76 



INTERMEDIARY METABOLISM AND GROWTH 



1955). It has been shown that rat liver extracts (Matsiio et al., 1956) convert 

 homoserine-2-^'*C to labelled a-ketobutyrate, a-hydroxybutyrate, a-amino- 

 butyrate, and propionic acid (Fig. 33). 



Partially pvu-ified enzyme preparations, however, formed a-ketobutyrate as 

 virtually the only radioactive product of the reaction. The formation of a-amino- 

 butyric acid is apparently attributable to a transamination reaction, since the 

 presence of glutamic acid greatly increased the radioactivity of the former sub- 

 stance. An a-aminobutyric transaminase also occurs in heart muscle. 



77?^ conversion of threonine to cL-ketobutyrate 



a-Ketobutyrate can also be formed from threonine. The conversion of threonine 

 to a-aminobutyrate and of the latter to a-ketobutyrate and propionate has been 

 demonstrated in liver extracts (Kinnory et al., 1955) (Fig. 33). 



HOCH2— CH2 CH COOH 



I 

 NH2 



Homoserine 

 ♦ 



CH,— CHo— CH — COOH- 



OH 



a- Hydroxy buty rate 



,— CH,— ^C 



CH3— CH2— CO— COOH 

 a-Ketobutyrate 



(CH 



CH3— CH2-CH— COOH 



NH2 



a-Aminobutyrate 



* 

 ■COOH 



Propionate 

 Krebs cycle 



Fig. 33. Metabolism of homoserine, a-ketobutyrate, and threonine. 



[g] Lysine biosynthesis 



Two pathways exist for the biosynthesis of lysine. In Neurospora and Tondopsis, 

 a-aminoadipic acid is an intermediate in lysine synthesis, while in E. coli and 

 possibly other bacteria, diaminopimelic acid is the lysine precursor. 



Amino adipic acid pathway, a-aminoadipate, a constituent of corn protein, is a growth 

 factor for certain lysine auxotrophs o{ Neurospora (Windsor, 1951). This substance, 

 labelled with ''*C on the epsilon carbon atom, is incorporated into the lysine of 

 Neurospora protein. 



a-Aminoadipate is probably derived from intermediates of the tricarboxylic acid cycle. It 

 has been shown that uniformly labelled glutamic acid but not glutamic- i-i-tC is incorporated 

 into the lysine oi Neurospora and Tondopsis protein (Abelson and Vogel, 1955). Glutamic- '■^C 

 is better incorporated than aspartic-i^C. As in the case of leucine, acetate is a precursor 

 of the carboxyl and alpha carbons of lysine (Strassman and Weinhouse, 1953). These 

 results suggest that acetate may condense with a-ketoglutarate to form homocitrate and 



