140
ERNEST SCHOFFENIELS
extensively reviewed and the reader is referred to Volume II of this
treatise (Chapters 8 and 10) for a more complete analysis of the subject.
2. Organic Ions
a. Amino Acids. Since Van Slyke and Meyer (4) showed half a
century ago, that amino acids enter tissues against apparent concentration gradients, our knowledge of the intimate nature of the process has
not progressed much. Most of the work devoted to the matter was little
more than a refinement of the original observation. Considering the
results accumulated during the last decade, it seems that cells can be
classified into three categories with respect to amino acids: (a) cells
where an accumulation of amino acids from the extracellular medium
can be demonstrated (monocellular organism, cells of the central nervous
system, muscle fibers, etc.); (b) cells exhibiting a transcellular transport
of amino acids—intestinal and renal epithelium, Ehrlich cells in certain
experimental conditions (5); and (c) cells showing a regulation of the
intracellular amino acid pool mainly through a balance between synthesis and catabolism of amino acids—nerve and muscle fibers in invertebrates, for instance.
If we consider the first two categories defined above (a and b), it is
only in a very limited number of cases, surprisingly enough, that a direct
demonstration of the nature of the process responsible for the flux of
amino acids observed has been given. This is mainly owing to the
technical difficulties encountered in the determination of the various
parameters required to ascertain the nature of the forces involved. In
the case of accumulation of amino acids by cells, dependence of the
process on other metabolic events has been demonstrated with few
exceptions (6). Glycolysis as well as respiration can provide the necessary energy (6-9).
Experiments with animal cells and tissues have shown that amino
acids with rather different structures compete with one another in the
uptake process (10-16). D-Amino acids seem also to be accumulated in
certain specific cases (17, 18), but generally they inhibit the accumulation of the L form. In the cell nuclei, the D form is never accumulated
and does not interfere with the accumulation process of the L form (19).
Each of the above arguments is, however, of little value in establishing the nature of the force involved (see Section II, A, 1), and one has
to rely on various lines of evidence to consider that the uptake is indeed
active. On the other hand, when we are dealing with transcellular accumulation, the nature of the forces responsible can be more readily
defined. Various techniques have been proposed, the most generally used
being those of the everted isolated segments of small intestine (20), the
ERNEST SCHOFFENIELS
extensively reviewed and the reader is referred to Volume II of this
treatise (Chapters 8 and 10) for a more complete analysis of the subject.
2. Organic Ions
a. Amino Acids. Since Van Slyke and Meyer (4) showed half a
century ago, that amino acids enter tissues against apparent concentration gradients, our knowledge of the intimate nature of the process has
not progressed much. Most of the work devoted to the matter was little
more than a refinement of the original observation. Considering the
results accumulated during the last decade, it seems that cells can be
classified into three categories with respect to amino acids: (a) cells
where an accumulation of amino acids from the extracellular medium
can be demonstrated (monocellular organism, cells of the central nervous
system, muscle fibers, etc.); (b) cells exhibiting a transcellular transport
of amino acids—intestinal and renal epithelium, Ehrlich cells in certain
experimental conditions (5); and (c) cells showing a regulation of the
intracellular amino acid pool mainly through a balance between synthesis and catabolism of amino acids—nerve and muscle fibers in invertebrates, for instance.
If we consider the first two categories defined above (a and b), it is
only in a very limited number of cases, surprisingly enough, that a direct
demonstration of the nature of the process responsible for the flux of
amino acids observed has been given. This is mainly owing to the
technical difficulties encountered in the determination of the various
parameters required to ascertain the nature of the forces involved. In
the case of accumulation of amino acids by cells, dependence of the
process on other metabolic events has been demonstrated with few
exceptions (6). Glycolysis as well as respiration can provide the necessary energy (6-9).
Experiments with animal cells and tissues have shown that amino
acids with rather different structures compete with one another in the
uptake process (10-16). D-Amino acids seem also to be accumulated in
certain specific cases (17, 18), but generally they inhibit the accumulation of the L form. In the cell nuclei, the D form is never accumulated
and does not interfere with the accumulation process of the L form (19).
Each of the above arguments is, however, of little value in establishing the nature of the force involved (see Section II, A, 1), and one has
to rely on various lines of evidence to consider that the uptake is indeed
active. On the other hand, when we are dealing with transcellular accumulation, the nature of the forces responsible can be more readily
defined. Various techniques have been proposed, the most generally used
being those of the everted isolated segments of small intestine (20), the