192 D. RiTTENBERG 



be excreted. We measured this partition of dietary nitrogen 

 by adding to a normal diet a small amount of ^^N labelled 

 amino-acid. These experiments using ^^N labelled glycine 

 indicated that in a normal adult human about 1 • 3 grams of 

 protein per kilo weight were daily being synthesized. 



At the same time the magnitude of the metabolic pool was 

 calculated. This pool was defined as being all those nitro- 

 genous substances which arise either from the diet or from 

 the degradation of tissue proteins which can be used for the 

 synthesis of tissue proteins. The size of the metabolic pool 

 was found to be 0-5 g. N/kilo weight. This metabolic pool 

 is really a mathematical construct, since we are unable to 

 define exactly its chemical composition or even its location 

 in the organism. It appears to be large, for it would require 

 that the metabolic pool of the normal human adult contain 

 about 35 g. of nitrogen. This value is more than 10 times 

 the total free amino-acid nitrogen ('^2-5 g. N). While it is 

 to be expected that the metabolic pool should be greater than 

 the total free amino-acid nitrogen, such a large difference is 

 surprising. Data similar to that which we have obtained 

 have been reported by White and Parson (1950), who fed 

 ^^N labelled glycine and ^^N labelled yeast, and by Wu and 

 Snyderman (1950) who fed labelled L-aspartic acid. 



The most obvious defect of the theoretical system elaborated 

 by Sprinson and Rittenberg is their tacit assumption that the 

 rate of urea excretion is not the rate determining step. Dr. 

 San Pietro and I have re-analysed the problem without 

 making this assumption. 



The system we assume is graphically indicated in Fig. 1. 

 The metabolic pool consists of organic compounds, previously 

 defined, containing P mg. of nitrogen. Dietary nitrogen 

 enters the metabolic pool at the rate of D mg. per day. Some 

 of the components of the metabolic pool are used for protein 

 synthesis at the rate of S mg. N per day; another part, £„ mg. 

 N per day, is converted to urea and mixes with urea already 

 present. The remainder of the nitrogen excreted is denoted 

 by Ejj. We assume E, to be negligible as compared to E^. 



