Farm Animal Metabolism and Nutrition

tertiary transporter in terms of energy
expenditure (Ganapathy et al., 1994).
Most primary, secondary and/or
tertiary transporters can accumulate sub-
strates against substrate concentration
gradients. In contrast, ‘facilitative’ trans-
porters (transporter-mediated absorption
that is driven by the osmotic gradient
and/or electrical charge of the substrate)
usually are not capable of concentrating
substrates. Instead, facilitative transporters
mediate the ‘downhill’ passage of sub-
strates across membranes. For example,
amino acid transport systems asc, L and
xcdo not couple the transport of sub-
strates with ions, and therefore are not

considered to require the expenditure of

respiration energy in order to function.

Although not usually discussed in these

terms, facilitative transport also depends

on the generation of membrane potential

energy for transporter movement within

the membrane. This is because the amount

of electrochemical energy required to

reactivate the transporter is thought to be

derived from the membrane potential,

which is a function of the steady-state

non-equilibrium ion gradients that are

generated by primary transporters (Na+/K+

ATPase, H+ATPase and Ca2+ATPase) and

the antagonistic functioning of trans-

membrane leak channels (Na+, K+, H+, Ca2+,

Amino Acid and Peptide Transport Systems 5

Table 1.1.
-Amino acid transport systems that have been identified in selected tissues, cells, and apical
(Ap) and basolateral (Bs) membranes.

Skeletal Placental Pancreatic

Transport Coupled

Fibroblastamuscleb Hepatocytesc Enterocytesd scyncytiae acinarf

system ions Substrates Plasma Plasma Bs Canalicular Ap Bs Ap Bs Ap Bs

ANa+ All neutrals, X X X X XXXXX
Pro
ASC Na+ Neutral (pH 7.5), X X X X X X X
Anionic (pH 5.5)
asc Neutrals X X X X
B (NBB, Na+ Most neutrals X
Bo)
Bo,+ Na+g Neutrals and X X Xh Xh
cationics
bo,+ Same as Bo,+ XX
y+LNa+i Neutrals, cationics, X X
but not cystine
y+ Cationics X X X XXXX X
L Large branched X X Xj Xj XXXXX
neutrals
NNa+ Asn, Gln, His Xk XX X
XAG Na+, K+ D-,L-Asp, L-Glu X X Xl XX XX
xc Anionics, cystine X Xm XX
GLY Na+, Cl Gly X
IMINO Na+, Cl Pro, hydroxy-Pro X X X
T Tyr, aromatics X

aKilberg and Haussinger, 1992; bMackenzie et al., 1992; cMailliard et al., 1995; dGanapathy et al., 1994; Mailliard et al., 1995;
eMatthews et al., 1998b; fMailliard et al., 1995.
gMay also be Cldependent (Munck, 1997).
hPresent in rat but not human placentas.
iNeutral amino acids only.
jIn rat hepatocytes, system L has both high-affinity (L1) and low-affinity (L2) components. Adult rats demonstrate primarily L2 activity
in canalicular and basal membranes whereas L1 activity predominates in both membranes in suckling pups.
kMuscle variation of liver system N.
lPrimarily pericentral hepatocytes.
mPrimarily periportal hepatocytes.