SYNTHESIS OF AMINO-ACIDS 77 



leucine, a,y5-dihydroxy-/?-ethylbutyric acid, a-aminobutyric 

 acid, a-ketobutyric acid or threonine. Hence, the metabo- 

 lism of isoleucine, the sulphur amino-acids and threonine 

 is closely related and may proceed from a common C4- 

 precursor. Isoleucine may arise from the latter by the 

 addition and reduction of an acetyl group [ib] (see Fig. 5.3). 

 The keto-acids of valine and isoleucine have been identified 

 in the culture filtrate of an isoleucinless mutant of Esch. 

 coli [47]. 



Isotopes 



Isotopes of carbon are proving useful in tracing the 

 origin of the various carbon atoms of amino-acids, and their 

 application to the study of syntheses in micro-organisms 

 (Torulopsis utilis, Esch. coli, N. crassa) is mainly due to 

 Ehrensvard and his colleagues [6, 12, 13]. By using acetate 

 as a sole source of carbon and labelling the carbon in the 

 two positions with different isotopes (C^^HgC^^OOH), it is 

 possible to determine whether a particular C atom is derived 

 more or less directly from the CH3 or the COOH group. 

 With the yeast T. utilis adapted to growth on acetate, it was 

 found that two acetate carboxyl groups were liberated as 

 respiratory CO 2 for every methyl group. After hydro- 

 lysing the yeast with acid, the amino-acids were isolated 

 by means of electro-dialysis and chromatography on ion 

 exchange resins. The ratio of C^* to C^^ in the carboxyls 

 of most of the amino-acids was the same as that in the 

 respiratory CO 2 , thus demonstrating that CO 2 fixation had 

 taken place. In general, all the alpha-C atoms and many of 

 those in the side chains were derived from the methyl group 

 of the acetate. It was concluded that glutamic acid is a 

 precursor of arginine (cf. p. 70) and that lysine is synthe- 

 sized by the head to tail condensation of acetyl radicals. 

 When NHa-CHgC^^OOH was used as a sole N source the 

 C^* was finally located mainly in the glycine, serine and 

 proline of the proteins. Thus in Torulopsis, as in bacteria 

 (p. 149) and animals [cf. 14], glycine is a precursor of serine. 

 By growing cultures of Sac. cerevisiae, T. utilis and Ps. 

 fluorescens in the presence of HC^*OOH, it has been shown 



