126 Comparative Animal Physiology 



proper growth and for normal skin, hair, and hoofs. ^°''' Fur-bearing mammals 

 such as the fox require riboflavin for normal sleekness and color of their 

 fur. "^' "" Manifestations of deficiency of riboflavin in man appear as lesions 

 of skin, mouth, or eyes, and the recommended intake is 2.7 mg./day. Monkeys 

 deficient in riboflavin fail to grow normally, develop an anemia and dermatitis, 

 and on autopsy have fatty livers; they need 25-30 />,g./kg./day. ^'- 



It is difficult to know the true body requirement for riboflavin since it is 

 synthesized by digestive microorganisms. When these microorganisms are in 

 the cecum or colon riboflavin may not be absorbed in sufficient quantity and 

 more may be lost in feces than need be taken by mouth. In the rabbit, for 

 example, the total excretion (largely fecal) of riboflavin is 10 to 15 times the 

 intake, and rabbits do well on very low dietary riboflavin, either because of 

 cecal absorption or from eating feces. ^^^ Ruminants obtain sufficient ribo- 

 flavin by the synthetic process of rumen organisms. In sheep the dried rumen 

 contents contained 32 /xg./gm. as compared with 0.3 /xg./gm. in the food, 

 whereas the milk of cows contained 16-18 mg. per day compared with an 

 intake of 1.8 mg. in the food. ^^ Calves lacking ruminant organisms require 

 riboflavin in the diet. ^'^'^ The amount of riboflavin produced by organisms 

 in the digestive tract varies according to the carbohydrates in the diet, and 

 the riboflavin excreted in rats on a high lactose diet is doubled because of 

 stimulation of specific flora. ^-^ 



Niacin (Nicotinic Acid). Niacin is the third vitamin which is known to 

 function in cellular respiration. With adenine, a pentose, and phosphoric acid, 

 nicotinic acid amide (niacinamide) constitutes coenzymes I and II which with 

 specific proteins are dehydrogenases. The structure of niacin is: 



H 



HC C-COOH 



II 1 



HC^ CH 



Niacin is synthesized by many microorganisms, yeasts, bacteria, and flagellates, 

 and it is probably required by all metabolizing cells, hence by all animals. Its 

 synthesis in amounts sufficient for good growth and multiplication has been 

 demonstrated in Chilonionas. •''**" Niacin is required by the ciliates Tetra- 

 hymena and Colpoda diiodenaria. 



Niacin is required by a variety of insects, but it is produced by the symbiotic 

 microorganisms in some of them (Table 24). Turkeys deficient in niacin 

 develop a perosis or scaly skin lesions. *''^ In mammals also the microorganisms 

 of the digestive tract synthesize niacin (or nicotinamide), and even in man the 

 body requirement may be partly filled by synthesis from tryptophane. Enough 

 is formed and absorbed in the hamster that none need be added to the diet. 

 In rats, mice, and man tryptophane is converted to niacin, partly by digestive 

 microorganisms but largely in body tissues, particularly the liver, since some 

 additional niacin is still excreted when the flora is poisoned by sulfathiazole 

 or when tryptophane is injected intravenously. Synthesis is increased by add- 

 ing tryptophane to the diet and favored by the presence of pyridoxine."^' '■"' 



