202 INTERMEDIATE METABOLISM 



We have, indeed, no direct evidence of the transforma- 

 tion of protein into fat, except the fact that some amino- 

 acids yield /?-oxybutyric acid on administration to the 

 diabetic animal. But we know that protein can be con- 

 verted into carbohydrate and that carbohydrate can be 

 converted into fat. There is therefore no reason why 

 protein should not indirectly be converted into fat when- 

 ever fat is being rapidly laid down. 



Oxidation of Fats 



The first step, as we have seen, appears to be an intro- 

 duction of double hnkages, forming acids of the unsaturated 

 series. It is now universally beheved that oxidation of 

 fatty acids occurs in the /5-position — that is to say, that the 

 carbon atoms in the chain (and they are always straight 

 chains) are spUt off two at a time. This is the evidence : — 



1. In animal fats only those fatty acids occur which 

 have an even number of carbon atoms. 



2. In butter all the even series are present from those 

 containing eighteen to those containing four carbon atoms. 



3. When fats are burnt incompletely, as in diabetes, we 

 can detect substances partially oxidised in the /5-position : 

 /3-hydroxybutyric acid, CHg CHOH CHg COOH, and aceto- 

 acetic acid, CHg CO CHg COOH. 



4. On perfusion of the hver with various fatty acids 

 the formation of acetoacetic acid occurs only when the 

 fatty acids have an even number of carbon atoms. 



5. Knoojps Experiment.— When benzoic acid is adminis- 

 tered it is excreted combined with glycine in the form of 

 hippuric acid — 



CeHgCOOH + NH2CH2COOH= CgHsCONHCHaCOOH. 



When the next homologue, phenyl-acetic acid, is given, 

 this, too, is combined with glycine, w^th formation of 

 phenaceturic acid^ — 



CfiH.CHoCOOH + NH0CH2COOH = 



C«H.CH,CONHCH,COOH, 



