ORGANIC CATION TRANSPORTERS - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 13

Extracellular
Intracellular
FIGURE 2.1. Secondary structure predicted for OCT1. OCT1 shares a similar transmembrane
topology to the other OCTs, consisting of 12 transmembrane domains (TMDs), and a large
extracellular loop between TMD1 and TMD2 which contains several putative N-glycosylation
sites. TMD4 and TMD10 have been shown to contribute to substrate recognition by OCT1.
and carboxy termini. The OCTs share a large extracellular loop between TMDs 1 and
2 which contains potential N-glycosylation sites, as well as a large intracellular loop
between TMDs 6 and 7 which includes predicted phosphorylation sites. The poten-
tial importance of N-glycosylation and phosphorylation for OCT activity is discussed
below. Figure 2.1 shows the transmembrane topology of OCT1, which is typical of
the OCTs. The transmembrane domains are believed to be important for substrate
recognition by the OCTs; specifically, recent evidence suggests that the fourth and
tenth transmembrane domains are critically involved in substrate recognition by the
OCTs, 21 , 22 and differences between isoforms in terms of substrate specificity (see
Section 2.3) may be related to differences in these critical regions. However, it is im-
portant to note that given the broad substrate selectivity of the OCTs, the key domains
or residues involved in substrate recognition may be different from one substrate to
another, even within the same protein. For example, in mutational analysis studies of
rat OCT1, mutation of two residues in the fourth TMD (Trp218Tyr and Tyr222Leu)
resulted in increased affinity for both tetraethylammonium (TEA) and 1-methyl-4-
phenylpyridinium (MPP + ), whereas a third mutant (Thr226Ala) had increased affinity
for MPP + but no change in affinity for TEA. 21 This suggests that OCT1, and probably
all of the OCTs, contain multiple overlapping but nonidentical recognition sites for
the various structurally diverse substrates.
Transport of organic cations by the OCTs occurs by facilitated diffusion and is
driven by the inside-negative membrane potential. 14 − 20 Positively charged cations
are taken up into cells according to the electrochemical gradient (see Figure 2.2),
and this process is membrane potential sensitive [i.e., artificially reducing the mem-
brane potential (as through replacement of extracellular Na + with K + , or treatment
with ionophores such as valinomycin) reduces the rate of transport by OCTs]. 2 , 9