. 



The connection between PAB and the synthesis of the 

 group of substances now designated as vitamin B12 (the 

 cobalamins) began with the observation that the anaemia 

 produced by feeding an animal large amounts of sulpha- 

 thiazole was like pernicious anaemia and could be relieved 

 by large doses of synthetic folic acid or concentrates of sub- 

 stances isolated from the liver of normal animals. These 

 liver substances were not of the folic acid type and functioned 

 as growth factors for Lb. leichmannii and Lb. lactis. The key 

 compound, vitamin B^g, active both as a growth factor and in 

 the treatment of pernicious anaemia, has now been iso- 

 lated in the crystalline state and is composed of 5:6-di- 

 methylbenzimidazole - 1 - a - D -ribofuranoside-3 -phosphate 



