SPECIFICITY OF TRANSPORT 



relatively low affinities for C that, ordinarily, only when a large 

 gradient has been established in favor of the cell interior is their 

 exodus able to keep up with entry. 



The scheme of Figure 22 should be related to an earlier one 

 proposed by Kepes (1960) for the transport by E. coll of a group 

 of three thio-/?-galactosides, which proves to be quite similar. Kepes' 

 scheme has been redrawn here (Figure 23) to facilitate comparison. 

 Accumulation is considered to occur because an extra amount of 

 the carrier-solute complex CA is formed outside the osmotic barrier 

 by reaction of the external solute with a reactive form of the carrier, 

 specified here to be C ~ R. The solute displaces the radical R. The 

 dissociation of CA may occur spontaneously. For the particular 

 case of amino acid transport, the scheme of Figure 22 avoids the 

 problem of retaining the high chemical potential of CA outside the 

 osmotic barrier, in view of the leak represented by its direct outward 

 release of A, by showing the carrier-solute complex transferred 

 across the osmotic barrier as an immediate consequence of complex 

 formation. This scheme also places the exergonic process driving 

 uphill transport inside the cell. A direct stoichiometric linkage be- 

 tween ATP breakdown and activation of the carrier site should 

 not be implied; such a direct linkage would probably be prohibi- 

 tively wasteful since each ATP breakdown supplies several times 

 the energy presumably required for transfer uphill of each amino 

 acid molecule into the Ehrlich cell (Heinz and Patlak, 1960). 



An alternative possibility is that the two chemically and func- 

 tionally distinguishable mediators for the transfer of neutral amino 

 acids are not two forms of the same carrier but are instead unrelated. 

 Horecker et al. (I960; 1961) secured the induced formation of an 

 exit process for galactose by an E. coll strain lacking a galactokinase. 

 This bacterial strain has a constitutive, concentrative uptake process 

 for galactose. The fact that the entry process could be present 

 in the absence of the exit process appears discouraging to the pos- 

 sible view that the exit mediation was a part or a manifestation 

 of the entry process. As the uptake process was suppressed by 

 2,4-dinitrophenol, however, the exit rate was increased. This be- 

 havior, which has also been observed for amino acids in the 

 Ehrlich cell (cf. Christensen et al., 1962), suggests that more of 

 the exit-mediator is present when less energy is available for con- 

 verting it into a form serving for uphill entry, as illustrated in 

 Figure 22. 



63 



