Biosynthesis of Branched-Chain Compounds 23 



into the formation of the branched portions of isoleucine, leucine, or 

 valine. For the moment we shall merely take note of the fact that, 

 for the .synthesis of branched chains, generally speaking, there exist 

 at least two mechanisms which are entirely separate. Those aspects 

 of the biogenesis of valine and leucine which are pertinent to this 

 discussion will be briefly considered later on. 



Active interest in the biosynthesis of branched carbon chains arose 

 from the concern with two seemingly unrelated processes, the forma- 

 tion of steroids, which Rittenberg and Schoenheimer ' began to study 

 in 1 ( J37 in the Columbia Laboratories, and of rubber, which Bonner 

 and his associates 2 have investigated more recently. The first point 

 of contact between the two areas was made by the observations that 

 in both processes acetic acid serves as the carbon source. Later, in 

 the light of the suggestive relationship of squalene and cholesterol, 

 the link between isoprene derivatives and steroids was more firmly 

 established, and even more recently the biogenesis of carotene 3 and 

 of geraniol 4 have provided additional examples in the terpene field 

 of what appears to be the same basic process, namely, the transforma- 

 tion of acetate units to branched 5-carbon chains. If one accepts the 

 thesis that those natural products which obey the isoprene rule, i.e., 

 which can be formally constructed from isoprene units, also belong 

 biogenetically to the isoprene family, then the number and variety of 

 cell constituents which are products of this special type of acetate 

 metabolism becomes impressively large (carotenes, phytol, mono- and 

 sesquiterpenes, tetra- and pentacyclic triterpenes) . 



The identity of the repeating units in the terpenes and carotenes 

 has inspired numerous hypotheses dealing with the origin of this class 

 of compounds. A recent formulation is that of Bonner, who, on the 

 basis of two sets of observations, (1) the incorporation of acetate 

 carbon into rubber by isolated guayule leaves, and (2) the capacity 

 of acetate, acetone, acetoacetate, and /?-dimethylacrylic acid to en- 

 hance net rubber production, postulated the following series of reac- 

 tions leading to the formation of 5-carbon units: 



2CH 3 COOH -> CH 3 COCH 2 COOH -> CH3COCII3 + C0 2 



CH 3 CH 3 



\C0 + CH3COOH -> \c=CH— OOOH -> 



CH 3 CH 3 



CH 3 



— CH 2 — C=CH— CH 2 — 



