532 H. N. CHRISTENSEN et al. 
over into the D-area (as in a-aminoisobutyric acid and cycloleucine) appears to permit 
more effective depletion of affinity. Accordingly the D-amino acids show low affinities 
for influx but are eventually concentrated almost as well as the L-forms, apparently 
because they show proportionally as large an affinity difference between the sites 
predominating for influx and efflux, respectively. Perhaps it will be possible to reach 
by this pathway a complementary picture of the character of a site deformation that 
tends to dislodge amino acids. 
If we suppose that an identical parcel of energy serves in the preparation of the 
transport site for amino acid binding, no matter which neutral amino acid is to be 
20/— Cycloleucine 


Methionine 
Distribution ratio 
uo 
Leucine 
a l | [es ee) eek ee ee 
O 10 2ObR Tso 40 50 60 
Minutes 


Fig. 2. Progressive uptake of several amino acids by Ehrlich ascites tumor cells at 37°. Initial 
level of the test amino acid in Krebs—Ringer—bicarbonate medium, 1 mW; 25 vol. of suspending 
medium used. 
carried, energy wastage might be anticipated if considerable affinity is retained on 
site modification. Until we know better the character of the energy exchange, 
however, this concern is perhaps premature. It is of course possible that the apparent 
second affinity arises from efflux by an entirely distinct site; the results in our tests 
so far would be the same, although we would then have to explain why one site is 
available mainly for exodus, another for entry. In the present formulation we sup- 
pose that the preferred site is eliminated almost as fast as it appears at the inside 
limit of the membrane; whereas the degraded site is reactivated as quickly as it 
reaches the outer limit. 
The speed of the uptake of the leucines and valine in comparison with methionine, 
histidine and other amino acids, out of all proportion to the extent to which they 
can be concentrated by the Ehrlich cell, does not arise merely from their suscep- 
tibility to uptake by exchange, since these rate superiorities are retained when cells 
have been depleted of amino acids, and are observed for erythrocytes, which have 
very small quantities of endogenous amino acids available for exchange. Note that 
the uptake of glycine and a-aminoisobutyrate reaches its relatively lowest rate in 
the erythrocyte, supporting the view that with this cell we are measuring mainly 
the affinities for a single unmodified site. 
Other evidence also supports the existence of a mediated exodus process for 
solutes from cells, a process which can be modified without modifying the mediated 
entry. HoRECKER et al.8 detected an inducible exit process for galactose in FE. colt. 
References p. 538 
