DYNAMIC ASPECTS — AMINO ACID POOL TURNOVER Hoy 
DISCUSSION 
Chairman: HARRY EAGLE 
EaGLe: Dr. WESTALL, would you care to open the discussion? 
WESTALL: There is one thing that is rather interesting, not with respect to the leucine, iso- 
leucine and valine, where their high plasma levels build up so dramatically in maple-syrup disease, 
but in the case of argininosuccinic aciduria. When we first started to study the disease we were 
still of the opinion that urea production was a hepatic function only and we were very interested 
to hear of some results of SporN, I think it was, and his collaborators, who seemed to be able to 
show that many of the enzymes necessary for urea production were present in brain tissue. 
L. MILLER: With respect to SPORN AND DINGMAN’s observations, one has only to look at their 
data in terms of quantitation to appreciate the fact that although the brain may produce urea, 
it could produce urea only from labeled arginine. I was not convinced that they showed any kind 
of net urea synthesis, labeled or otherwise, from carbon dioxide and ammonia. In terms of the 
total mass of urea produced in any one day to maintain normal nitrogen economy, the brain 
plays a very insignificant role. 
EaG Le: Dr. MILLER, your experiments offer a very good explanation for the fact that in cell cul- 
tures most of the essential amino acids, in general, are not metabolized to any significant degree. I 
am puzzled, however, as to why isoleucine, leucine, and valine are not metabolized if their metab- 
olism is not limited to the liver but seems to be as general as your results would indicate. 
L. Miter: Do I understand you correctly to state that you have recorded observations on the 
failure of cells in tissue culture to oxidize leucine, isoleucine, and valine? I can only tell you that 
of these three we have studied only the metabolism of randomly labeled leucine, but here again, 
not with any of the cells that you have used. We have used a strain of a Walker tumor grown in 
cell culture under sterile conditions and we find that one gets very definite oxidation to COg. 
It is hard to compare it with what goes on in the intact animal, but in terms of a relatively small 
mass of tissue this was significant. We have also observed that the same Walker tumor cell pre- 
paration would not oxidize !C-lysine in cell culture. 
EaGLe: We had better go back and take another look at it. 
