B.— CHEMISTRY. 59 



break down the corresponding a-hydroxy acid ; or he might do a Hofmann 

 degradation on the amide. In either case he would get an acid with 17 

 carbon atoms, and have to repeat the degradation in order to obtain 

 palmitic acid. Knoop has shown, however, that in the animal organism 

 the p-carbon atom is attacked so that the chain is shortened by two 

 carbon atoms at a time, an acid with 18 carbon atoms being converted 

 .successively into one with 16, 14, 12, etc. In accordance with this 

 scheme the principal fatty acids in nature are those with an even number 

 of carbon atoms. Knoop established this by feeding to-phenyl fatty 

 acids ; those with an even number of methylene groups in the side chain 

 were converted into benzoic acid and appeared in the urine as hippuric, 

 those with an odd number of methylene groups yielded phenyl acetic 

 acid and were excreted as phenaceturic acid. 



Instead of using a resistant phenyl group and the whole animal, fatty 

 acids themselves and the isolated liver may be used to establish the same 

 result. Embden perfused the ten lowest members of the series of fatty 

 acids through a surviving liver and obtained acetone formation only with 

 those having an even number of carbon atoms ; they are converted by 

 p-oxidation finally to acetoacetic acid, from which acetone results. Dakin 

 found in hydrogen peroxide an oxidising agent which closely imitates the 

 biochemical method in vitro and at a low temperature ; thus butyric 

 acid gives acetoacetic, and the higher fatty acids give methyl ketones, 

 with loss of carbon dioxide. 



The same method of oxidation seems to occur in the vegetable kingdom, 

 for plants are apt to contain ketones with an even number of carbon 

 atoms in addition to a methyl group. Thus methylamyl-, methylheptyl- 

 and methylnonyl-ketones of essential oils doubtless result from the 

 decarboxylation of p-keto acids, as in Dakin's experiment, with hydrogen 

 peroxide. 



The degradation of amino-acids in the body also proceeds contrary to 

 the expectations of the organic chemist. If he were asked to bring about 

 the degradation by gentle stages he would doubtless first convert the 

 a-amino into the a-hydroxy acid. The organism forms the a-keto acid, 

 however, as shown by Neubauer and by Knoop. This biochemical result 

 suggested to Knoop an interesting and unlooked for synthesis of amino- 

 acids in vitro, by reversing the normal breakdown. He shook solutions 

 of a-keto acids, containing also ammonia and platinum black, in a 

 hydrogen atmosphere when the corresponding amino-acid resulted by 

 the reduction of the hypothetical imino compound. 



The transformation of tryptophane into kynurenic acid may be quoted 

 as a particular problem of metabolism to which a good deal of organic 

 chemistry has been applied. When large amounts of tryptophane, or 

 of meat containing this amino acid, are given to dogs they excrete in 

 their urine kynurenic-acid, a quinoline derivative 



N=C.COOH 



/ >CH 

 f^\ ,|-CH2.CH(NH2)COOH -> ff \ COH 



