348 



CELLS, TISSUES, AND ORGANISMS 



involvement of malonyl CoA in the synthesis of fatty acids, and the 

 formation of malonyl CoA by biotin-induced carboxylation of acetyl 

 CoA, are clues to the known relation of biotin to the biological syn- 

 thesis of fatty acids. 



The transfer of amino groups in the keto acid-amino acid trans- 

 amination process is brought about by a pyridoxamine-pyridoxal en- 

 zymatic interchange. 



Various examples of the carrier functions of vitamins are summa- 

 rized in Table I. These examples of reactions in which small molecular 

 units are transferred serve to illustrate one of the most important func- 

 tions of vitamins in the growth of animals. 



TABLE I 

 Carrier Functions of Vitamins 



Antibiotics and growth 



The effect of antibiotics in inceasing the growth rate of animals 

 has focused attention on the fact that animals live in a state of nutri- 

 tional equilibrium with many billions of bacteria in their intestinal 

 tracts. These bacteria may consume part of the food supply of the host, 

 and they may produce toxic substances such as decarboxylated amino 

 acids, which may or may not enter the bloodstream. The bacteria may 

 produce vitamins, such as biotin and vitamin K, which may be ab- 

 sorbed by the host directly through the intestinal wall or secondarily 

 through coprophagy. 



Various studies with germ-free animals kept in sterile environ- 

 ments have indicated that the absence of intestinal bacteria leads to a 

 somewhat more rapid growth rate in rats and chicks, provided that the 

 animals have a well-supplemented diet. The addition of common anti- 

 biotics to the diet under these conditions does not affect the rate of 

 growth. When the same antibiotics are added to the diet of apparently 

 healthy animals under conventional conditions, a growth response is 

 observed. This is due not to a wholesale elimination of the population 



