IV BIOSYNTHESIS OF AMINO ACIDS 7I 



negligible radioactivity was found in dihydroxyisovalerate, the valine precursor. 

 Non-labelled threonine prevented the incorporation of labelled acetate into car- 

 bons I, 2, 4 and 5 of a, [3-dihydroxy-[3-methylvalerate. 



The following further evidence has been obtained by Weinhouse and coworkers 

 (Strassman et aL, 1954, 1956): (/) the carboxyl group of lactate is only slightly 

 incorporated into yeast isoleucine, (2) the alpha carbon of lactate contributes to 

 carbons i , 5, and carbon 3, (j) the lactate beta carbon gives rise to isoleucine carbon 

 atoms I, 2, 4, 5, and carbon 6, {4) label from, acetate- i-''*C is present exclusively in 

 carbons i and 5 of isoleucine, (5) label from acetate-2-^'*C is found in isoleucine 

 carbons 2 and 4 with a specific activity twice that of carbons i and 5. 



These are the results to be anticipated if acetate or lactate is metabolized through 

 the tricarboxylic acid cycle to form oxalacetate, a precursor of aspartate, threonine, 

 and a-ketobutyrate. It is also to be recalled that the carboxy Jcarbon of lactate is 

 lost during the conversion of that substance to acetaldehyde. Fig. 30 summarizes 



(16117-iv-1) 



■Pyruvate ^[f^] ^DHV- 



Homoserine 



-35423 threo 



Threonine 



a-Ketobutyrate 



-[Ce]— ^DHI- 



-KV ^Valine 



-*-K I »- Isoleucine 



a-Aminobutyrate 

 Fig. 30. Relationship between valine aud isoleucine biosanthesis in JVeurospora. 



the relationship between valine and isoleucine in Keiirospora. The accumulation 

 of DHI and DHV by mutant 16117-iv-i is due to the inability of that organism 

 to effect the conversion of the dihydroxy acids to the keto acids. In an interesting 

 experiment, strain 16177-iv-i was genetically combined with the threonine re- 

 quiring mutant (35423 threo). The washed mycelial pads of the double mutant 

 and the single mutant were then compared for DHI and DHV accumulation in 

 the presence and the absence of threonine (Adelberg et aL, 1955). As predicted in 

 the diagram, the introduction of threonine increased the accumulation of DHI 

 but did not cause a parallel increase in DHV. 



Leucine. By isotope competition experiments, it was shown that non-labelled py- 

 ruvate markedly reduced the incorporation of uniformly labelled glucose into the 

 leucine o^ Neurospora and E. coli proteins (Abelson, 1954; Abelson and Vogel, 1955). 

 Furthermore, a-ketoisovalerate, valine, and a-ketoisocaproate reduced the label- 

 ling of £■. coli leucine. Moreover, alanine-2-''*C and valine-2-''*C were incorporated 

 into the leucine oi'E. coli and Neurospora respectively. Acetate- i-''*C is incorporated 

 exclusively into the carboxyl group of the leucine of yeast and bacterial protein 

 (Reed et aL, 1954). This suggests that leucine is synthesized by a condensation of 

 acetate with the a-ketoisovalerate (Fig. 28). 



Yeast cells were also grown in the presence of pyruvate-2-'''C and the radio- 



Literature p. 124 



