ORGANIC ANION TRANSPORTERS - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 59

FIGURE 4.5. Schematic illustrating the transmembrane topology of organic anion trans-
porters. Hydropathy analyses indicate that the OATs comprise 12 TMDs (numbered in the fig-
ure). The large loops between TMDs 1 and 2 (extracellular) and TMDs 6 and 7 (intracellular)
contain several consensus glycosylation (G) and PKC phosphorylation (P) sites, respectively.
(From ref. 13.)

Despite the fact that PKC down-regulates OAT function, the phosphorylation sites of
the second loop are probably not involved directly in this down-regulation. 35 , 38 − 41
OATs are multispecific transporters that bind a vast array of substrates, with binding
being based on general substrate physicochemical properties (charge, hydrophobic-
ity, hydrogen-bonding ability) rather than distinctive molecular characteristics. 42 , 43
However, there are no direct data on the molecular interactions of substrate ions with
the amino acid residues lining the channel of the OATs which might indicate the
exact properties that determine substrate binding and transport. Mutagenesis studies
have been conducted targeting highly conserved amino acid residues believed to be
essential for transport function. These studies have identified conserved aromatic and
cationic residues, which appear to interact, respectively, with hydrophobic and an-
ionic moieties of substrates. 44 − 46 For example, in rat OAT3, five conserved aromatic
residues (located in TMDs 7 and 8) and three conserved basic residues (in TMDs
1, 8, and 11) are required for transport activity, as identified by expression studies
in Xenopus oocytes. 45 Importantly, in the OCTs (cation-transporting homologs of
the OATs), acidic or neutral residues are located in the positions corresponding to
the critical basic residues in the OATs. Therefore, these conserved basic residues are
believed to determine the substrate charge specificity of the OATs. Remarkably, a rat
OAT3 double mutant with the Lys370 and Arg454 residues substituted by one neutral
and one acidic residue (K370A/R454D) has been reported to change its substrate
orientation from anions to cations. 44
Although the exact mechanism for substrate transport is not known, considerable
data indicate that it is based on ion exchange across the membranes of the tubular cell.
Basolateral transport is driven by the concentration gradient of intracellular dicarboxy-
lates, mostly α -ketoglutarate ( α KG), which are exchanged for extracellular anions