710 METABOLISM 



of Knoop and of Embden and his coworkers. Knoop conceived the idea 

 of introducing into the fatty-acid molecule some group which is resistant 

 to oxidation in the body. The phenyl group (C 6 H 5 ) was found to have 

 this effect. By feeding an animal with the phenyl derivatives of acetic, 

 propionic, butyric, and valeric acids, it was found that the urine con- 

 tained either hippuric (see page 630) or phenaceturic acid. Both of 

 these are compounds of aromatic acids with glycocoll or aminoacetic 

 acid (CH 2 NH 2 C'OOH), one of the protein building-stones and always 

 available in the organism to form such compounds, thus: 



(1) C 8 H 5 COOH + CH 2 NILCOOHz=C 6 H/X)NHCH 2 COOH. 



(benzoic (glycocoll) (hippuric acid) 



acid) 



(2) C 6 H 6 CH 2 COOH + CHJSTH^COOH = C 6 H 5 CH 2 CONHCH 2 COOH. 

 (phenylacetic (glycocoll) (phenaceturic acid) 



acid) 



When either benzoic acid (C 6 H 5 COOH) or phenylacetic acid (C G H 5 CH 2 - 

 COOH) is formed in the body as a result of the oxidation of phenyl 

 derivatives of the higher fatty acids, the acid combines with glycocoll 

 according to the above equations. From this it folloAvs that if oxidation 

 occurs so that two C-atoms are thrown off at a time (/^-oxidation), fatty 

 acids with an even C-atom chain should yield hippuric acid, and those 

 with an uneven chain, phenaceturic. This was found to be the case, as 

 the accompanying table shows. 



acid 



Phenylwopioiiic acid, C 8 H, . CH, . CH, . COOH C 6 H 5 .COOH Hippuric acid 



Phenylbutyric acid, C e H 5 . CH, . CH 2 . CH 2 . COOH C 6 H 3 . CH 2 . COOH Phenaceturic 



acid 

 Phenyl valeric acid, C 6 H 5 . CH 2 . CH 2 . CH . CH 2 . COOH C 6 H C . COOH Hippuric acid 



(From Dakin.) 



Embden 's experiments are equally convincing. He studied the forma- 

 tion of acetone in defibrinated blood perfused through the freshly excised 

 liver. Normally only a trace of this substance is formed, but when fatty 

 acids with an even number of carbon atoms were added to the blood, 

 they gave rise to a marked increase in acetone, whereas those with an 

 uneven chain failed to cause any change. The acetone was found to be 

 derived immediately from acetoacetic acid. The following table shows 

 the results. 



