Problems in Lipide Metabolism 137 



those tested that undoubtedly incorporate acetate into fat have been 

 mammary gland, intestinal mucosa, lung, heart, spleen, testes, ovaries, 

 adrenals, and adipose tissue. It should be pointed out that such posi- 

 tive results simply indicate that the above-mentioned tissues contain 

 enzyme systems capable of transforming precursor carbons into fatty 

 acid carbon atoms. The results do not imply that net synthesis has 

 occurred, nor do they provide much information concerning the driving 

 forces necessary to reverse the oxidative pathway in the direction of 

 lipogenesis. 



It is clear that lipogenesis from acetyl CoA is an endergonic and 

 reductive process. Although oxidation of fatty acids may be demon- 

 strated in mitochondria or mitochondrial extracts, it has not yet been 

 possible to obtain incorporation of labeled pyruvate or acetyl CoA in 

 such preparations. The soluble supernatant fluid is required in addi- 

 tion to mitochondrial extracts before this result can be achieved. 



A study of the cofactor requirements of the complete water-soluble 

 system 18 indicates that it requires all of the known cofactors of 

 glycolysis, Mg++, ATP, and DPN. In addition, it has been possible 

 to demonstrate a dependency upon CoA. Although there is no doubt 

 that flavoprotein is also required, this has not been demonstrated in 

 such systems. The addition of citrate in relatively high concentra- 

 tions (0.1-0.2 M) is also necessary for maximal lipogenesis in these 

 systems, whereas equivalent concentrations of glutamate, oxalacetate, 

 and a-ketoglutarate have proved to be not nearly as effective. Al- 

 though the reason for this stimulating effect of citrate upon lipogenesis 

 is not clear, there is some evidence that the presence or absence of 

 citrate influences profoundly the amount of labeled acetate incorpo- 

 rated into acetoacetate and /3-hydroxybutyrate. 



Attempts to replace the supernatant fluid with a source of glycolytic 

 enzymes (rabbit-muscle acetone powder) and suitable substrates have 

 been unsuccessful. Although it is obvious that both high-energy com- 

 pounds and hydrogen donors are essential for lipogenesis, DPNH and 

 ATP have not proved satisfactory nor has it been possible to obtain 

 lipogenesis by enzymatic generation of DPNH and ATP. 



Relatively inactive supernatant fluids are obtained following brief 

 dialysis; they are also inactive when prepared from livers of previ- 

 ously fasted or alloxanized rats. Mitochondria or mitochondrial ex- 

 tracts obtained from such animals appear to be essentially normal and 

 can be supplemented with supernatant fluid obtained from livers of 

 normal well-fed rats. 



