ISOTOPE TECHNIQUE 



the Lsolopc clilnlion iiuiliod li;is I)C(mi ils use in (Iclcniiiiiing whether 

 or not certain siihstances fire iinoKccI in aniinal inctabohsm. For 

 example, it has been rlemonslialed that large amounts of acetate are 

 produced daily in the rat thongh its (oneentratinn is never great enough 

 to he measured directly (4). 



The role of r/-ainino acid oxidase in the animal organism is 

 obscure, though it appeared possible that its function was to deaminate 

 <f-amino acids that might arise from a partially or completely symmet- 

 rical synthesis of amino acids from a-keto acids (19). The demon- 

 stration that </-amino acids are not formed in the animal organism 

 would be very difficult with ordinary techniques. If, however, a 

 substance labeled with an isotope is administered to an animal and 

 subsequently isolated either from the tissues or the urine, then the 

 isotope concentration of the recovered compound will indicate whether 

 or not it has been diluted by some of the same nonisotopic substance 

 synthesized by the animal. f/jZ-Glutamic acid and f/,/-tyrosine labeled 

 with N'^ were administered to rats and the (/-amino acids isolated from 

 the urine. Since the isotopic content of the isolated (/-amino acid was 

 the same as the amino acid administered, it would appear that (/-amino 

 acids are not formed in the animal organism (35). By a similar pro- 

 cedure Bernhard (2) demonstrated that the long-chain dicarboxylic 

 acids are not intermediates in the metabolism of the fatty acids, as had 

 been suggested by Vei'kade (46). Bernhard used deuteriodicarboxylic 

 acids, and isolated from the urine of the test animal fatty acids con- 

 taining the same deuterium concentration as the sample that had been 

 fed. 



The finding of N^^ in the a-amino group of amino acids of an 

 animal fed either labeled ammonia or an amino acid demonstrates 

 that the organism can synthesize these amino acids from the corre- 

 sponding keto acids. Such evidence throws no light on the question 

 as to whether an amino acid is essential or nonessential from the stand- 

 point of nutrition, since, while the cell may be able to convert a keto 

 acid to the required amino acid, it may not be able to synthesize the 

 appropriate keto acid. This seems to be true for most essential amino 

 acids. On the other hand, if, under these circumstances an amino 

 acid contains no labeled nitrogen, then we can conclude that the animal 

 cannot synthesize it, even though the carbon skeleton is available. 

 Examples of these two cases, in the rat, are leucine (24) and lysine 



