566 DYNAMIC ASPECTS — PERMEABILITY AND TRANSPORT 
TRANSPORT AND ACCUMULATION OF AMINO ACIDS 
BY MICROORGANISMS* 
JOSEPH T. HOLDEN 
Department of Biochemistry, Medical Research Institute, City of Hope Medical Center, 
Duarte, Calif. (U.S.A.) 
INTRODUCTION 
The discovery of freely extractable amino acids in microorganisms by GALE and 
his associates?®. 27 (cf. HOLDEN, p. 73) was followed by the pioneering studies of this 
group on the uptake and accumulation of amino acids by Streptococcus faecalis and 
Staphylococcus aureus. These studies, which have been summarized extensively by 
GALE”, 31, 32, focused attention to this problem and demonstrated that microorga- 
nisms might be advantageously used in studies of transport. This review will sum- 
marize some of the commonly described properties of representative microbial amino 
acid accumulation systems subsequently studied and with this as a base discuss 
current views concerning the mechanism of this process. 
At the outset it should be understood that there is considerable confusion and 
inaccuracy in terminology used by workers in this field. (cf. ROSENBERG! and 
CHRISTENSEN!® for discussions of nomenclature). It is not always appreciated that 
the existence of a permeability barrier or the interaction of cell components with 
solute during uptake even in a catalytic manner are not necessarily indicative of 
active transport, nor is the additional demonstration of an energy requirement. The 
term “active transport” should be reserved to describe situations in which the solute 
studied occurs in the same state in the intra- and extracellular compartments and is 
present at a higher concentration in the compartment into which it has 
moved, 7.e. movement from a lower to a higher chemical or electrochemical potential 
has occurred. While it has become customary, with considerable justification, to use 
the term transport in discussing many cases of amino acid accumulation, it should be 
understood that evidence sufficient to demonstrate active transport has not been 
presented yet for any specific microbial amino acid accumulation system. The infer- 
ential evidence in some cases is very strong, as will be seen for the systems studied in 
this laboratory. Nevertheless this evidence appears incomplete, largely because the 
intracellularly accumulated amino acid still has not been demonstrated beyond all 
doubt to be in the free state. Furthermore, some investigators after extensive studies 
retain the belief that the accumulations they have observed cannot be attributed 
entirely, if at all, to a classical active transport process. In view of this uncertainty, 
* The following abbreviations will be used: DNP, 2,4-dinitrophenol; HB, cells, high vitamin 
B, cells, nutritionally normal cells grown in complete medium, containing an excess of this vitamin 
(cf. ref. 47); LB, cells, low vitamin B, cells grown in media containing none, o1 trace amounts, 
of vitamin B,. 
References p. 592/594 
