METABOLISM OF THE B VITAMINS 351 



nicotinic acid predominates in plant tissues. In animal tissues and yeast 

 pyridoxal and pyridoxamine predominate, while pyridoxine is present in 

 larger amounts in plants. 55 Of theoretical interest in this regard is the 

 fact that oxybiotin functions per se in animal tissues and is not converted 

 to biotin. 56 Its storage in tissues parallels closely that of biotin. 57 Vitamin 

 storage in animals is a limited process, being influenced by many factors, 

 most frequently by the protein intake, as previously mentioned (p. 277) . 

 Whereas storage doubtless occurs throughout the body, the liver seems to 

 be particularly effective as a storage depot in many cases. As previously 

 noted, rats on a low-protein diet are not able to store riboflavin, regard- 

 less of the intake, but excrete unused excesses promptly, whereas high 

 riboflavin intakes on a high-protein diet bring about prompt liver storage 

 of riboflavin. 58 Similar relationships have been observed for other vita- 

 mins, 59 although storage abilities vary among species and for each 

 vitamin. Thus the depletion of some vitamin in one species {e.g., thiamine 

 in the rat) may cover a long period of progressive pathology, while in 

 others (e. g., thiamine in the mouse), the depletion is so rapid that death 

 is one of the first observable symptoms. 60 For this reason, the general 

 nature of and trends in B vitamin storage are so erratic as to make 

 generalization impossible for practical purposes. At best, the possibilities 

 of building up extensive B vitamin reserves are limited, unlike the fat- 

 soluble vitamins, and current trends in nutritional thought along these 

 lines should be considered critically in regard to both humans and farm 

 animals. 61 The desirability of increasing the vitamin level in meats in 

 certain areas is great, and it has been shown that it is possible to increase 

 the thiamine level in hog tissues by feeding higher levels of thiamine in 

 the diet. 62 It should be noted that the hearts and livers of hogs on lower 

 thiamine levels, however, had more riboflavin than did those tissues from 

 animals receiving adequate thiamine. In the chicken, increases in muscle 

 pantothenic acid cannot be achieved by dietary supplementation with 

 pantothenic acid levels above those required for adequate nutrition, how- 

 ever. A considerable amount of data has recently shown that within limits 

 the riboflavin content of hens' eggs may be increased by increasing the 

 dietary riboflavin, and in such cases when the efficiency and limits of 

 effectiveness are known, higher feeding levels are obviously desirable 

 in fowls. 63-66 



Synthesis and Catabolism in Animals 



The biogenesis of the B vitamins has been discussed at some length m 

 an earlier chapter, but certain considerations make it seem appropriate 

 to review some aspects of this subject as it applies to the higher animals. 

 Whereas most animals have the ability to convert the commonly recog- 



