150 NITROGEN METABOLISM 



combined with an unidentified organic molecule contain- 

 ing cobalt. Though the B^g requirements of a micro-organ- 

 ism cannot be replaced by any of the known folic acids, 

 either synthetic or natural, they are replaceable by thymi- 

 dine and also in most organisms, by the deoxyribosides of 

 purines. This is to be contrasted with the fact that though 

 thymidine replaces part of the PAB or folic acid require- 

 ments of an organism, all other deoxyribosides are inactive. 

 Vitamin B^g contains an aromatic ring, yet it did not stimu- 

 late the growth of mutants with a multiple requirement for 

 aromatic compounds. Davis therefore suggested that it was 

 either derived from one of the aromatic substances required 

 by such mutants or else it was synthesized by a totally 

 different route or from an intermediate prior to the 

 genetically blocked reaction. Vitamin B^g exerts a sparing 

 effect on the PAB requirement of a PAB auxotroph of Esch. 

 coli, and since the amount of vitamin B12 required is only 

 one fiftieth of the amount of PAB, it is conceivable that the 

 ring of PAB is used directly in the synthesis of B^g [3]. On 

 the other hand, folic acid is known to be associated with the 

 synthesis of purines, which, like B^gj contain an imidazole 

 ring, and this may be the reason for the close relationship 

 between these two co-factors. At the present time, the 

 details of the relationship are but vaguely understood and 

 have only been explored in one species, Esch. coli. 



From these and other studies, it has become evident that 

 the ultimate co-factor form of PAB participates in the 

 methylation of homocysteine, the introduction of carbon 

 into position 2 of the purine ring (pp. 1 37-8), and the synthesis 

 of serine and thymine; in other words, this co-factor is con- 

 cerned with the intermediary metabolism of one carbon 

 units (cf. Co.A, the ultimate co-factor form of pantothenic 

 acid, and its function in the metabolism of acyl units [20]). 

 There is also reason to believe that Bjg is involved in at 

 least some of these reactions, e.g. the synthesis of methionine 

 and purines, and it is to be noted that the presence of B^g 

 increases the amount of sulphanilamide required to induce 

 bacteriostasis of Esch. coli growing in the presence of one 

 or more of the sulphonamide antagonists methionine, 



