24 



Essays in Biochemistry 



Once the distribution of acetate carbon in the isooctyl side chain of 

 cholesterol had been ascertained, it became immediately clear that the 

 isotopic pattern observed in the sterol could be readily accounted for 

 by the series of reactions shown above. It thus became a logical step 

 to postulate the occurrence of parallel events at some stage of rubber 

 and steroid biogenesis. The details of the above scheme, in particular 

 the suggestion that branching was achieved by a biochemical equiva- 

 lent of the Reformatskii reaction, have not withstood later scrutiny 

 mainly because acetone fails to show the properties of a specific steroid 

 precursor. 5,6 In the systems investigated so far, i.e., in the intact 

 animal and in the liver slice, acetone at best equals the efficiency of 

 acetate, a result which reflects the rapid oxidation of this 3-carbon 

 compound to acetate and "formate." Moreover, when Brady and 

 Gurin found that l-C 14 -acetoacetate was incorporated into cholesterol 

 apparently without fragmentation of the carbon chain, 7 participation 

 of acetone seemed clearly ruled out. However, it has lately become 

 less certain that the results with labeled acetoacetate were interpreted 

 correctly, and the possibility that acetone participates in a direct way 

 cannot as yet be dismissed entirely. 



Searching for an alternative mechanism which would permit the 

 assembly of acetate units to a branched chain intermediate, we have 

 suggested /?-hydroxy-/?-methylglutaric acid (I) as an intermediate that 

 could be formed from acetoacetate acid and acetate, or acetyl CoA. s 

 This condensation would bear some resemblance to the formation of 

 citrate (II) from acetyl CoA and oxaloacetate. The dihvdroxy-6- 



CH 2 COOH 



CH3CO 

 + 

 CH3COOH 



I 

 CH 2 COOH 



CH3COH 



CH 2 COOH 



CHoCOOH 



HOOC— CO 

 + 

 CH3COOH 



I 

 CH 2 COOH 



HOOC— COH 



I 



CH 2 COOH 

 n 



COH— COOH 



carbon dicarboxylic acid (III) which has been implicated by the work 

 of Tatum and Adelberg as the common precursor of valine and iso- 

 leucine in Neurospora 9 is most likely formed by an analogous reaction, 



