70 NITROGEN METABOLISM 



and two in the conversion of ornithine to citruUine. In 

 Hver, the synthesis of citrulUne from ornithine, CO 2 and 

 NH3 proceeds by a mechanism utiUzing metaboHc energy 

 and with carbamylglutamic acid as an essential co-factor. 

 Contrary to expectation, the latter does not function by 

 transferring the carbamyl group directly to ornithine. In the 

 presence of ATP and Mg"^"^, the citrulline combines with 

 aspartic acid and the product subsequently undergoes 

 hydrolysis to yield arginine and malic acid [38]. Comparable 

 systems have not yet been described in micro-organisms. In 

 Lb. arabinosus glutamine appears to play an essential role in 

 arginine synthesis, the amide group being used in the 

 formation of citrulline from ornithine [37^]. 



A mutant of P. notatum grew on arginine, citrulline, 

 ornithine or proline, whilst another grew on either of these 

 amino-acids or glutamic acid. A third was known to grow 

 only on proline, indicating that proline is not itself on the 

 direct route of arginine synthesis but is probably related 

 to a precursor of ornithine [3, also cf. 43]. The possible 

 relationships between glutamic acid, proline and the ar- 

 ginine cycle are shown in Fig. 5.1. Studies of the nutri- 

 tion of naturally occurring strains of lactobacilli [50] and 

 mutants of Esch, coli provided evidence that the mechanism 

 of arginine synthesis in bacteria is the same as in the fungi. 

 Using mutants of Esch. coli, Davis and his colleagues [8] 

 have shown that proline is formed by the reduction of 

 A^-pyrroline-5-carboxylic acid (PCA), a compound formed 

 from the y-semialdehyde of glutamic acid (GSA). 



CH2 — CH.2 CH2 — Crl2 



II II -H,0 



HOOC CH.COOH >CHO CH.COOH 



/ / 



NH2 NH2 



Glutamic acid (GSA) 



CH2 — Cri2 CHg — Cri2 



II +2H I I 



CH CH.COOH > CH2 CH.COOH 



\/ \/ 



N NH 



(PCA) Proline 



