BIOGENESIS OF THE B VITAMINS 83 



pea embryos, 33 and while it is presumed that green plants synthesize 

 nicotinic acid, little attention has been paid to the problem. 



The relatively recent demonstrations that nicotinic acid is nutritionally 

 replaced by tryptophan in higher animals (p. 279) has, of course, an 

 important bearing on the problem of its biogenesis. While intestinal syn- 

 thesis of nicotinic acid in rats has been demonstrated, 34 it is not known 

 how important such synthesis is because nicotinic acid is also reported 

 to be formed in rat tissues. 35 Human beings also excrete more nicotinic 

 acid degradation products when tryptophan is fed. 36 It appears certain 

 that the production of nicotinic acid from tryptophan in animals is im- 

 portant in the economy of nature, whether or not bacteria play an impor- 

 tant role symbiotically in the transformation. 



The precise mode in which nicotinic acid is synthesized is at present 

 under intensive investigation, and our insight into this problem is ham- 

 pered more by the vast amount of conflicting information than by lack 

 of data. Present trends tend to emphasize the biosynthetic sequence start- 

 ing from tryptophan and proceeding as follows: 



O 

 — -.— CH 2 — CH— COOH ^N— C— CH 2 — CH— COOH 



. N J NH 2 " k^-NH, NH 2 



Tryptophan Kynurenin 



—COOH r |T~ C00H 



OH 



6-Hydroxy-anthranilic acid Nicotinic acid 



This plan has much to support it; it is discussed in greater detail in the 

 sections that follow (pp. 279, 353). It is, however, well known that both 

 ornithine and proline are involved in niacin biosynthesis, and these facts 

 are completely neglected in the above scheme. Proline, ornithine and glu- 

 tamic acid have all been shown to be effective in increasing the production 

 of trigonellin by rice plants. 



or 



CH, 



Trigonellin 



