590 J. T. HOLDEN 
amino acid transport in mammalian tissue, CHRISTENSEN now also has concluded, 
that in the Ehrlich ascites tumor cell the vitamin does not participate in the entry 
reaction)». 
On the basis of evidence indicating that bacteria are bounded by a membrane 
which is virtually impermeable to many solutes in the resting state and the finding 
that bacterial protoplast membranes contain a considerable number of en- 
zymes**; 88, 110. MITCHELL has proposed that solute penetration is achieved by a trans- 
location of chemical groups catalyzed by enzymes in the membrane for which the 
compound in question is a normal substrate’? 8% 84, 86, 87, 9. He has proposed the 
name “translocases” for such systems. The most convincing evidence for the involve- 
ment of enzymatic or, at least, protein catalysts in solute accumulation has come 
from Monop, COHEN and their associates who proposed the permease concept (cf. 20, 
gi, 98.) It was demonstrated that galactoside accumulation in EF. coli occurs with 
the intervention of an inducible, specific catalyst whose formation can be prevented 
by interfering with protein synthesis. Kinetic evidence was relied on originally to 
exclude stoichiometric binding as the mechanism of accumulation. Subsequently, 
SistRoM!°? demonstrated that protoplasts accumulating galactoside undergo swelling, 
suggesting that this substance is osmotically active and not bound internally. Thus, 
it was proposed that the cell contains protein catalysts presumably within the 
membrane serving to promote the entry of specific organic solutes. Although originally 
envisaged as being distinct from metabolic catalysts, ZABIN ef al.“” presented pre- 
liminary evidence suggesting the possible identity of the so-called galactoside per- 
mease with an enzyme which acetylates thiomethylgalactosides. Subsequent ex- 
periments summarized elsewhere in this volume (ZABIN, p. 613) now make this 
appear unlikely. 
A completely analogous set of observations for amino acid accumulation has not 
yet been reported. These systems appear to be constitutive in the organisms investi- 
gated so far, although Borzzi AND DE Moss’ recently have reported that the tryp- 
tophane accumulation system in EF. coli might be inducible. SCHWARTZ et al.1°° and 
LuBIN et al.®® have described the isolation of viable mutants of E. col lacking specific 
amino acid accumulation systems. It is not yet clear whether these mutants lack a 
protein which is the accumulation catalyst or a protein which synthesizes some 
essential component of a catalyst. Further study of such mutants offers one of the 
best available tools for identifying the components of the accumulation system. 
Additional comments on the possible identity of such systems must be based largely 
on speculations with little or no experimental support. The activating enzymes 
presumed to be involved in protein synthesis have been repeatedly proposed as 
possible accumulation catalysts (e.g. ref. 84). This is supported by reports that they 
may occur in the protoplast membrane®*: %; %, (cf. ref. 81) although a conflicting 
recent report has appeared™?. One would expect that their loss by mutation would 
be lethal in view of their apparent involvement in protein synthesis. However 
MITCHELL’s cautioning suggestion®® might be appropriately mentioned here, namely 
that transport mutants could result simply by a change in location of the catalyst in 
the cell. In this specific case the activating enzymes might be synthesized but not 
incorporated into the membrane. Their participation in accumulation might thereby 
be prevented without significantly reducing their effectiveness for protein synthesis. 
Perhaps the most impressive argument against the possibility that the well-known 
References p. 592/594 
