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ABSORPTION, DISTRIBUTION, METABOLISM, AND EXCRETION
Departments of Pharmacology and Toxicology (E.M.L., S.P.C.C.) and Chemistry (R.J.B.) and the Cancer Research Laboratories (E.M.L., R.G.D., S.P.C.C.), Queen's University, Kingston, Ontario, Canada
The human multidrug resistance protein 1 (MRP1) is a primary active
transporter of reduced (GSH) and oxidized glutathione, as well as GSH-,
glucuronate-, and sulfate-conjugated organic anions. In addition, the
transport of certain MRP1 substrates is stimulated by the presence of GSH. To
evaluate the structural features of GSH required for interaction with the
protein, we investigated the ability of a series of GSH analogs to enhance GSH
stimulatable transport of [3H]estrone 3-sulfate
(E1SO4). We found that substitution of the
-Glu
residue with Gly,
-Asp, and
-Glu resulted in complete loss of
transport stimulation. In contrast, substitution of Gly with Glu or
-Ala
resulted in only a partial loss of stimulatory activity.
E1SO4 transport activity sur-passed GSH-stimulated
levels in the presence of tripeptides in which Cys was substituted with the
hydrophobic amino acids Leu, Phe, and homo-Phe. Moreover, polar substitutions
of Cys did not enhance transport to the same extent as nonpolar substitutions
of comparable size.
-Glu-Leu-Gly was 1.6-fold more effective than GSH
in stimulating E1SO4 uptake, and kinetic analysis
indicated this was due to an increased Vmax. In addition,
this tripeptide was shown to be a competitive inhibitor of apigenin-stimulated
GSH transport (Ki value of 14 µM), confirming that it
either interacts with the same site on MRP1 as GSH or that the binding of the
two tripeptides is mutually exclusive. These data provide insight into the
architecture of the GSH binding domain of MRP1.
Address correspondence to: Dr. Susan P. C. Cole, Cancer Research Laboratories, Room 328, Botterell Hall, Queen's University, Kingston, ON, Canada, K7L 3N6. E-mail: coles{at}post.queensu.ca
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