ANIMAL OXIDATIONS. 455 



isobutyl acetic acid itself is not broken down into acetone, while this will 

 be the case with isovaleric acid, as the following formulas show: 



CH 3 CH 3 



\ 



C 

 CH 2 



CH 8 CH 3 



%'$ CH 3 CH 3 x . / 



8 N / CO 



(0) CH 



(a) CH 2 (a) CH 2 



COOH COOH 



Isobutylacetic acid Isovaleric acid Acetone 



Direct experiment confirms this explanation. The decomposition of 

 leucine, therefore, may be represented as taking place in the following 



stages : 



In place of the isovaleric acid, naturally the corresponding aldehyde or 

 alcohol may appear: 



CH 3 CH 3 CH 3 CH 3 



\ / \ / 



CH CH 



CH 



CHO CH 2 OH 



Isovaleric aldehyde Isoamylalcohol 



We have intentionally gone into this hypothesis at length in order to 

 emphasize the complexity of such a process in contrast to the simple idea 

 of combustion in the tissues that is generally assumed. Many observa- 

 tions speak for the above conception of the breaking down of the amino 

 acids. We have discussed this with tyrosine and phenylalanine. On the 

 other hand, we must admit that we are not yet able to understand why 

 the animal organism can split off a carboxyl group from leucine, but not 



