NATURE OF FAT SYNTHESIS 91 



The Nature of Fat Synthesis and Degradation 



The following group of facts needs to be considered in rela- 

 tion to fat synthesis and degradation: 



1. Long-chain aldehydes are intimately connected with fat metabolism 

 in the liver. 



2. The fat droplets found in tumours are also commonly associated with 



long-chain aldehyde. 



3. When fat appears in tissue cultures the droplets are commonly sur- 

 rounded by a shell of aldehyde. 



4. Biochemical evidence shows that changes in fat are mainly mediated 

 by loss or addition, not of CO2, but of 2-carbon fragments. 



5. Whereas free fatty acid is not attacked by liver extracts, the acyl 

 phosphates are readily attacked by such extracts (Lehninger, 1945). 



There are a number of questions the answers to which are far 

 from clear at the present time in connection with the function 

 of the aldehyde. Thus we do not know whether the aldehyde 

 involved occurs as a true intermediate, as an end-product or as a 

 by-product of processes occurring in the liver. If it is a true 

 intermediate, we must envisage a series of reactions in which 

 aldehydes, or compounds such as acetals which readily give rise 

 to aldehydes, are involved in synthesis and degradation of at least 

 the higher fatty acids (Danielli, 1950b), and occur as part of the 

 process of ^-oxidation. 



It must, of course, be pointed out that we do not know whether 

 the aldehyde which is involved in fat metabolism occurs as free 

 aldehyde, as aldehyde acetal, or as some other compound of alde- 

 hydes. One possibility is that the true intermediate is aldehyde 

 phosphate such as R-CHOH-OP0 3 H 2 . This last compound 

 would be highly reactive and unstable in the presence of water, 

 and so it would be difficult to discover in tissues. If it occurred 

 as a true intermediate, one would probably find by cytochemical 

 processes either free aldehyde derived from its decomposition, or, 

 as we shall see later, an acetals It is clear that there is room 

 for a considerable amount of work, particularly on the enzymes 

 which might be associated with reactions involving aldehydes. 

 At the present no methods have been worked out for the cyto- 

 chemical localization of such enzymes. But it is interesting to 

 note that Worden (1943) has found that there is a very high con- 

 centration of aldehyde oxidase present in adsorbed form on the 

 surface of the fat droplets of milk. 



