348 E. ROBERTS AND D. G. SIMONSEN 
transformations in a tissue may be extensive our comprehension of the kinetics of 
these processes within living cells is negligible. We are thus faced with a situation in 
animal cells in which a variety of experimental conditions have been employed 
which probably have had great effects on turnover rates of the free amino acids 
while having minimal effects on their steady-state concentrations. If the above inter- 
pretation is correct, further analyses of the servo-mechanisms involved should be 
carried out by studying intensively the rates of the various processes involved in 
the maintenance of the concentration of single constituents and not by trying to 
find a general answer which would fit all cases. Perhaps the most favorable sub- 
stances with which to begin would be those that have the smallest number of alternate 
metabolic pathways within the cells employed for study. 
In this report no attempt has been made to cover the entire literature dealing 
with the occurrence of free amino acids in animal tissues. Those aspects of our own 
work and that of others were discussed which could be woven into a meaningful 
pattern without infringing on the material to be reported by other speakers at the 
Symposium. 
ACKNOWLEDGEMENTS 
This investigation was supported in part by research grant C-2568 from the National 
Cancer Institute, and grant B-1615 from the National Institutes of Neurological 
Diseases and Blindness, National Institutes of Health; grant 3001 (00) from the 
Office of Naval Research; and a grant from the National Association for Mental 
Health. 
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