V. Gorboulev et al., Selectivity of the polyspecific cation transporter rOCT1 is changed by mutation of aspartate 475 to glutamate, MOLEC PHARM, 56(6), 1999, pp. 1254-1261
After site-directed mutagenesis, the organic cation transporter rOCT1 was e
xpressed in Xenopus laevis oocytes or human embryonic kidney cells and func
tionally characterized. rOCT1 belongs to a new family of polyspecific trans
porters that includes transporters for organic cations and anions and the N
a+-carnitine cotransporter. When glutamate was substituted for Asp475 (midd
le of the proposed 11th transmembrane alpha-helix), the Vmax values for cho
line, tetraethylammonium (TEA), N-1-methylnicotinamide, and 1-methyl-4-phen
ylpyridinium were reduced by 89 to 98%. The apparent K-m values were also d
ecreased (choline by 15-fold, TEA by 8-fold, N-1-methylnicotinamide by 4-fo
ld) or remained constant (1-methyl-4-phenylpyridinium). After the mutation,
the membrane potential dependence of the K-m value for [H-3] choline uptak
e was abolished. The affinity of n-tetraalkyl ammonium compounds to inhibit
TEA uptake was increased. This affinity and its increase by the D475E muta
tion were increased with the length of the n-alkyl chains. After expression
in X. laevis oocytes, the IC50 ratios of wild-type and D475E mutant were 1
.7 (tetramethylammonium), 4.3 (TEA), 5.0 (tetrapropylammonium), 5.0 (tetrab
utylammonium), and 65 (tetrapentylammonium). Cationic inhibitors with ring
structures were differentially affected: the IC50 value for TEA inhibition
by cyanine 863 remained unchanged, whereas it was increased for quinine. Th
e data suggest that rOCT1 contains a large cation-binding pocket with sever
al interaction domains that may be responsible for high-affinity binding of
structurally different cations and that Asp475 is located close to one of
these interaction domains.