FREE NITROGEN COMPOUNDS IN PLANTS 673 
different situations, as for example by the use of both exogenous urea and carbon 
dioxide?’ °°, the carbon of asparagine is almost impossible to label intensely. For 
example, in the white Lupin (Lupinus), which is a classical asparagine plant, the 
asparagine seems not to be readily labeled either from applied CO, or even from 
radioactive sugar applied exogenously”. The interpretation here is that the as- 
paragine arises not directly from sugar but by the reworking of the products of protein 
breakdown. 
In recent experiments with wheat leaves it has been possible to obtain a small 
amount of radioactively labeled asparagine which, however, contrasts very markedly 
with the massive synthesis of glutamine which also takes place. The labeling 
TABLED 
DISTRIBUTION OF RADIOACTIVITY IN SOLUBLE AMINO ACIDS 
Wheat leaves supplied with [1,4-“C]succinic acid (Data of BIDWELL, unpublished) and 
(2) bean leaves supplied with CO, (Data of NELSON AND KrotTKov?®), 

% of radioactivity found in 


Amino acid Carbon atom(s) Wheat leaf Dane 
(1,4-MC) Succinic 14CO, 
Aspartic acid c,+C, — 32 
Asparagine Gi 50 206 
Glutamic acid Ga 75 4 
Glutamine G 75 10 
Alanine Cc 122 34 

of the asparagine was accomplished by the use of succinate radioactively labeled 
in the two carboxyl groups. The resultant 'abeling of the carbon in the asparagine 
corresponded precisely to the labeling in the succinic acid supplied, showing that 
there must have been a direct conversion via fumaric acid or oxaloacetate to aspartic 
acid and then to asparagine (BIDWELL, unpublished results). By contrast, however, 
in bean leaves this seems not to be so (see Table I). NELSON AND KrotTKov” allowed 
bean leaves to photosynthesize in CO, for 15 min. The small amount of radio- 
active asparagine when isolated proved to have 75% of its MC in the two middle 
carbons. This result contrasts sharply with that obtained with the wheat leaves and 
does not lead to any readily understood line of synthesis of asparagine via a Krebs’ 
cycle intermediate. Perhaps an even more striking example is the labeling to be 
found in alanine when [1,4-!C|succinic acid is supplied to wheat leaves. In the 
absence of ammonia a small amount of radioactive alanine was formed which had 
96% of the label in the carboxyl group. When ammonia was added to the leaves 
together with the labeled succinate, somewhat more radioactive alanine was formed, 
but the carboxyl carbon now only contained 2°, of the label. 
Therefore, with alanine as with asparagine and glutamine, one is forced to the same 
conclusion; namely that the same compound may arise in different situations by 
quite different routes, and one should not, therefore, be obsessed with the more 
obvious reactions but try to find out what actually does occur in these different 
systems. 
References p. 692/693 
