ROUND TABLE DISCUSSION 523 
wondering, when you were feeding those rats on a complete synthetic amino acid mixture and 
then withdrew one essential amino acid, how long would it be before they stopped eating? 
Winitz: This would depend upon the particular essential amino acid you chose to withdraw. 
H. RosENBERG: Let’s say the most severe condition. 
Winitz: Withdrawal of leucine, isoleucine or valine lead to the most severe conditions. But 
with regard to the animal not eating a diet, it is not necessary to leave an essential amino acid 
out of the diet. The same effect can be achieved by using a particular non-essential amino acid 
as the source of non-essential nitrogen. When we studied the ability of different non-essential 
amino acids to serve as the sole source of dietary non-essential nitrogen—t-alanine, D-alanine, 
glycine, serine, hydroxyproline and various others—we found that the growth response of the 
animals varied considerably with the amino acid employed. We further found that certain amino 
acids, as in the specific instance of hydroxyproline, were extremely toxic. The animals rejected 
the diets where this material was employed and consequently their weight fell off very markedly. 
The apparent toxicity of hydroxyproline in the diet was probably due to the inability of the animal 
to metabolize it effectively in large quantities. In any case, the animals refused the diet almost 
immediately. They would take only a few sips and that’s all. 
E. Roserts: In line with that, is feeding collagen toxic to animals? 
WIniTz: No. 
E Roserts: Well, how would we square that with hydroxyproline being toxic, and not collagen? 
Winitz: This I don’t know, except that it may be a matter of hydrolytic rate—just how 
much the organism is actually exposed to at any given time. The collagen is very, very slowly 
hydrolyzed by the proteases of the gut as opposed to other proteins, probably due to the very 
fact that it contains so many hydroxyproline residues, and this may be part of the answer. 
SCHREIER: In studies in human beings, methionine seems to be the most toxic amino acid of all. 
Dr. SouPART, would you like to add something to this subject? 
Soupart: The only things I know about amino acid imbalance deal with protein in malnutri- 
tion, especially of the kwashiorkor type. When there is a slight protein deficiency, as in mild 
cases of this disease, the only abnormalities that are found in urine are high excretion levels of 
taurine and /-aminoisobutyric acid. Both of these are end-products of metabolism, and in order 
to find hyperexcretion of other amino acids, one must investigate the more severe cases of protein 
malnutrition. This probably results from the fact that the kidney itself is affected in severe cases; 
in mild cases, however, when high protein diets are given to the patients, the result of this 
therapy is to disclose the hyper-amino acid excretion. For example, when milk protein is given 
to children with kwashiorkor, a hyperexcretion of all of the other amino acids occurs for some 
time. This therapy is really effective, and taurine excretion disappears very quickly, in most 
instances in 24 hours. The other amino acids are elevated in the urine but return to a normal 
level within one or two weeks; the only one which remains elevated for a longer period in the 
urine is 6-aminoisobutyric acid. 
I would ask for suggestions as to the interpretation of these findings. Do the high taurine and 
f-aminoisobutyric acid levels result from a disturbance of the permeability of the cells? Do they 
result from the fact that there is a higher number of cells dying as a result of starvation? Or is 
there another interpretation of these facts? 
WEsTALL: With regard to f-aminoisobutyric acid, I would be inclined to think that it is probably 
due to an increased number of cells dying. In the examination of routine chromatograms over a 
number of years, we have been rather shaken by the alteration in taurine, and Dr. DENT has come 
to the general conclusion that if the patient has a fever it is usually reflected in an increasing 
taurine excretion in the urine. 
To say something rather different, in general terms, I would like to make a plea here that 
studies in diseases and nutrition stress the condition of the amino acids in the blood. This, in my 
opinion, is far more important than what is shown in the urine. The intervention of the kidney 
tends, in a way, to give a false picture of what may be happening, because different amino acids 
have different rates of clearance. It is much easier, for example, to force histidine out into the 
urine than it is to force out isoleucine, so that the isoleucine level of the blood may be up three 
times and it would never be seen in the urine. 
Winitz: I most fully agree with Dr. WESTALL’s statement concerning the importance of the blood 
amino acid picture. Recently, we have had occasion to use the fasting amino acid plasma levels 
of the rat to determine its essential amino acid requirements in only a few days with a greater 
degree of precision than can be achieved by the conventional long and tedious growth or nitrogen 
balance studies. The method we employed is extremely simple. The animals, toog rats in this 
case, were fasted for 24 h, blood samples were taken, pooled and deproteinized, and the amino 
acid composition of the plasma determined with the aid of the amino acid analyzer. Chemically 
defined diets were then prepared which contained the essential amino acids in exactly the same 
molar proportion as they appeared in the fasting plasma. These diets, when compared with the 
best chemically defined diets we had been previously been able to devise over a period of six 
References p. 524 
