Biosynthesis of Branched-Chain Compounds 33 



poses of the present discussion that in the synthesis of the carbon 

 chains of valine and leucine pyruvic acid does not serve as a source 

 of acetate (or acetyl CoA) but enters as such into the condensation 

 reactions. 



Leaving aside details of the synthetic mechanisms, we may compare 

 the structures of two of the 5-carbon compounds which have been 

 implicated in steroid biosynthesis (IV and V), and hence are formed 

 in the animal body as well as in the plant and microbial cell, with two 

 precursors of the amino acid valine (VI and VII i : 



Clearly a transformation of IV or V, to VI or VII does not occur in 

 the animal body, or else valine would not be an essential amino acid. 

 The action of crotonase, which catalyzes the addition of water to x,fi 

 unsaturated acids of type IV is evidently restricted to the formation 

 of /^-hydroxy acids, because the a-hydroxy acid, if formed, should be 

 readily oxidized to a-ketoisovalerate, a compound which is converted 

 to valine in the animal body. One may argue then that as a result 

 of the specificity of crotonase the synthetic pathways leading to 

 isoprenoid intermediates are useless as far as valine and leucine syn- 

 thesis are concerned, and that as a consequence a synthetic pathway 

 for these amino acids has evolved that is distinct both with respect 

 to the mechanism of condensation and with respect to the carbon 

 sources which it uses. 



It appears to be equally true that the valine precursors VI and VII 

 are not convertible to IV and V since in microorganisms such as N&uro- 

 spora the sole carbon source for ergosterol is acetate, which is used for 

 the synthesis of IV and V but not of VI and VII. Thus, even in the 

 absence of the restrictions which exist in the higher animal, the separa- 

 tion of synthetic pathways for two closely similar structures is rigidly 

 maintained. 



