26 Essays in Biochemistry- 



analogous to the events which are known to occur with the straight- 

 chain fatty acids, and hence their reversibility may be anticipated. 

 In this event the above scheme would provide for the formation of 

 hydroxymethylglutarate by two pathways: (a) by a C 2 + C 4 con- 

 densation and (b) by C0 2 fixation of a 5-carbon acid. The second 

 reaction is perhaps not quantitatively important at least in animal 

 tissues, since the supply of isovaleric acid would appear to be limited 

 by the rate of oxidation of leucine which is an essential amino acid. 

 At any rate reactions have now been shown to occur which afford 

 branched 5- or 6-carbon acids, and assuming these acids to be inter- 

 mediates rather than metabolic end products we may now consider 

 their relation to terpene and steroid biogenesis, the more central issue 

 of our discussion. 



When we first took up this problem in 1944 it seemed reasonable 

 to pose the question whether the preformed chains of leucine or valine 

 might serve as carbon sources for some portion of the steroid molecule, 

 and in fact deuterioleucine and deuterioisovaleric acid proved to be 

 efficient precursors of cholesterol. Later, with the finding that only 

 acetate carbons make up the skeleton of ergosterol and cholesterol, 

 it became clear that a carbon source which originated from an indis- 

 pensable amino acid could not be an obligatory intermediate, at least 

 in the animal body. Nevertheless, it appeared possible that the 

 metabolism of leucine, for example, led to a product that was identical 

 with one of the intermediates in the acetate-sterol conversion, and 

 hence experiments with leucine or isovaleric acid seemed worth pursu- 

 ing. With the aid of isotopic carbon Zabin 6 showed isovaleric acid 

 to be several times more effective than acetate for cholesterol synthesis. 

 This was true, however, only when the precursor was labeled in the 

 isopropyl portion of the molecule. Carboxyl-labeled isovalerate un- 

 expectedly gave results that were indistinguishable from those obtained 

 with l-C 14 -acetate. 



More recently, we have tested additional branched-chain acids as 

 cholesterol precursors with results that have been both encouraging 

 and puzzling. HMG, HIV, and DMA when labeled at the tertiary 

 carbon atom were incorporated into cholesterol, but only with DMA 

 as the substrate was the transformation extensive enough to indicate 

 specific conversion. On the other hand, partial degradation of the 

 cholesterol samples from the three experiments indicated that in all 

 cases C 14 was present only at those six positions which one would 

 expect to be labeled if the carbon chains of the acids had remained 

 intact during condensation to the triterpenoid intermediate (Fig. 2). 



