Farm Animal Metabolism and Nutrition

involve substrate recognition. Typically,
the rate of non-mediated amino acid
and peptide absorption is much lower than
that by transporter-mediated processes.
However, at any given time, the relative
contribution that transporter-independent
absorption makes to the total absorption of
amino acids and peptides by a cell
depends on extra- and intracellular sub-
strate concentrations, the magnitude of
signals affecting membrane endocytosis
and structural protein function, substrate
size and charge (relative to protein channel
charge and size) and intracellular energy
levels. A thorough discussion of cell-
ular mechanisms by which transporter-
independent substrate absorption can
occur is available (Gardner, 1994).

Biochemically Characterized Amino

Acid and Peptide Transport Systems

Amino acid transporters

The study of how cells absorb
-amino
acid nitrogen historically has followed the
study of free or peptide-bound amino
acids. The work of Halvor Christensen and
colleagues (Kilberg and Haussinger, 1992)
has resulted in the characterization of a
number of separate amino acid transport
systems in epithelial and non-epithelial
cells (Table 1.1). These transport activities
typically are categorized according to
their required energy sources, substrate
specificities and kinetics of absorption. The
characterization of amino acid transport
processes has resulted in the paradigm that
translocation of free
-amino acids across
cellular membranes occurs by multiple
transport activities, often with overlapping
substrate specificities for amino acids of
the same and/or different class (cationic,
anionic or zwitterionic). For example,
lysine and leucine are each recognized by
at least four biochemically distinct
-
amino acid transport systems, with three of
these transport systems (Bo,+, bo,+and y+L)
recognizing both substrates.
-Amino acid
transport systems display varying degrees
of substrate specificities, ranging from

system IMINO, which is specific for proline,

to system Bo,+, which accepts most dipolar

and cationic amino acids. Free 
-amino acid

transporters, however, do not recognize -

amino acids (e.g. taurine). Recognition of

amino acids by transport proteins is thought

to be dependent on the 
-amino or 
-imino

group (for proline) and a carboxyl group,

with the size, charge and/or configuration of

the side chains acting as important determi-

nants for substrate transport (Christensen,

1984).

It is clear from reported free amino acid

transport activities of mammalian fibro-

blasts, skeletal muscle, hepatocytes, entero-

cytes, placental trophoblasts and pancreatic

acinar cells (Table 1.1) that the expression of

transporter activities differs among types of

cells (e.g. B, Bo,+, N and y+L) and between

the membranes of cells (e.g. asc, B and Bo,+).

It is also apparent that several transport

activities are expressed in most types of

cells (A, ASC, y+, L and XAG). Part of this

heterogeneity in amino acid absorption

capacity appears to be associated with

substrate supply. For example, only the

intestinal lumen-facing brush border mem-

branes of enterocytes and the bile-facing

canalicular membranes of hepatocytes do

not express systems A and ASC and,

instead, rely on systems B and Bo,+to absorb

dipolar amino acids. Additionally, the

expression of system Bo,+confers an added

capacity for cationic amino acid transport,

as compared with other cells and mem-

branes which only express system y+.

Interestingly, enterocytes also express

system IMINO for the absorption of proline

in the apical membrane, which is readily

transported by system A in the basolateral

membrane and in cells other than

enterocytes. System L, a predominant Na+-

independent transporter of large hydropho-

bic dipolar amino acids, also is not

expressed in the apical membrane of entero-

cytes, but is in the basolateral membranes.

Peptide transporters

As with free amino acids, the absorption of

peptide-bound amino acids is a universal

Amino Acid and Peptide Transport Systems 7