106 



BIOCHEMICAL SYSTEMATICS 



CH2 — CH2 



CH2 CHCOOH 

 H 



proline 



Finally, azetidine-2-carboxylic acid which is a lower homolog 

 of prohne, seems to be typical of the Lihaceae where it is of rather 

 widespread occurrence. This fact is not evident from Reuter's lists, 

 but Fowden and Steward (1957a) reported the presence of this acid 

 in seventeen of fifty-six genera of Liliaceae tested. Renter also shows 

 it as the principal amino acid in roots of Convallaria majalis (some- 

 times treated as a separate tribe [Hutchinson, 1959] or family [Gates, 

 1918] of the Liliaceae). 



/^. 



CH2 CH2COOH 



\ V 

 N 

 H 



azetidine-2-carboxylic acid 



Renter has described a scheme of probable inter-conversion 

 for the acids just considered which, if correct, suggests a rather close 

 biochemical affinity for all. It is not hkely that the distributions of 

 these acids are of great taxonomic importance in themselves. The 

 scheme of probable inter-conversion (shghtly modified from Renter) 

 appears below: 



glutamine y, citrulline -^ arginine 



glutamic -^ glutamic semialdehyde -^ ornithine^ 



^ proline 6 acetylornithine 



aspartic -^ aspartic semialdehyde ^ azetidine-2-carboxylic acid 

 asparagine 



When two acids such as prohne and azetidine-2-carboxylic 

 acid probably arise by analogous reactions of precursors differing by 

 a single carbon, comparison of the enzymes involved should prove 

 interesting. There is some hkehhood that two enzymes responsible 

 for such equivalent reactions are structurally related, perhaps even 

 phylogenetically related (homologous). The same enzyme may catalyze 

 both reactions, of course. Such a phenomenon exists in valine-iso- 

 leucine synthesis in Neurospora. 



A particularly interesting study of plant amino acids is that 

 of Fowden and Steward (1957) who studied the amino acids of 

 eighty-nine species representing fifty-six genera of the Family 



