AMINO ACID TRANSPORT IN MICROORGANISMS 507 
the more general terms, “accumulation” and “transport” will be retained in referring 
to this phenomenon. 
Presently held views on the mechanism of amino acid accumulation have been 
influenced strongly by many exciting, provocative findings with other classes of 
solutes, in bacterial cytology, on the properties of protoplasts, etc. As a result, the 
specific status of the amino acid transport problem is not always clearly evident. 
This review, therefore, will discuss principally experimental observations obtained 
with amino acids, and their implications as they relate to amino acid transport. 
Findings from other systems, most of which have been elegantly reviewed by others 
(see below) will be introduced only when they are crucial to an understanding of this 
problem. Hopefully, this more exclusive treatment will permit a clearer understand- 
ing of the strengths and limitations of the evidence available as support for currently 
popular concepts depicting the mechanism of amino acid transport and accumulation 
in microorganisins. 
Additional discussions of amino acid permeability and accumulation in micro- 
organisms can be found in the reviews of MITCHELL® 84, CoHEN AND MoNnop??, 
CHRISTENSEN), and BRITTEN AND McCLuRE®™. Other aspects of microbial permeabil- 
ity and transport have been reviewed by MITCHELL”: 81, 87, 99, ROTHSTEIN1©3, Monop”!, 
Davis?3, 24, CrrtLLo!84 and others®® and will be discussed in a forthcoming review by 
COHEN AND KEpPEs!?. 
PROPERTIES OF AMINO ACID ACCUMULATION SYSTEMS 
Explorations of various properties of amino acid accumulation systems has permitted 
some limited deductions concerning the possible nature of this process. Thus, the 
operation of a simple passive diffusion process has generally been excluded by 
showing a non-linear relation between rate and external concentration, competition 
among structurally related substrates and a high temperature coefficient. A reliance 
on cell metabolism generally has been observed most often as a dependence on energy- 
yielding reactions. Finally high concentration gradients are usually reported although 
little evidence exists to show that these are true gradients involving solutes in the 
same state within and outside the cell. Examples of such studies will be considered 
below. 
Methods 
Experimental procedures sufficient to demonstrate accumulations are extremely 
simple, requiring only the exposure of cells to a solution of the test substance, sepa- 
ration of cells and medium after an appropriate time interval, and analysis of the 
cells (or extract thereof) and/or the supernatant fluid for the solute. Some investi- 
gators use exponentially growing cells in the steady state whereas others, following 
GALE’s example, use relatively dilute suspensions of resting cells incubated in buffers 
or growth media rendered incapable of supporting cell division. Thick suspensions in 
which intra- and extracellular volumes are roughly equivalent are seldom used for 
accumulation studies, although they are frequently used in studies of permeability 
properties. By employing isotopically labeled amino acids and measuring isotope and 
the L-amino acid in both intra- and extracellular compartments it is possible to insure 
the absence of extensive metabolism of the accumulated amino acid which can 
References p. 592/594 
