characterized, which transports long-chain,
branched-chain and aromatic zwitterionic
amino acids in an Na+-independent
manner, also has not been cloned. Because
system L is thought to be the primary
mechanism for release of neutral amino
acids from most mammalian cell types,
understanding the molecular structure and
patterns of regulation may yield important
knowledge of how the transmembrane flux
of amino acids is moderated to achieve
intracellular amino acid homeostasis.
System N is the primary Na+-
dependent transport activity for glutamine,
asparagine and histidine in the liver. The
heterogeneous absorption of anionic amino
acids by the liver (system xc, periportal
hepatocytes; system XAG, pericentral
hepatocytes) is important to whole-animal
nitrogen balance (Haussinger, 1990).
Therefore, when combined with our ability
to measure the expression of system XAG
proteins, the cloning of systems N and xc
will provide the ability to evaluate factors
which affect the whole-body nitrogen
homeostasis at the molecular level.
Similarly, the elucidation of system A trans-
port protein(s) and gene(s) will yield
invaluable information as a model for the
regulation of transporter expression.
Normally, system A activity is low in most
cells. However, in response to hormones,
growth factors, cell division and/or sub-
strate supply, system A activity increases
from 2- to 50-fold. Accordingly, system A
has been one of the most extensively bio-
chemically characterized amino acid trans-
port systems. Despite this fact, and the use
of many imaginative protocols, including
methodology successfully used for other
transporters, the protein(s) responsible for
system A activity has yet to be cloned. In
terms of reported peptide transport activi-
ties, the protein(s) responsible for the puta-
tive basolateral membrane peptide
transporter has yet to be identified. Once
identified, two important questions to be
answered using functional expression stud-
ies are what is the magnitude of the pH gra-
dient required for transport and what
regulates the capacity for peptide transport
across the basolateral membrane.
Characterization of Gastrointestinal
Tract Amino Acid and Peptide
Transporters in Farm Animals
ChickensIt has been known for some time that the
absorption of dietary amino acids across
the chicken small intestine occurs by
mediated processes, that neutral amino
acids are transported more rapidly than
cationic or anionic amino acids and that
peptide absorption accounts for a substan-
tial proportion of the total amount of amino
acids absorbed (Duke, 1984). In a series of
experiments designed to identify the trans-
port system responsible for methionine and
lysine uptake, the amino acid transport
systems B, bo,+, y+ and L have been
identified in the jejunal BBM of chicks
(Torras-Llort et al., 1996; Soriano-Garcia et
al., 1998). Methionine was transported by
all four systems, whereas lysine uptake is
reported to be by systems bo,+and y+. In a
subsequent trial, it was determined that
feeding a lysine-enriched diet (68 g kg^1
versus 48 g kg^1 ) resulted in the increased
activity of systems bo,+and y+(Torras-Llort
et al., 1998). For system y+, only the
velocity of transport was increased,
indicating that the activity of resident
system y+ transport protein increased
and/or more transport proteins were pre-
sent. In contrast, both the velocity and
specificity of lysine transport by system
bo,+were increased. It is important to note
from these studies that methionine was
recognized by system y+and that system L
was not expressed in jejunal BBM. In other
species, system y+is reported to transport
only cationic amino acids. Accordingly, the
eventual molecular characterization of
the complement of transport proteins
expressed in chicken jejunum should
either confirm this assignment, identify a
new isoform of the CAT transporters
(which may be unique to chickens) or
demonstrate that methionine is actually
transported by other Na+-independent
transport systems that recognize cationic
and neutral amino acids (such as system
y+L/4F2hc). Additionally, once the genes16 J.C. Matthews