BIOGENESIS OF THE B VITAMINS 87 



tion with the other B vitamins. Tomato roots, for example, are stimulated 

 in growth by pyridoxine, 44 and in this respect pyridoxine shows a re- 

 semblance to some of the other B vitamins. 



From recent work it appears that alanine is not, as formerly supposed, 45 

 a direct precursor of vitamin B and that D-alanine, which is reported 

 to be a consistent cell constituent for several organisms, can replace B 6 

 in culture media without giving rise to additional B 6 . 45a 



Biotin 



The bacterial synthesis of biotin undoubtedly constitutes an im- 

 portant natural source. While biotin deficiency has often been induced 

 in animals it is usually by introducing egg white or avidin into the diet, 

 or administering sulfa drugs, or eliminating bacteria from the intestinal 

 tract 44 that it is accomplished (see p. 428). Usually feeding a biotin- 

 deficient diet is itself ineffective, though not necessarily so with baby 

 chicks. Peterson and Peterson * list 21 bacteria which produce biotin, 

 among them several common intestinal bacteria. The synthesis of biotin 

 is not limited to the bacteria which fail to respond to it; thus Rhizobia 

 which are stimulated by biotin (coenzyme R) can be cultured under 

 conditions in which preformed biotin is excluded. 46 As Knight 47 has aptly 

 pointed out, very limited growth of a microorganism may occur in a 

 biotin-deficient medium even though the organism is able to synthesize 

 biotin. The rate of growth may under these conditions be limited by the 

 rate of synthesis, in which case the addition of biotin to the culture 

 medium may greatly accelerate growth. 



The synthesis of biotin by bacteria is attended often by a large amount 

 of release into the culture medium. In four organisms studied by Thomp- 

 son 5 the amount of biotin found in the medium averaged about 8 times 

 that in the bacterial cells. This release into the medium is not due to 

 the autolysis of dead cells as it occurs progressively in a rapidly growing 

 culture. 



The bacterial production of biotin in the rumen of cattle has been 

 demonstrated, 48 - 49 and rats on a biotin-deficient ration excrete much 

 more biotin than they take in, 50 as do also human beings. 51 The presence 

 of relatively large amounts of biotin in bacterial culture media which 

 have accidentally been contaminated by organisms from the air bespeaks 

 the ubiquity of organisms capable of producing this vitamin. 



Yeasts and lower fungi as well as bacteria produce biotin. While many 

 yeasts are stimulated by biotin they often are capable of its synthesis. 2, s 



Knight 52 lists 12 lower fungi, including Aspergilli and Penicillia, which 

 do not require biotin, and it is relatively safe to conclude that they 

 synthesize it. One notable genus of molds that requires biotin, however, 



