272 G. PopjAK 



of Schoenheimer and Rittenberg (1937) and of Stetten and 

 Schoenheimer (1940) with deuterium-labelled fatty acids. 

 The elongation of deuterio-lauric (dodecanoic) and deuterio- 

 myristic (tetradecanoic) acids to deuterio-palmitic and 

 deuterio-stearic acids has been shown to occur also (Klem, 

 1943). Although in the quoted experiments w4th deuterium- 

 labelled fatty acids it was not known whether the lengthening 

 or shortening of the chains occurred at the a>- or carboxvl-end, 

 it seems safe to assume that it took place at the carboxyl 

 end. Thus the elongation of fatty acids by the addition of a 

 Cg unit is well established for the longer acids which are 

 constituents of body fats. 



The short-chain acids, which were shown in our investiga- 

 tions to be formed likewise by the lengthening of a still 

 shorter acid bv the addition of an acetate molecule, and which 

 might be expected as intermediates in the total synthesis of 

 the longer ones, are however thought to be absent from the 

 cells of internal organs. Moreover, Rittenberg, Schoenheimer 

 and Evans (1937) failed to find any evidence that deuterium- 

 labelled butyric and caproic acids could be used for the syn- 

 thesis of higher acids. Therefore a scheme of simple reversal 

 of jS-oxidation as a general mechanism of fatty acid synthesis 

 in the tissues (with the exception of the mammary gland) 

 could not be supported by experimental evidence. An 

 alternative hypothesis of simultaneous condensation of Cg 

 units, giving long-chain acids directly, has been considered 

 (e.g. by Peters and van Slyke, 1946, p. 408). It should, 

 however, be pointed out that the failure of deuterio-butyric 

 and deuterio-caproic acids to give rise to longer-chain fatty 

 acids labelled with deuterium may not constitute an absolute 

 proof against the possibility of the carbon-chain of these 

 acids having been utilized in the synthesis of higher acids. 



The question of the general applicability of the mechanism 

 of fatty acid synthesis as revealed in the mammary gland, 

 hinges on the relationship which exists between the fatty acids 

 of milk triglycerides and the various intermediates which are 

 undergoing metabolism in the cell. This relationship is a 



