82 THE BIOCHEMISTRY OF B VITAMINS 



riboflavin by Aspergillus niger is more effective when the culture medium 

 is relatively deficient in magnesium. 26 



The importance of bacteria and fungi as producers of riboflavin in 

 nature cannot seriously be doubted. The demonstration is particularly 

 clear in connection with the action of intestinal bacteria and of the or- 

 ganisms inhabiting the rumen of cattle and sheep. 27 Milk is a highly im- 

 portant nutritional source of riboflavin, and it is clear that the feed which 

 the cow consumes is not the only source of the vitamin, since the output 

 of riboflavin in the milk may be ten times as great as the intake in the 

 food. The riboflavin output in the milk of cows and goats is independent 

 of the content of the feed, and the rumen content of riboflavin may be 

 100 times that of the feed. Intestinal synthesis of riboflavin has been 

 demonstrated in rats, 28 fowls 29 and in man, 30 and presumably takes place 

 in animals generally. In ruminants the production takes place higher in 

 the intestinal tract and the utilization is therefore expedited. Intestinal 

 synthesis in animals and in man is not sufficient, of course, to insure 

 against riboflavin deficiency. 



The production of riboflavin during the growth of green plants is indi- 

 cated by the fact that leafy vegetables are generally good food sources, 

 but little definite information is available as to the locus of the synthesis 

 or whether symbiotic microorganisms are important. The discussion of 

 the general topic of B vitamin synthesis in the previous section on 

 thiamine is applicable also at this point. 



Nicotinic Acid 



The synthesis of nicotinic acid by bacteria has been observed to be far 

 less widespread than, for example, the synthesis of riboflavin. In the list 

 of organisms having growth factor requirements previously mentioned, 1 

 48 bacteria are stimulated by or require nicotinic acid, 9 do not require 

 it (and presumably carry out its synthesis), for 3 bacteria its synthesis 

 has been demonstrated, and no information was available concerning 76 

 others. In the same review, 14 organisms which synthesize nicotinic acid 

 are listed in another table. Here, as in other cases, some strains of a given 

 species may have a requirement while other strains do not. Thompson's 

 study 5 showed that five diverse organisms which are capable of growing 

 on a relatively simple medium all carry out nicotinic acid synthesis. Very 

 little attention has been paid to the question of the production of nicotinic 

 acid by yeasts and fungi, though unquestionably those which can grow 

 on simple media carry out its synthesis. 



While some observations have been made with respect to the stimu- 

 latory action of nicotinic acid or nicotinamide on plant roots 81 - 32 and 



