Biosynthesis of Branched-Chain Compounds 



27 



Metabolic breakdown of the precursors to acetate or acetoacctate 

 would have caused the appearance of labeled carbon in twelve positions 

 of the steroid molecule. Although the relatively low overall efficiency 

 of HMG and HIV as cholesterol precursors might be ascribed to rate 

 differences in the transformation of the free acids to activated deriva- 

 tives (admittedly a restatement of the problem rather than an ex- 

 planation) a more discordant result came to light when 1-C 14 DMA 

 was tested. If it is valid to assume that the intact DMA molecule 

 enters into the condensation reaction, then the conversion efficiency 



C? 



6X 



C - C 



Fig. 2. 



Postulated distribution of C 14 in triterpenoid precursor formed from 

 branched acids labeled at carbon 3. 



should be independent of the location of the carbon label in the pre- 

 cursor. This was not true for 1-C 14 DMA. Moreover, C 14 from 1-C 14 

 DMA entered the C-25 position of the cholesterol side chain at a 

 level indicating that extensive breakdown to 1-C 14 acetate had oc- 

 curred. This result in fact confirms our earlier experiences with 

 isovaleric acid and leads to the anomalous situation in which the 

 branched-chain acids behave as if they were direct cholesterol pre- 

 cursors only when they are labeled in the isopropyl group. The forma- 

 tion of C 2 units from carbon atoms 1 and 2 of isovalerate and of DMA 

 is readily explained by cleavage of the molecule between carbon atoms 

 2 and 3, perhaps by the reactions suggested by Coon (Fig. 1), i.e., with 

 concomitant C0 2 fixation. On the other hand, the route taken by the 

 isopropyl portion of isovalerate or dimethylacrylate during conversion 

 to steroids is not immediately apparent. Acetoacetate formation from 

 the isopropyl portion as depicted in Fig. 1 cannot be the explanation 

 because the observed isotope-distribution pattern in cholesterol 16 dif- 

 fers so markedly from what is found with acetate or acetoacetate 17 

 as precursors. 



