The human bile salt export pump: characterization of substrate specificity and identification of inhibitors.
OBJECTIVE
The bile salt export pump (BSEP) is the major bile salt transporter in the liver canalicular membrane. Our aim was to determine the affinity of the human BSEP for bile salts and identify inhibitors.


METHODS
Human BSEP was expressed in insect cells. Adenosine triphosphatase (ATPase) assays were performed, and bile salt transport studies were undertaken.


RESULTS
The BSEP gene, ABCB11, was cloned and a recombinant baculovirus was generated. Infected insect cells expressed a 140-kilodalton protein that was absent in uninfected and in mock-infected cells. An ATPase assay showed BSEP to have a high basal ATPase activity. Transport assays were used to determine the Michaelis constant for taurocholate as 4.25 micromol/L, with a maximum velocity of 200 pmol x min(-1) x mg(-1) protein. Inhibition constant values for other bile salts were 11 micromol/L for glycocholate, 7 micromol/L for glycochenodeoxycholate, and 28 micromol/L for taurochenodeoxycholate. Cyclosporin A, rifampicin, and glibenclamide were proved to be competitive inhibitors of BSEP taurocholate transport, with inhibition constant values of 9.5 micromol/L, 31 micromol/L, and 27.5 micromol/L, respectively. Progesterone and tamoxifen did not inhibit BSEP.


CONCLUSIONS
The human BSEP is a high-affinity bile salt transporter. The relative affinities for the major bile salts differ from those seen in rodents and reflect the different bile salt pools. BSEP is competitively inhibited by therapeutic drugs. This is a potentially significant mechanism for drug-induced cholestasis.
Byrne
Jane A
JA
Institute of Liver Studies, King's College Hospital, Guy's, King's, and St. Thomas' School of Medicine, London, England.
Strautnieks
Sandra S
SS
Mieli-Vergani
Giorgina
G
Higgins
Christopher F
CF
Linton
Kenneth J
KJ
Thompson
Richard J
RJ
D000074020:ATP Binding Cassette Transporter, Subfamily B, Member 11; D018528:ATP-Binding Cassette Transporters / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism; D001647:Bile Acids and Salts / Q000378:metabolism; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000378:metabolism; D003001:Cloning, Molecular; D016572:Cyclosporine / Q000494:pharmacology; D018076:DNA, Complementary / Q000235:genetics; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005905:Glyburide / Q000494:pharmacology; D006801:Humans; D012293:Rifampin / Q000494:pharmacology; D013379:Substrate Specificity
C093491:ABCB11 protein, human; D000074020:ATP Binding Cassette Transporter, Subfamily B, Member 11; D018528:ATP-Binding Cassette Transporters; D001647:Bile Acids and Salts; D002352:Carrier Proteins; D018076:DNA, Complementary; D004791:Enzyme Inhibitors; D016572:Cyclosporine; D000251:Adenosine Triphosphatases; D005905:Glyburide; D012293:Rifampin