792 PHYSIOLOGY 



have discussed in the previous section, do not involve any large changes 

 of energy. Weight for weight, butyric acid with its 4 carbon atoms has 

 practically the same heat- value as stearic acid with its 18 carbon atoms, 

 or stearine with its 57 carbon atoms. We have therefore to determine 

 what changes the great fat molecule undergoes before it is brought into a 

 condition in which it may undergo oxidation and set free the energy required 

 for the purposes of the body. The general tendency of metabolic research 

 of recent years is to show that the living cell is in a position to effect all 

 changes which do not involve a large evolution or absorption of energy in 

 either direction. In the plant cell, at any rate, the fatty acids may be 

 converted into amino-acids, or the latter may be deaminised, as occurs 

 in the liver, into fatty or oxyacids. Dextrose may pass into maltose 

 and starch, or starch may be converted into maltose or dextrose. If 

 therefore fats are constantly being made from carbohydrates, or from 

 the lower molecules such as aldehyde, by a process of repeated addition 

 of a group containing two carbon atoms, it is probable that the same change 

 will go on in a reverse direction when fats are broken down previous to 

 oxidation. 



In the germination of oily seeds the utilisation of the fat is preceded 

 by the splitting of the higher fatty acids into acids of lower molecular 

 weight. Although we cannot trace out in the animal body the stages in 

 the breakdown of a large fatty acid, such as stearic acid, we can, by a certain 

 artifice much used in metabolic experimentation, bring forward evidence 

 in favour of the view that the breakdown, like the building up of fats, 

 occurs by two carbon atoms at a time. When, in the process of breaking 

 down, a fat finally arrives at the four- or two-carbon stage, it is quickly 

 oxidised and is therefore not traceable in the excretions or in the fluids of 

 the body. This end stage may, however, be preserved from oxidation 

 by hanging it, so to speak, on to an aromatic ring. If acetic acid or ethyl 

 alcohol be administered in small quantities, it is entirely oxidised. If, 

 however, these bodies be attached to a benzene ring and be administered 

 as a phenacetic acid or phenylethyl alcohol, they are excreted in the oxidised 

 form of phenaceturic acid, which is simply a combination of phenacetic 

 acid with glycine. In the same way benzoic acid and benzyl alcohol are 

 excreted in the form of hippuric acid, thus : 



C 6 H 5 .COOH + NH 2 .CH 2 .COOH = C 6 H 5 .CO.NH.CH 2 .COOH + H 2 O 

 Benzoic acid Glycine Hippuric acid 



Phenacetic acid, C 6 H 5 .CH 2 COOH, is excreted as C 6 H 5 .CH 2 .- 

 CO.NH.CH 2 COOH. In each case the fatty side-chain is protected from 

 further oxidation by its attachment to the benzene ring and by the tacking 

 on of the glycine molecule. 



With phenylpropionic acid two carbon atoms of the side- chain are 

 oxidised, and the remaining benzoic acid excreted as hippuric acid. Phenyl- 

 butyric acid undergoes a similar change ; two carbon atoms are oxidised 

 away, leaving phenylacetic acid, which is excreted as phenylaceturic acid. 





