28 Essays in Biochemistry 



There is one further observation which should be mentioned before 

 an interpretation of the relevant data is attempted. Labeled C0 2 when 

 administered to rats is incorporated into cholesterol at a level that is 

 barely detectable. The incorporation values can, however, be sub- 

 stantially increased by the simultaneous feeding of normal isovaleric 

 acid. At the same time the concentration of C 14 in the acetate pool 

 is raised only slightly as judged by the C 14 content of the acetyl groups 

 of N-acetylphenylaminobutyric acid. Hence the C0 2 -fixing step does 

 not give rise to labeled acetic acid. One is inclined to interpret this 

 result in accordance with Coon's scheme, as reflecting a C 5 + Ci con- 

 densation to form a dicarboxylic acid which is subsequently trans- 

 formed into the steroid precursor. The structure of the substrate that 

 fixes C0 2 is, however, not defined except that it would appear to be 

 a transformation product of isovaleric acid. 



Any scheme designed to harmonize the available experimental find- 

 ings must be capable of accounting for the following observations: 

 (1) the ease of conversion to cholesterol of DMA as compared to other 

 branched acids, at least in the intact animal; (2) the preferential 

 utilization of the isopropyl portions of DMA and isovalerate; (3) the 

 C0 2 fixation into cholesterol, which is promoted by isovaleric acid. 

 Some of the experimental facts enumerated here can be fitted into a 

 scheme that is highly speculative to be sure, but since it is susceptible 

 to experimental test it may prove to be of temporary value as a 

 working hypothesis. It should be understood that at the present time 

 the concept which visualizes terpene and steroid biogenesis as a process 

 involving the multiple condensation of C 5 (or perhaps C 6 ) units has 

 merely the status of an hypothesis. There are experimental observa- 

 tions which tend to support it, but no proof exists that the basic 

 principle, i.e., polymerization of monomeric units to polyisoprenoids, 

 has biological reality. The speculations which follow are designed to 

 reconcile some of the conflicting experimental data and thereby to 

 strengthen the underlying hypotheses. 



As the initial step in the metabolism of DMA, using the 1-C 14 com- 

 pound for purposes of illustration, we wish to propose (Fig. 3) a shift 

 of the double bond to the exomethylene position, a reaction which 

 could either proceed directly or by way of /?-hydroxyisovaleric acid. 

 Coon 15 regards the hydroxy acid as the substrate for C0 2 fixation 

 because in his enzyme system DMA formed labeled acetoacetate only 

 in the presence of crotonase. It is possible, however, that this finding 

 is the result of a more complex set of events, such as the hydration 

 of DMA to the ^-hydroxy acid followed by dehydration to the /?-y 



