l82 A. I. VIRTANEN, J. K. MIETTINEN VOL. 12 (1953) 



amides is high, asparagine is usually found 5 times more than glutamine. In no parts 

 of alder have these amides been found. On the other hand, we^^'^^ have lound l(-|-)- 

 citrulline more than any other free amino acid in the root nodules as well as in other 

 parts of alder, whereas it has never been found in any parts of pea including the nodules, 

 and to date it has never been found in any higher plants except the water-melon (Wada^^). 

 The amount of citruUine in the root nodules of alder is in summer about 0.2 % of the dry 

 weight. After the leaves had fallen in October the amount was already about 0.9 % of 

 the dry weight in A. incana. The citrulline content of A. glutinosa nodules was nearly 

 2 % of the dry weight in January. Citrulline must therefore be regarded as a nitrogen 

 store which is rapidly used in the spring when growth begins but which is continuously 

 found, although to a lesser extent, during the growth period. There are only traces of 

 arginine and ornithine in the nodules, whereas they as well as citrulline are abundant 

 in the parts of the roots nearest to nodules. It is remarkable that we^^ have not been 

 able to find any arginase effect in the nodules, roots and leaves of alder, accordmgly, 

 the decomposition of arginine to ornithine and urea does not seem likely in alder. In 

 its action citrulline may be in some way comparable with glutamine and asparagine, 

 which are abundantly found in legumes, and also may act as store and carrier of nitrogen. 

 Because citrulline is also found in alder, which has grown with combined nitrogen without 

 nodules, its formation is not linked to the molecular nitrogen fixation. 



In pea plants we have recently found, as mentioned above, considerable quantities 

 of homoserine which is not found in alder. After its existence had been indicated paper 

 chromatographically homoserine was isolated from pea plants as follows^^. 



Protein-free extract was boiled under a reflux condenser for 3 hours with i N HCl 

 for decomposition of amides, potysaccharides etc. after which the coloured impurities 

 were removed by active carbon. The amino acids were divided into groups by alkaline 

 ion exchange resin Dowex-2. The group of neutral amino acids was further fractioned 

 with Dowex-50 resin according to Stein and Moore^^ by using elution analysis with 

 hydrochloric acid. 



A part of the homoserine could be eluted by the acid cycle coming through before 

 alanine, another part remaining firmly bound in the column evidently as the basic 

 lactone form. This part was recovered only by displacement with ammonia. 



At the moment it is unknown why some amino acids, e.g., citrulline and homoserine, 

 which are not known as common protein components, may occur in some plants even 

 in surprisingly large quantities, in others again not at all. Other examples are y- 

 methylene-glutamine in Arachis hypogaea}^<'^^ and hydroxy proline in Santalum alhum^^. 

 The common occurrence of free y-aminobutyric acid in plants is evidently due to its 

 formation as a decarboxylation product of glutamic acid. Its possible role in the meta- 

 bolism of plants is unknown. Citrulline is an intermediate product in the synthesis of 

 arginine. Whether it has some other role is unknown. Homoserine acts as a precursor 

 in the synthesis of methionine and threonine. In addition, canavanine is a derivative of 

 homoserine. It is possible that amino acids, which are absent from the proteins but 

 present in the soluble nitrogen fraction, function in the biosynthesis of alkaloids and 

 other substances. 



Effect of light on the occurrence of free amino acids 



By using the paper chromatographic semiquantitative method we have followed 

 the amount of free amino acids in pea at different ages (after 12, 30, 43, and 61 days). 



References p. i8y. 



