FREE NITROGEN COMPOUNDS IN PLANTS 675 
was much higher when sugar was the source of labeled carbon than when labeled 
amino acids were supplied. This is shown in Table II. Furthermore, the labeling of 
the carbon dioxide was more closely parallel to the labeling which occurred in the 
protein than to that in the free compounds; this suggests that carbon dioxide emerged 
from the products of protein breakdown (Table III). Not only this, but as may be 
TABLE TI 
SPECIFIC ACTIVITIES OF FREE AND PROTEIN-BOUND AMINO ACIDS IN CARROT EXPLANTS 
AFTER SUPPLY OF C!4-LABELED GLUTAMINE, y-AMINOBUTYRIC ACID OR GLUCOSE 
Data of STEWARD, BIDWELL AND YEMM. Specific activities based on the specific activity 
of the compound supplied = too. N.D., not determined. —, no detectable activity. 

4C-Substrate supplied 
Amino acid Glutamine y-Aminobutyric acid 

Glucose 



Soluble Bound Soluble Bound Soluble Bound 
Glutamic acid 81 28 26 8.5 100 40 
Aspartic acid 22 — 10.2 INeD: 43 
Threonine —- 0.4 12 4.9 NED: 26 
Alanine NSD: Pals N.D — 100 50 
Proline 8 14 os BT -= 



seen from Table II the specific activity of protein-bound amino acids did not relate 
in any way to the specific activities of their counterparts in the soluble pool. 
This type of result led to a scheme* in which the absorbed and labeled sugar enters 
into those phases in the cell which are intimately associated with the anabolic 
reactions, in which the Krebs’ cycle operates as a line of synthesis and, therefore, the 
carbon of sugar enters promptly into the protein. By contrast, the stored amino 
TABLE Ill 
SPECIFIC ACTIVITIES OF CARBON IN SOLUBLE AND PROTEIN-BOUND COMPOUNDS AND 
IN RESPIRED CO,g FROM CARROT EXPLANTS SUPPLIED WITH 14C-LABELED COMPOUNDS 
Data from STEWARD, BIDWELL AND YEmMM. N.D., not determined. 
, no detectable activity. 

Specific activity of C (Supplied compounds = roo) 

u 0 tate of growth Time < : : 
( ears Ra a Ms (hy ald Glutamic acid oat 
Soluble Bound 
Glucose Rapid 22 17-38 — N.D 41 
Glucose Rapid 109 71 100 40 74 
Glucose Slow 40 26-29 NaI: 34 
Glucose Slow oI 29-33 59 24 35 
Glutamine Rapid 40 oO 84 22 18.4 
Glutamine Rapid 109 1.4 81 28 24.0 
y-AB Rapid 40 e) 33 2.6 G7 
y-AB Rapid 109 iad 26 8.5 14.1 
References p. 692/693 
