AMINO ACID TRANSPORT INTO CELLS 533 
Growth on mannose or succinate plus to-° M galactose increased the exit rate for 
galactose, thereby decreasing the equilibrium internal galactose level. 2,4-Dini- 
trophenol blocked the entry process so that the exit process could be studied sepa- 
rately. OSBORN AND MCLELLAN® have reported that dinitrophenol also accelerates 
the exit process in the same strain, an effect which we suggest could arise from an 
interconvertibility of the transport mediator. 
@ 
e) 
{e) 
1mM DNP 
o 
[e) 
oO 
Control 
iS 
{e) 
(e) 
| 
a-Aminoisobutyrate in medium, counts/min 
ye) 
fe) 
fe) 

Minutes 
Fig. 3. Increase by 2,4-dinitrophenol (1 mM) of the exit rate for previously accumulated labeled 
a-aminoisobutyrate. The Ehrlich cells had been incubated 40 min in 20 mM a-aminoisobutyrate, 
washed in cold media, and then transferred to a large volume of Krebs—Ringer—bicarbonate 
buffer held at 37°. 
OXENDER!? and OXENDER AND Royer!! have shown that pyridoxal phosphate 
and related agents stimulate the extent of amino acid uptake by cells by slowing a 
mediated efflux, and not by accelerating the mediated influx. In the Ehrlich cell we 
find that dinitrophenol distinctly accelerates the efflux of either valine or a-amino- 
isobutyrate (Fig. 3) supporting the idea that the exit site may be an inactive or 
inferior form of the entry site, which is present in increased amounts when the energy 
supply of the cell is curtailed. 
The intestinal transport of amino acids shows a remarkable resemblance to the 
behavior I have described so far. At low levels the leucines and valine are found to 
be most rapidly transported and most strongly concentrated into the everted gut 
sac of the rat, using the technique of CRANE AND Witson!?. With shorter side- 
chains slower rates are encountered. The amino acids with lipophilic side-chains are 
also the strongest inhibitors of transport. In fact they are such good inhibitors that 
at 20-mM levels their transport becomes the slowest, as WISEMAN showed in 1953, 
not the fastest. The intestinal transport of glycine and a-aminoisobutyrate can be 
extensively suppressed by valine or methionine, but unreasonably high levels are 
required, and the intestinal transport of these two symmetrical amino acids can 
scarcely be saturated by raising their levels (H. AKEDO, cited by CHRISTENSEN’). 
Hence, although glycine and aminoisobutyrate (AIB) apparently use the ordinary 
transport site, they are not displaced from it by other amino acids as readily as 
would be expected from their low transport rates. Again departure from a single 
affinity series is evident. 
References p. 538 
