104 NITROGEN METABOLISM 



1 



[see 32]. Dried cell preparations of Clostridium kluyveri cata- 

 lyse the acetylatioh of amino-acids by acetylphosphate, a 

 reaction which, although analogous to the synthesis of 

 acetylsulphanilamide, only occurs in the presence of o-i m. 

 cyanide. Acetylated amino-acids contain a peptidic bond, 

 and it is feasible that peptides can be formed by transfer 

 reactions in which the acetyl group is replaced by an amino- 

 acid [40]. Cell-free extracts of Staph, aureus [9] and a number 

 of other bacteria [13] and Sac. cerevisiae catalyse the syn- 

 thesis of glutamine from glutamic acid and ammonia in the 

 presence of ATP and Mg"^"^ or Mn"^"^. For each mole of 

 amide synthesized, one mole of inorganic orthophosphate 

 is liberated, and if ammonia is replaced by NHgOH the pro- 

 duct is y-glutamylhydroxamic acid. By analogy with the role 

 of Co. A as a carrier of acetyl groups in the synthesis of 

 acetylsulphanilamide, it is tempting to suggest that glut- 

 amine synthesis involves the formation of the corresponding 

 y-glutamyl compound, but all attempts to obtain supporting 

 evidence have failed. Though both glutamine and gluta- 

 thione contain the y-glutamyl radical, there is no proof that 

 their synthesis involves a common enzyme system or that 

 the former participates in the synthesis of the latter. When 

 incubated with ATP, K"^, Mg"^"*", phosphate and hexose 

 diphosphate, cell-free extracts oiEsch. co/z synthesized GSH 

 from glutamic acid, glycine and cysteine [37]. Whilst earlier 

 and similar experiments with preparations of rat liver 

 showed that the enzymes which synthesized GSH are dis- 

 tinct from those catalysing hydrolysis, the individual steps 

 in the biosynthesis are not yet known; the first one may be 

 the formation of y-glutamylcysteine [38]. 



In addition to synthesis at the direct expense of metabolic 

 energy, new peptide bonds may be formed by transfer 

 (transpeptidation) reactions of the type: 



XCO.NH.R+NH2R1 ^ X.CO.NH.Ri+NHoR 



where XCOOH, R.NHg and R^.NHa represent amino-acids 

 or peptides. Such reactions do not result in an overall in- 

 crease in the number of peptide bonds, and since the type 



