540 E. HEINZ 
dynamic efficiency of the transport system’. It must be accounted for in any model 
which is to represent the carrier mechanism. The following model (Fig. 3) which has 
been based on the kinetic observations, seems to meet this requirement?. The ex- 
change diffusion requires that steps 1-4 are reversible. Since inhibitors of energy 
supply affect only the net transport and not the exchange diffusion’, energetic coupling 
probably does not concern these steps, but rather step 5 in which the carrier is re- 
activated. JACQUEZ arrives at a similar conclusion through a mathematic analysis of 

uM 
*2min uM 
g aA V max=633 4.3 min 
a Cells preloaded (0) 
[Glycine];,= 63mM 
x] 40 as 
2 
re = pM 
£" 30 Vmax 31.5 52min 
e 
Q 20 Cells not preloaded (+) 
Gy [Glycine]; ,~ 6mM 
10 K_=31mM 
m 
Ke 5 10 15 mM 
Extracellular glycine 
Fig. 2. Influx of [1-“C]glycine at various extracellular glycine concentrations. Upper curve, 
cells preloaded by preincubation with unlabeled glycine. Lower curve, cells not preloaded. 
the kinetic data available!. More recently we have tried to modify the above model 
along the lines of MITCHELL’s hypothesis of group transfer“. Evidence, however, 
does not favor the intermediate activation of the amino acid by a covalent linkage, 
as seems to be implied by such a group transfer. Carboxyl-oxygen does not exchange 
during the transport!, nor is the NH, group likely to be linked to a carbonyl group 
of the carrier by formation of a Schiff base}. 
A major contribution concerning the amino acid uptake by Ehrlich mouse-ascites 
Medium Transport region Cell 

ot nie, yee 
zi 
Leshan aa usakeges tae 
ular sabe 
Fig. 3. Schematic model of active transport, associated with exchange diffusion; ay and a, refer 
to extracellular and intracellular substrates, respectively. x is the active; y the inactivated 
carrier; ax is the carrier-substrate complex; ~ P and —P stand for reactant and product, re- 
spectively, of any exergonie reaction, energerically coupled to transport mechanism. (From 
HEINZ AND WALSH?, with permission). 
References p. 544 
