CDNA CLONING AND EXPRESSION OF THE HUMAN HEPATIC BILE ACID-BINDING PROTEIN - A MEMBER OF THE MONOMERIC REDUCTASE GENE FAMILY

In human liver, we previously identified one isoform of dihydrodiol de hydrogenase activity that expresses high affinity bile acid binding (H BAB) with minimal 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) act ivity for bile acids. This protein may assist in the rapid intracellul ar transport of bile acids from the sinusoidal to the canalicular pole of the cell. We now report the cDNA cloning and bacterial expression of this novel, multifunctional protein. A 1252-base pair HBAB cDNA was cloned from a HepG2 lambdaGT11 library using a rat hepatic bile acid binder cDNA probe. Bacterial expressed recombinant HBAB oxidized racem ic trans dihydrodiol benzene (0.455 mumol NADPH/mg/min) with minimal 3 alpha-HSD activity for bile acids (<0.003 mumol NADPH/mg/min). Lithoch olic acid and chenodeoxycholic acid dissociation constants as determin ed by displacement of the fluorescent probe, bis-1-anilino-8 sulfonate , were higher than those previously reported for the native protein (1 muM versus 10 nM). Significant amino acid sequence homology was found with the human chlordecone reductase, bovine prostaglandin F syntheta se, and rat hepatic-3alpha-HSD suggesting, that HBAB is also a member of the recently identified, monomeric oxidoreductase gene family. Futu re studies will define the physiologic significance of this novel, mul tifunctional protein in bile acid transport and xenobiotic metabolism.