148
ERNEST SCHOFFENIELS
this time is variable, since in some experiments it was found that influx
reaches a constant value after 75 minutes, (b) With the small intestine
the influx values are of an order of magnitude higher than those of the
outflux. (c) In the colon, the flux values are very similar and always
smaller than the values found with the small intestine. Thus the results
demonstrate unambiguously that glycine is actively transported across
the epithelium of the small intestine whereas it moves passively across
the colon.
Table IX shows the results obtained with glutamate. It can be seen
that in the colon, as well as in the small intestine, the flux ratio is very
close to 1, thus demonstrating the passive behavior of this amino acid.
It is also interesting to note that the permeability of the colon to
glutamate is lower than the permeability of the small intestine for the
same compound. Using this technique it has also been demonstrated
that L-glutamate and L-arginine are without effect on the influx of
glycine across the small intestine and the colon. On the contrary, L-alanine
reduces the influx of glycine across the small intestine. This result is
interpreted as showing that a common step is involved in the active
transport of L-alanine and glycine. Other experiments are, however,
necessary to decide whether or not we are dealing with a true competitive phenomenon obeying Michaelis-Menten kinetics.
It may thus be concluded that a number of L-amino acids are transported by the intestine against a concentration gradient, whereas the
D-isomers tested are not.
Among several amino acids transported, certain ones inhibit the
transport of others. L-Methionine was found to be the most potent
inhibitor in this respect. Although the intestinal transport system has
an unambiguous requirement for the stereo configuration at the a carbon
of the amino acid, its specificity for the side chain is difficult to define.
Thus the transport of both L-tyrosine and glycine were inhibited by
L-methionine, suggesting a common carrier for these compounds, although they have very different side chains. However, in the studies
on L-tyrosine analog (DL-ra-tyrosine, DL-o-tyrosine, 3,5-diiodotyrosine),
slight alterations in the ring of the side chain greatly affects the rate of
transport (25).
The different results on the specificity of the amino acid transport
system in these two kinds of experiments might be explained if both
entrance and exit permeations are selective and have different structural
requirements.
It has been shown repeatedly that the cells of the Ehrlich mouse
ascites tumor transfer amino acids into their interior as do most cells of
pluricellular organisms. There is, however, a difficulty when one has to
ERNEST SCHOFFENIELS
this time is variable, since in some experiments it was found that influx
reaches a constant value after 75 minutes, (b) With the small intestine
the influx values are of an order of magnitude higher than those of the
outflux. (c) In the colon, the flux values are very similar and always
smaller than the values found with the small intestine. Thus the results
demonstrate unambiguously that glycine is actively transported across
the epithelium of the small intestine whereas it moves passively across
the colon.
Table IX shows the results obtained with glutamate. It can be seen
that in the colon, as well as in the small intestine, the flux ratio is very
close to 1, thus demonstrating the passive behavior of this amino acid.
It is also interesting to note that the permeability of the colon to
glutamate is lower than the permeability of the small intestine for the
same compound. Using this technique it has also been demonstrated
that L-glutamate and L-arginine are without effect on the influx of
glycine across the small intestine and the colon. On the contrary, L-alanine
reduces the influx of glycine across the small intestine. This result is
interpreted as showing that a common step is involved in the active
transport of L-alanine and glycine. Other experiments are, however,
necessary to decide whether or not we are dealing with a true competitive phenomenon obeying Michaelis-Menten kinetics.
It may thus be concluded that a number of L-amino acids are transported by the intestine against a concentration gradient, whereas the
D-isomers tested are not.
Among several amino acids transported, certain ones inhibit the
transport of others. L-Methionine was found to be the most potent
inhibitor in this respect. Although the intestinal transport system has
an unambiguous requirement for the stereo configuration at the a carbon
of the amino acid, its specificity for the side chain is difficult to define.
Thus the transport of both L-tyrosine and glycine were inhibited by
L-methionine, suggesting a common carrier for these compounds, although they have very different side chains. However, in the studies
on L-tyrosine analog (DL-ra-tyrosine, DL-o-tyrosine, 3,5-diiodotyrosine),
slight alterations in the ring of the side chain greatly affects the rate of
transport (25).
The different results on the specificity of the amino acid transport
system in these two kinds of experiments might be explained if both
entrance and exit permeations are selective and have different structural
requirements.
It has been shown repeatedly that the cells of the Ehrlich mouse
ascites tumor transfer amino acids into their interior as do most cells of
pluricellular organisms. There is, however, a difficulty when one has to