578 J. T. HOLDEN 
BOEZzI AND DE Moss’ have observed a ten-fold increase in tryptophane accumula- 
tion capacity when E. coli is grown with a complete amino acid mix including high 
levels of tryptophane. Glucose inhibits the activity of the accumulation system but 
not this apparently adaptive increase. 
Studies on solute penetration monitored by its consumption intracellularly have 
been described occasionally (e.g. refs. 74, 116). Unfortunately, interpretation of such 
findings is greatly complicated by the necessity to dissociate the properties of the 
transport and the metabolizing systems. They appear more useful in studies on the 
functional aspects of accumulation systems, as, for example, in investigation of 
crypticity. 
MECHANISM OF AMINO ACID ACCUMULATION 
The mechanism by which exogenous amino acids are taken into the cell in opposi- 
tion to apparent large concentration gradients still remains in doubt, although the 
active transport hypothesis is favored by most workers. The differences in findings 
with different organisms raises the possibility that more than a single mechanism 
may be found to operate. Speculations regarding the nature of this phenomenon 
have centered on two very different views of the cell and the state of its amino acid 
pool. In the first instance it has been proposed that intracellular pools are adsorbed 
or associated with internal polymers”: 1°! (cf. BRITTEN, p. 595 and CowlE, p. 633) 
in a cell which is readily penetrated by a wide range of nutrients and metabolites. 
The more frequently encountered proposal is that intracellular solutes are free within 
cells which are enclosed by a permeability barrier capable of retaining these solutes. 
In the latter instance, entry would be achieved by specific catalysts which permit net 
movement across the membrane into the cell even when the intracellular amino acid 
concentration exceeds that of the extracellular environment. In the most recent 
formulation of the adsorption site hypothesis BRITTEN AND MCCLURE have suggested 
that a mobile carrier catalyzes the reaction of exogenously derived amino acid with 
adsorption sites within the cell? (cf. BRITTEN, p.595). Beyond establishing the 
metabolic dependency of accumulation phenomena and suggesting that specific 
catalysts function in this process, the properties considered in previous sections have 
had limited value in evaluating this question. A choice between these models could 
be made by demonstrating beyond doubt the state of the accumulated amino acid. 
State of intracellular amino acid pools 
If amino acids occur within the cell largely in a free form, unassociated with intra- 
cellular polymers, a permeability barrier must exist at the cell surface or at the 
surface of that portion of the cell which contains the amino acids, and the accumula- 
tion phenomena would then clearly involve active transport in view of the large 
concentration gradients which must consequently exist. If the pool is largely associ- 
ated with adsorption sites a different type of catalytic reaction can be expected and 
the occurrence of a functional permeability barrier might not be required, although 
it could still exist and possibly even play a role in the accumulation process. There- 
fore, dependence of accumulation on an effective permeability barrier cannot by 
itself exclude the adsorption site mechanism, but is a necessary condition if the pool 
is in a free state. On the other hand, existence of a major portion of the pool in a 
References p. 592/594 
