CARBOHYDRATES, FATS AND PROTEINS 109 



Removal of two carbon atoms at a time can give aceto- 

 acetic acid only in those cases where there are an even num- 

 ber of carbon atoms in a chain of four carbon atoms or more, 

 that is in the cases of Butyric,Caproic, Octoic and Decoic Acids. 



Aceto-acetic acid readily decomposes into acetone, and it 

 is also reduced to /3-hydroxybutyric acid. These three 

 substances are usually found together and are spoken of as 

 " acetone bodies." 



The /^-oxidation hypothesis is a curious instance where 

 the biological chemist seems to have developed a line apart 

 from the organic chemist. At one time objection was made 

 to this hypothesis that in all in vitro oxidations the a or 

 carbon atom next to the carboxyl group was the one 

 attacked. Dakin, on the other hand, was able to show that 

 one oxidising agent was capable of producing /9-oxidation. 

 On neutralising a fatty acid and digesting it at 37 C. with 

 hydrogen peroxide, oxidation occurred in the /^-position,* 

 thus the chemical analogy was developed by a biological 

 chemist in support of an hypothesis which was almost 

 discredited because it was contradictory to the chemical 

 facts known at the time of its promulgation. 



The unsaturated fatty acids probably take up water-forming 

 hydroxy acids and these are oxidised like the hydroxy acids 

 formed from saturated acids. The double bond is frequently 

 shifted before oxidation occurs. 



Branched chains lose their side chains and undergo 

 oxidation along similar lines to those of the corresponding 

 straight chain acids. 



We can sum up the oxidation of fatty acids by saying that 

 they are oxidised in the fi position probably with the formation 

 of a ketone acid. The ketone acid is reduced asymmetrically 

 to the optically active hydroxy acid. The liver is capable of 

 reducing the ketone acid to hydroxy acid, and of oxidising the 

 hydroxy acid, thus indicating the probable presence of an 

 enzyme which accelerates the two reactions. 



The absence of fatty acids with an odd number of carbon 

 atoms indicates that the synthesis as well as the oxidation 

 involves two carbon atoms at a time. 



Raperf has suggested that this synthesis is due to aldol 

 condensation, in which two molecules of acetaldehyde condense 

 to form aldol which, by simultaneous oxidation and reduction, 

 yields butyric acid. 



* H. D. Dakin, Journ. Biol. Chem., 1908, vol. 4, p. 77. 



t H. S. Raper, Journ. Chem. Soc., 1907, vol. 91, pp. 1831-1838. 



