The Nitrogen-Sparing Effect 

 of Glucose 



HENRY D. HOBERMAN 



The utilization of dietary nitrogen in the synthesis of body proteins 

 is more efficient when starch, sucrose, or some other readily metabolized 

 carbohydrate is present in the diet. This action of carbohydrates on 

 the metabolism of nitrogen is commonly called the "nitrogen-sparing" 

 effect. The retention of nitrogen induced in this way is a consequence 

 of interactions in the intermediary metabolism of carbohydrates and 

 amino acids and not of proteins. This follows from the fact that, 

 when a small amount of N 15 -glycine or N 15 -aspartic acid is adminis- 

 tered to fasting rats and to rats ingesting a solution of 30% glucose, 

 40% less N 15 -urea and 40% less N 14 -urea are excreted by the glucose- 

 fed than by the fasting animals. 1 The fall in the total urea-nitrogen 

 output is quantitatively accounted for in terms of reactions at the 

 amino acid level, for almost all of the excreted N 15 -urea was formed 

 from the administered amino acid concurrently with and not after the 

 incorporation of N 15 into the body proteins. In similar experiments 

 performed with N 15 -ammonia it was shown that the amount of N 15 

 appearing in the urinary urea of animals receiving N 15 -ammonium 

 citrate was 35% less when a solution of 30% glucose was ingested than 

 when the animals were fasted. 2 These results are interpreted to mean 

 that in the sharing of common pathways of metabolism glucose sup- 

 presses the oxidative deamination of amino acids and accelerates the 

 synthesis of amino acids from ammonia, and/or that glucose inhibits 

 the formation of urea by interfering with the operation of the Krebs- 

 Henseleit cycle. In the following discussion these hypotheses will be 

 examined in relation to our present knowledge of the reactions leading 

 from the oxidative deamination of amino acids to the synthesis of urea. 



In mammalian liver the oxidative deamination of amino acids results 

 from the activities of two enzymatic processes, i.e., direct oxidative 



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