70 



INTERMEDIARY METABOLISM AND GROWTH 



l) The carboxyl group of acetate, lactate, glycine and the third and fourth 

 carbons of glucose are precursors of the carboxyl group of valine (carbon i), 

 (Strassmanf^«/., 1953, 1955; McManus, 1954; Rafelson, 1955a), 2) the alpha car- 

 bon atoms of the above compounds are precursors of carbons two and carbon 

 three of valine, j) glucose- i-'"'C is converted to the methyl groups of valine. As 

 a result of the reactions of glycolysis glucose-3,4-''*C gives rise to the carboxyl 

 carbon of pyruvate while glucose- i-^'*C is a precursor of the beta carbon of 

 pyruvate and of the alpha carbon of acetaldehyde. 



The synthesis of isoleucine takes place by a siniilar pinacol rearrangement 

 except that a-ketobutyrate replaces pyruvate as the compound which condenses 

 with acetaldehyde (Strassman et al., 1954, 1956; Fig. 29). 



CH3 



CHj 



HO-C — CH3 



CO 



COOH_ 

 2H 



V 



(5) 



C^ (4) (6) 



CH2 ,CH3 glutamic 



CH{3) "■ 



(2)CH — NHj 



CH3 

 CH2 

 HC — CH3 

 CO 



CH, 



-H,0 



-CH2 ^CH. 

 C-OH 



HC-OH 



I 



COOH 



(l)COOH 

 Isoleucine 



COOH 



Ketoisoleucine 



Fig. 29. Biosynthesis of isoleucine 



(DHI) 



a, ft - Dihydroxy-/3 - methyl 

 valerate 



The [3-ketobutyrate, which is required for isoleucine synthesis, is derived from 

 threonine or homoserine. These substances are in turn formed from aspartic acid. 

 The reactions of Fig. 29 are consistent with the results of the isotope competition 

 experiments of Abelson and coworkers. Thus, non-labelled aspartate, homoserine, 

 threonine, a-ketobutyrate, a-aminobutyrate, a-keto-p-methyl-valerate, and iso- 

 leucine markedly reduced the incorporation of labelled glucose into the isoleucine 

 of E. coli protein. Likewise, non-labelled a-ketobutyrate, and a-keto-[i-methyl- 

 valerate reduced the conversion of labelled glucose and homoserine, threonine, 

 and isoleucine suppressed the incorporation of ^''CO, into the isoleucine of jV>z/- 

 rospora protein (Abelson and Vogel, 1955; Abelson, 1954). 



Threonine i,2-^''C was also converted by Neurospora mutants to the carboxyl and 

 alpha carbons of the dihydroxy analog of isoleucine with 73% of the specific activity 

 of the threonine precursor (Adelberg, 1954; Adelberg et al., 1955). However, 



