Sheep and cattleIntestinal absorption of amino acids
and peptides
As with chickens and pigs, several charac-
terized amino acid and peptide transport
system activities have been identified in
ruminants. Also, as in monogastrics, the
duodenal, jejunal and ileal regions of the
ruminant small intestine appear to have
different abilities to absorb amino acids,
with essential amino acids being absorbed
preferentially over non-essential amino
acids and absorption capacities unequally
distributed throughout the length of the
intestine. In contrast to pigs and other
monogastrics, the ileal region of sheep and
cattle has been identified as possessing the
greatest potential for free amino acid
absorption (Johns and Bergen, 1973; Phillips
et al., 1979; Guerino and Baumrucker,
1987). These reports also indicate that
the relative contribution of transporter-
mediated amino acid absorption to total
amino acid absorption (versus that by non-
mediated diffusion) will vary, depending
on the concentration of luminal substrates.
For peripheral tissues, which are exposed
to much lower concentrations of amino
acids, the contribution of mediated trans-
port is expected to exceed that of
non-mediated diffusion (Kilberg and
Haussinger, 1992). For peptides, however,
research with cultured cell lines indicates
that a significant amount of non-mediated
flux of peptides may occur across the
plasma membranes of peripheral tissues
(Oehlke et al., 1997).
Although the specific transport
systems in ruminants have not been charac-
terized thoroughly, competitive uptake
experiments performed using ileal mucosal
strips isolated from beef and dairy cattle
determined that methionine and lysine
were absorbed by transport activities
exhibiting system A, ASC, L and/or y+
activities (Guerino and Baumrucker, 1987).
The observations that alanine, glycine,
leucine, lysine, phenylalanine and methio-
nine mutually inhibited the uptake of each
other across ileal BBM vesicles suggest
that the bovine small intestine additionally
expresses transporters capable of both neu-
tral and cationic amino acid uptake (Moe et
al., 1987). However, because transport only
was compared in the presence of Na+,
further delineation between the Na+-depen-
dent Bo,+and Na+-independent bo,+and y+L
transport systems cannot be determined.
Additionally, the uptake of proline by sys-
tem IMINO was indicated. Subsequently,
the total uptake of methionine and lysine
across ileal and jejunal BBM of steers was
characterized as having active, facilitative
and diffusional components (Wilson and
Webb, 1990). At both 1.25 and 7.5 mMcon-
centrations, a greater contribution to total
uptake occurred by diffusion than by Na+-
dependent or Na+-independent transport. It
also was observed that ileal and jejunal
tissue had lower affinities and higher
capacities for methionine than for lysine.
Collectively, though limited, the above
data suggest that the ruminant small
intestine expresses a variety of amino acid
transporter systems, similar to those
reported for monogastrics. However, the
membrane-specific localization has not
been determined adequately. For example,
on which membranes of enterocytes are
systems A, ASC, L and/or y+ activities
expressed? If on the apical, then trans-
porter expression in ruminants is dramatic-
ally different from that of monogastrics. As
the bovine homologues of amino acid
transporters are discovered, membrane-
specific assignments of transporter expres-
sion can be made. To date, only EAAT1,
EAAT2 and GLYT1 amino acid transporter
homologues have been cloned from bovine-
derived cell lines.
The expression of H+-dependent
peptide transport-like systems has been
reported (preliminarily) in the intestinal
epithelia of sheep and cattle. In one report,
the observed Km value of 0.005 mM for
Gly–Pro uptake by sheep duodenal BBM is
consistent with affinity constants typically
reported for the high-affinity, low-capacity
transporter (PepT2) (Backwell et al., 1995).
However, in a second report, velocities
measured for peptide transport in both
cattle and sheep proximal tissue are
consistent with those reported for low-18 J.C. Matthews