Leucine, synthesis

The conversion of [ C]valine to [ CJleucine would be predicted on the basis of the reversibility of most reactions catalyzed by aminotransferases and the fact that the oxoanalogue of valine is also a leucine precursor (Figs. 4 and 5). This has been observed to occur in flax and sorghum seedlings (Butler and Shen, 1963), and a duckweed (Borstlap, 1975). D,L-a-Isopropylmalate reduced the incorporation of from valine into bound (protein) but not free leucine (Butler and Shen, 1963). These results indicate that isopropylmalate is an intermediate in leucine synthesis and suggest that multiple pools of metabolites are present in plant cells (Section III,C). 

This is compatible with the accepted idea shown above that leucine is formed from the condensation of the isobutyryl moiety of valine with acetate. As Webb notes, interference by aminopterin with pantothenic acid synthesis would impede CoA synthesis, which would further cripple leucine synthesis. 

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