202 D. RiTTENBERG 



and I am afraid that I cannot answer them completely, but I should 

 like to present certain notions to you. 



Firstly, concerning the lack of homogeneity of the metabolic pool. 

 The mathematical treatment of this is extremely difficult, and I am 

 certain that at the present time it would not be worth the time it would 

 take to solve it, but intuitively you can discover some things which at 

 least are comforting. Suppose the pool contains Ni compartments 

 instead of one, and let's call compartment Ni, the liver pool. Let the i 

 compartments be connected with each other and let the rate of transfer 

 of constituents from one pool to the other be denoted by connection 



constants kNiN,- and kNjNi* Let us consider the situation which would 



arise where any set of k's (e.g. kNjNa and kNaNi) are very large or 

 infinite. Then whichever pools are connected by these infinite constants 

 are essentially identical pools. On the other hand, if any set of k's are 

 zero or close to zero then you can neglect that pool; it just doesn't 

 enter the problem. So at either end, at the boundaries, the situation 

 is satisfactory. But the question that you wished to have answered is, 

 "What happens in the middle?" In the middle, I don't know, but I 

 suspect that the answer is that in the middle the actual pool can be 

 replaced by another pool which is smaller. In other words, if pool Na 

 contains 4 grams of nitrogen, you replace it by a pool, say of 2 grams of 

 nitrogen, and treat it mathematically as if it has an infinite connection 

 constant. At some time in the not too distant future this problem will 

 have to be considered. 



The other question concerning the inhomogeneity of the labelled 

 materials is also a question which I find difficult to answer, but again 

 intuitively I feel that it is not a question which will create too great 

 difficulties. When Dr. Hsien Wu was at our laboratory he investigated 

 this excretion problem with L-Aspartic acid. L-Aspartic acid is quite 

 different from glycine, and its transfer rate of nitrogen to other amino 

 acids is quite different from that of glycine. Nevertheless, he gets almost 

 identical values with those we obtained. I think the answer to this is 

 that there are other things which are of more importance than the 

 question of transfer from one amino acid to the other. There are other 

 holes in this argument which unfortunately I cannot see at the present 

 time, but which I trust future work will bring to the surface. 



