The Nitrogen-Sparing Effect of Glucose 170 



Since s 2 /{s2 + s 4 ) is readily evaluated by measuring the fraction of 

 isotopic urea formed from a given amount of N 15 aspartic acid, 13 the 

 ratio s 3 /si may be calculated by substituting the appropriate data in 

 the following equation: 



S3/S1 = (4) 



(I /a) Am - (U/a) As 



where (U/a) Am is the fraction of N 15 given as ammonia which appears 

 in the urinary urea in an arbitrary time (2 days) and (U/a) A s is the 

 fraction of N 15 given as aspartic acid which appears in the urinary urea 

 during an equal interval. In Table 1 are the results of experiments 



Table 1. Influence of Fasting and Glucose on the Utilization of 

 N 15 -Labeled Precursors in the Synthesis of Urea and Amino Acids 



For definition of symbols see text. 



carried out, as indicated, on fasting rats and on animals ingesting a 

 solution of 30% glucose. 



Attention is directed first to the results of calculations which show 

 that glucose induces a substantial shift in the utilization of ammonia 

 from step I of the urea cycle to the synthesis of amino acids. Theo- 

 retically the proportion of amino acids formed from ammonia is 

 S3 (S3 -f- s 4 ). The calculated utilization of ammonia in the synthesis of 

 amino acids in animals ingesting glucose is thus two-thirds, and in 

 fasting animals one-half so that one-third more ammonia is trans- 

 formed to amino acids and one-third less is used in the formation of 

 urea. Although this accounts in large measure for the observed fall in 

 the output of the total urea nitrogen of glucose-treated animals, the 

 question which must be raised here is w r hether the increase in the ratio 

 of s 3 /si resulting from the ingestion of glucose represents an increase 

 in the absolute value of s 3 , or whether Si is decreased in relation to s 3 . 

 In accordance with earlier considerations, an increase in the absolute 

 value of s 3 may be expected to lead to a decrease in the concentration 

 of ammonia in the liver whereas a decline in the absolute value of s 3 

 may be expected to have the reverse effect. It was found that the con- 

 centration of ammonia in the liver of the glucose-fed rats was approxi- 

 mately 30% less than in fasting animals. This was indicated by the 



