ROUND TABLE DISCUSSION GIL. 
acids into cells and tissues. Very little has been said about excorporation of amino acids. In your 
presentation yesterday you showed rather impressively the uptake of tyrosine by brain tissues, 
but all of your slides seemed to end at the time of maximum uptake. I am interested what happens 
beyond this point. Is there any active excorporation of tyrosine out of brain tissue similar to that 
found by Dr. Layrua for lysine and leucine? 
GurorFF: I think Dr. LajTHAis probably more entitled to talk about efflux from the brain than I 
am. However, I can tell you that in vivo the endogenous tyrosine ratio between brain and plasma 
is about 1.3-1.4. This ratio is re-established in about 60-90 min when you have tyrosine in the 
blood at a high concentration. With the decrease of tyrosine concentration of the blood the tyrosine 
concentration of the brain also falls, but the endogenous ratio is maintained. That is, the brain 
remains higher than the plasma. After 4 h, the endogenous concentration had not been restored, 
but the pattern was clear. 
HoLpeEN: Do you find this for any other amino acid? 
GurRoFF: We have looked only very cursorily at the other aromatic acids, phenylalanine and 
tryptophane, and not in the efflux sense at all. 
Ho.LpEN: There is a report by LINDENBERG AND MassIn that different amounts of tyrosine 
can be extracted from yeast cells using hot water and cold TCA suggesting that the tyrosine pool 
is heterogenous and differs in this regard from that of the other amino acids. 
LajrHa: I would like to comment briefly on the important point brought up by Dr. BAXTER, 
concerning the direction of active transport. I think that at least in the brain levels of substrates 
are controlled by active processes working in two directions. Probably there is a number of systems 
where active transport out of the cell takes an important or even dominating role in homeostatic 
control. It would be important to learn more about influx and efflux, e.g. whether they refer to the 
same mechanism or to separate ones. 
Hoven: In closing I would like to remind you and those who read these proceedings that this 
session was held primarily to expose all of us as intimately as possible to the existing, and in some 
cases divergent, views and data regarding transport and accumulation phenomena. There was no 
expectation that major issues would be resolved. However, we did hope that such discussion and 
contact might reduce the time required to attain a clearer understanding of these processes. 
CowlE: I would like to say that it isn’t so much a matter of divergency. I think in the CARNEGIE 
group we do recognize that there are free amino acids and as well as bound amino acids; in other 
words, all we ask is that the so-called fvee amino acid advocates recognize that there might be 
some bound ones also. 
Hoven: I think that would be an appropriate note on which to end. Thank you very much. 
REFERENCES 
1D. L. OXENDER AND H. N. CHRISTENSEN, J. Biol. Chem., 234 (1959) 2321. 
2 V. KOEFOED-JOHNSON AND H. Ussinc, Acta Physiol. Scand., 42 (1958) 298. 
3 A. ABrams, J. Biol. Chem., 234 (1959) 383; 235 (1960) 1281. 
4 A. ABRAMS AND C. G. MACKENZIE, Federation Proc., 18 (1959) 178. 
5 A. ABRAMS, G. YAMASAKI, J. REDMAN AND C. G. MACKENZIE, Federation Proc., 19 (1960) 129. 
6 J. E. TREHERNE, J. Expil. Biol., 36 (1959) 533. 
7E. HEInz, J. Biol. Chem., 211 (1954) 781. 
8 EK. HEINZ AND P. M. Watsu, /. Biol. Chem., 233 (1958) 1488. 
® E. HEINZ AND R. P. Dursin, J. Gen. Physiol., 41 (1957) Iot. 
10 C. R. Park, R. L. Post, C. F. Karman, J. H. Wricut Jr, L. H. JOHNSON AND H. E. MoRGAN, 
Ciba Foundation Symposium: Endocrinol., 9 (1956) 257. 
11 T. ROSENBERG AND W. WILBRANDT, J. Gen. Physiol., 42 (1958) 280. 
12 EB. HEINZ, J. Biol. Chem., 225 (1957) 305. 
13K. A. C. ELLiotr AND N. M. VAN GELDER, J. Physiol. (London), 153 (1960) 423. 
144K. Sano AND E. RoBerts, Biochem. Biophys. Research Communs., 4 (1961) 358; unpublished 
results. 
