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DYNAMIC ASPECTS OF CELLULAR FREE AMINO ACID POOLS 
Round Table Discussion 
STATE OF THE INTRACELLULAR AMINO ACIDS 
Session Chairman: JOSEPH. T. HOLDEN 
Transcript Editor: ER1cH HEINZ 
HoLpeEn: It was apparent during the discussion last night that one of the most urgent problems 
in this field concerns the state of the intracellular amino acids. That is: are they in fact free or 
associated in some way with intracellular structures? I believe, this to be, therefore, the best 
place to begin this discussion. In view of his extensive experience with various aspects of this 
problem, I would like to ask Dr. CHRISTENSEN to express his views on the subject. 
CHRISTENSEN: I think that life tends to find a single answer only, or very few answers, to each 
of its problems. Accordingly I think that if we can prove that amino acids are concentrated by 
some organisms we may assume that the same thing occurs in many others, and probably by a 
similar process. No one can question that the animal organism is able to form an extracellular 
fluid and various secretions with compositions totally different from that of its aqueous environ- 
ment, and from each other. Further it is able to maintain these differences in spite of the more or 
less constant movement of water and solutes across the separating barriers of cells. The barriers 
that accomplish this effect may be illustrated by epithelial cell layers of the alimentary tract and 
of the renal tubule. This situation shows that at least specialized cells are able to transport solutes 
against concentration gradients, in some cases producing concentration ratios of ten million to 
one, as in the case of hydrogen ion secretion by the gastric glands. 
We might evade the apparently endless argument about the nature of solutions within an 
unharmed cell, by seeing whether cells not specialized for secretion can also generate gradients 
between two isolated segments of extracellular fluid. In this case two phases should be fully 
accessible to study, without such serious uncertainties as to activity coefficients. 
As I pointed out earlier in this meeting remarkably similar properties and similar affinity series 
are seen between, on the one hand, the intestinal mucosa in concentrating amino acids from the 
mucosal to the serosal phases, and, on the other hand, the Ehrlich cell in taking up amino acids. 
With the intestinal system we have two abundant extracellular phases that are highly susceptible 
to examination. Obviously one is dealing here with activities of the free amino acids and not with 
bound forms. Activities have indeed been measured for calcium ion transport across the intestinal 
mucosa, and for the hydrogen ion across the stomach wall. 
But perhaps more to the point is an experiment in which OXENDER and I arranged the Ehrlich 
cell as a barrier some three or four cells thick on a millipore filter, to form an artificial barrier or 
membrane’. We then put saline solution on the two sides of this membrane and placed a test 
amino acid, let’s say glycine, at equal levels in the two solutions. If we now added alanine to one 
side at a substantially higher level, we could observe the phenomenon of exchange. That is, while 
the alanine was moving down gradient, glycine was driven in the opposite direction so that the 
initially alanine-rich phase became enriched in glycine at the expense of the opposite phase. This 
effect persisted for more than an hour, until the two amino acids eventually came to be distributed 
uniformly between the two saline solutions. 
A glycine gradient could also be produced by placing pyridoxal on one side of the membrane. 
This agent appears to stimulate amino acid transport, for reasons I won’t discuss here; it caused 
glycine to be accumulated somewhat on the opposite side of the barrier. 
These results lead us to the hypothesis that secretion across membranes composed of cells 
probably occurs by the existence of an unidentified asymmetry between the two phases presented 
by the cells on the respective sides of the membrane. KOEFOED-JOHNSON AND UssING? earlier 
proposed the parallel idea that a cell layer of the frogskin transports Nat by extruding it more 
effectively to the inside than to the outside of the skin. The natural basis for such an asymmetry 
is not clear. In the intestinal mucosal cell the transport from the luminal side may well be faster 
because of the much greater surface area of the brush border, and the consequent presence of more 
transport sites at that surface of the cell. A type of profile of solute levels has repeatedly been 
References p. 777 
