84 S. S. COHEN 



It will be seen tliat this cycle can not only degrade a two-carbon fragment 

 to CO 2 and HgO but may provide moieties essential for the synthesis of 

 glutamate and many derived amino acids, the porphyrins, etc. In the life of 

 a cell, it is not possible to ascribe either a cataboHc or anabohc role to this 

 system, since it may fulfill both. Additional reactions of acetyl — SCoA follow: 



acetyl — SCoA + sulfanilamide ^ iV-acetyl sulfanilamide + coASH 



acetyl — SCoA + hexosamine ^ iV-acetyl hexosamine + coASH (2) 

 acetyl — SCoA + amino acid ^ iV-acetyl amino acid + coASH 



2 acetyl — SCoA ^ CoASH + acetoacetyl — SCoA (3) 



acetyl — SCoA + choline ^ acetyl choline + CoASH (4) 



acetyl — SCoA + fatty acid ^ acetate + fatty acyl — SCoA (5) 



+ bile acid ^ acetate + bile acyl — SCoA (5) 



In reaction 5, it can be seen that the acetyl moiety can be exchanged for 

 other acyl substituents. Mechanisms are then available for the synthesis of 

 esters, as in the synthesis of phosphatides, which occurs in mitochondria 

 (Romberg and Pricer, 1952). 



stearyl — SCoA + a-glycerophosphate -> monostearyl phosphatidate + CoASH (1) 

 stearyl — SCoA + monostearylphosphatidate -> distearylphosphatidate + CoASH (2) 



Kennedy (1953) has shown that incorporation of P^^ into phosphatide in 

 isolated mitochondria requires oxidative phosphorylation. Free glycerol was 

 stimulatory and the formation of the intermediate, a-glycerophosphate, was 

 strongly indicated. Again, enzyme fractions from the soluble supernatant 

 fraction were markedly stimulatory in this process. The synthesis of complete 

 phosphatides will be discussed in a later section. 



Although the conversion of acetate to cholesterol occurs to a slight degree, 

 if at all, in hver mitochondria (Bucher and McGarrahan, 1956), the oxidation 

 of cholesterol to acidic steroids such as cholic acid proceeds very well in these 

 organelles (Frederickson, 1956). The accumulation of cholesterol esters of 

 fatty acids was also noted in these isolated oxidative systems. 



The formation of acyl — SCoA derivatives also facihtates the synthesis of 

 some peptide bonds by mitochondria (Chantrenne, 1951). Thus, 



benzoyl — SCoA + glycine ^ benzoyl — glycine (hijipuric acid) + CoASH 



The possible significance of this reaction in the syntheses of peptides in 

 general will be considered in a later section. 



As shown in formula (II), succinyl — SCoA condenses with glycine to form 

 a-amino ^-keto adipic acid, which decarboxylates to a-amino levulinic acid. 

 Two moles of the latter condense to porphobilinogen, the immediate precursor 

 to porphyrins. 



