72 INTERMEDIARY METABOLISM AND GROWTH I 



activity of the protein leucine was determined. Radioactivity was observed in the 

 carboxyl group of leucine and in the beta and gamma carbon atoms. This is in 

 accord with the pathway postulated above since pyruvate-2-^'*G would be ex- 

 pected to form labelled a-ketoisovalerate-2,3-''*C in the course of valine synthesis. 

 Likewise, pyruvate-2-^'*C would give rise to acetate- i-'^^C by oxidative decarboxyl- 

 ation. As indicated above, the latter substance is the precursor of the leucine 

 carboxyl group. 



(/) Aspartate family [aspartic, methionine, threonine, isoleiicine) 



Aspartate 

 Aspartic acid is formed from oxalacetic acid by transamination: 

 Glutamic -f oxalacetic ♦ ^ aspartic + a-ketoglutarate 



Homoserine-threonine-methionine 



Aspartate to homoserine and threonine 



Aspartic acid is the precursor of threonine and methionine. There is increasing 

 evidence that homoserine is an intermediate in these transformations. Homoserine 

 can replace the methionine and threonine (Teas et al., 1948) which is needed for 

 the growth of a Neiirospora mutant. A mutant blocked in methionine synthesis 

 alone was found to accumulate homoserine as well as threonine. The results of 

 isotope competition experiments are consistent with these findings. Thus, in E. 

 coli and Neurospora, non-labelled homoserine prevents the incorporation of glu- 

 cose-^'*C into the threonine and methionine of the protein. Labelled aspartic 

 acid is converted to labelled threonine in both these organisms (Abelson, 1954; 

 Abelson and Vogel, 1955). Threonine and lysine partially spare the aspartic acid 

 requirements of certain Lactobacilli (Ravel et al., 1954). 



The conversion of aspartic acid to homoserine has been demonstrated with 

 partially purified yeast enzymes (Black and Wright, 1955a, b, c). The steps are 

 as follows: 



a) Aspartate + ATP — > ADP + [i-aspartyl phosphate 



b) P-Aspartyl phosphate + TPNH2 — > aspartic semialdehyde + TPN"^ + phosphate 



c) Aspartic semialdehyde + DPNH2 ((TPNH2) -* homoserine + DPN"- (TPN^) 



The conversion of aspartate to homoserine and to threonine has also been studied in 

 cell suspensions and extracts obtained from E. coli mutants (Hirsch and Cohen, 1954). 

 In the presence of glucose and phosphate, washed suspensions of a mutant, which was 

 blocked in threonine synthesis, aerobically synthesized L-homoserine from L-aspartate. 

 Suspensions of another mutant as well as of a wild type strain utilized L-homoserine for 

 threonine synthesis. Extracts obtained from a mutant capable of activating aspartic acid 

 in the presence of ATP were incapable of reducing the activated compound. Extracts from 

 a second mutant reduced aspartate in a TPNH2 dependent reaction but no homoserine 

 was formed. A third mutant yielded an extract which with ATP and TPNH2 formed 

 homoserine from aspartate. These results parallel those obtained by Black and Wright 

 with yeast cells. One difference was however noted. The activation of aspartate by E. coli 

 cell extracts was stimulated by coenzyme A as well as by ATP. It is therefore possible 



