HCO 3 and Cl). Therefore, the energy for
all facilitated transport ultimately depends
on cellular respiration. How energy is
coupled from the transmembrane potential
to drive reactivation of the transporter is
not understood.
Amino acids and peptides can be
absorbed without the aid of transport pro-
teins. However, because cell membranes
are relatively impermeable to these charged
hydrophilic molecules, the non-mediated
absorption of amino acids and small
peptides occurs by diffusion through
membrane-spanning channels and para-
cellular pathways, and/or by endocytosis
(membrane envelopment). Diffusion is a
substrate concentration-dependent event,
whereas endocytosis may additionally6 J.C. Matthews
Table 1.2.Affinity constants for mediated peptide transport in animal cells and tissues.
Experimental Co-transport
Animal Tissue model substrate Substrate Kt(mM)
Hamster Jejunum Everted H+ Val–Val 9.
rings Gly–Sar 6.
Leu–Leu 5.
Gly–Gly 5.
Ala–Ala 3.
Rabbit Small BBMVa None Gly–Sar 17.3 ± 1.
intestine H+ Gly–Sar 19.5 ± 2.
Rabbit Jejunum BLMVb H+ Gly–Pro 2.0 ± 0.
Tilapia (fish) Intestine BBMV None Gly–Phe 9.8 ± 3.
Intestine BBMV H+ Gly–Sar 0.56 ± 0.
BLMV H+ Gly–Sar 13.3 ± 3.
Rat Kidney: outer BBMV H+ Gly–Leu 0.101 ± 0.
medulla and cortical H+ Gly–Gln 0.003 ± 0.
Human Colon: Caco-2 cells Apical H+ Bestatin 0.
Basolateral H+ Bestatin 0.
Human Colon: Caco-2 cells Apical H+ Gly–Sar 1.1 ± 0.
Rat Lung: type II BBMV H+ D-Phe–L-Ala 2.
pneumocytes
Rat Liver pneumocytes Lysosome MV H+ Gly–Gln 4.67 ± 0.
Human Fibrosarcoma HT1080 cells H+ Gly–Sar 11.4 ± 3.
Rabbit Small PepT1 expressed H+ Cephalexin 4.
intestine in Xenopus laevis Cyclocillin 0.
oocytes
Rabbit Small PepT1 expressed H+ Gly–Sar 1.
intestine in oocytes H+ Gly–Leu 0.
H+ Gly–Glu 0.
H+ Gly–Lys 2.
2 H+ D-Phe–L-Glu 0.
H+ Gly–Gly 2.
H+ Gly–Gly–Gly 5.
H+ Gly–Gly–Gly–Gly 24.
Human Small PepT1 expressed H+ Gly–Sar 0.
intestine in HeLa cells
Human Kidney PepT2 expressed H+ Gly–Sar 0.
in HeLa cells
Sheep Omasum poly(A)+mRNA H+ Gly–Sar 0.
expressed in oocytes
Sheep Duodenum BBMV H+ Gly–Pro 0.005 ± 0.
aBrush border membrane vesicles; bbasolateral membrane vesicles.