Enzymatic characterization of a novel bovine liver dihydrodiol dehydrogenase--reaction mechanism and bile acid dehydrogenase activity

Mutational analyses of cysteine residues of bovine dihydrodiol dehydrogenase 3

The cloning, bacterial expression and purification of bovine liver cytosolic dihydrodiol dehydrogenase 3 (DD3) cDNA (1330 bp in full length) using the pKK223-3 expression vector has been reported previously. Recombinant DD3 (rDD3) was characterized in terms of its substrate specificity and inhibitor sensitivity [Terada et al., Adv. Exp. Biol. Res. 414 (1997) 543-553]. The nucleotide sequence of DD3 cDNA completely matched with that of bovine liver-type prostaglandin F synthase [Suzuki et al., J. Biol. Chem. 274 (1999) 241-248]. In the present study, we succeeded in high level expression of rDD3 in Escherichia coli BL21 (DE3) using the pET28a expression vector. rDD3 was easily and quickly purified to apparent homogeneity by one-step column chromatography using Ni(2+)-affinity resin. Furthermore, rDD3 showed essentially the same substrate specificity and inhibitor sensitivity to that of purified liver DD3. To analyze the role of cysteines (145, 154, 188, 193 and 206) in the enzymatic activity of DD3, site-directed mutagenesis of DD3 using the polymerase chain reaction method was performed. Mutants (C145S, C154S, C188S, C193S and C206S) were analyzed for substrate specificity, cofactor binding and inactivation by disulfide (dithio-bis(2-nitrobenzoic acid), alkylating reagent (N-ethylmaleimide) and oxidants (naphthoquinone and H(2)O(2)) Results indicated that these five cysteines of rDD3 may not be directly involved in substrate or cofactor binding. Mutant C193S showed strong resistance to SH-reagents unlike wild-type DD3 (WT) or other mutants. Both the WT and the other mutants showed essentially the same sensitivity to SH-reagents. Cofactor (NADP(+)) protected mutants C145S, C188S and C206S from inactivation as well as WT, while NAD(+) was not protective. Our present results indicate that Cys193, which is located close to the NADP(+)-binding site, may be involved in the alteration of enzymatic activity.

Site-directed mutagenesis studies of bovine liver cytosolic dihydrodiol dehydrogenase: the role of Asp-50, Tyr-55, Lys-84, His-117, Cys-145 and Cys-193 in enzymatic activity

A previous report on the cloning, bacterial expression and purification of bovine liver cytosolic dihydrodiol dehydrogenase (DD3) cDNA (1,330 bp in full length) using pKK223-3 expression vector characterize the properties of the recombinant DD3 in the aspects of substrate specificity and inhibitor sensitivity (Terada et al., Adv. Exp. Biol. Res. 414 (1997) 543-53). The nucleotide sequence of this DD3 cDNA completely matches that of bovine liver-type prostaglandin F synthase (PGFS) (Suzuki et al., J. Biol. Chem. 274 (1999) 241-8). In the present study, a large amount of recombinant DD3 (rDD3) was expressed in Escherichia coli BL21 (DE3) with a pET28a expression vector. The recombinant DD3 (rDD3) was easily and quickly purified to an apparent homogeneity with one step column chromatography of Ni(2+)-affinity resin. The rDD3 showed essentially the same substrate specificity and inhibitor sensitivity as purified liver DD3 (DD3). To analyze the role of amino acid residues of DD3 in its enzymatic activity, site-directed mutagenesis of DD3 with PCR method was performed. The results of the analyses of these mutants in the aspects of substrate specificity and cofactor-binding suggested a variety of functions in the enzymatic activity: as an active site Tyr-55 may act as a general acid and Asp-50, Lys-84 and His-117 may play an important role in the control of protonation of Tyr-55 as a general acid in the dehydrogenase activity under higher pH conditions, though these residues may not be involved in reductase activity under lower pH conditions. Though the mutated DD3s (Cys to Ser) did not show significant differences in their substrate specificities, these mutants showed different sensitivities to SH-reagents. Present results indicate that Cys-193 may play an important role in the modulation of enzymatic activity under redox conditions generated with GSH+GSSG among five cysteines in DD3.

Intracellular transport of bile acids

Bile acids originate from the liver and are transported via bile to the intestines where they perform an important role in the absorption of lipids and lipid-soluble nutrients. Most of the bile acids are reclaimed from the terminal ileum and returned to the liver via portal blood for reuse. The transport of bile acids is vectorial in both liver and intestinal cells, originating and terminating at opposite poles. Bile acids enter through the basolateral pole in liver cells, and through the apical pole in intestinal cells. During the past decade, much has been learned about the mechanisms by which bile acids enter and exit liver and intestinal cells. By contrast, the mechanisms by which bile acids are transported across cells remain poorly understood. The current body of evidence suggests that bile acids do not traverse the cell by vesicular transport. Although a carrier-mediated mechanism is a likely alternative, only a handful of intracellular proteins capable of binding bile acids have been described. The significance of these proteins in the intracellular transport of bile acids remains to be tested.

cDNA cloning, expression, and mutagenesis study of liver-type prostaglandin F synthase

Prostaglandin (PG) F synthase catalyzes the reduction of PGD2 to 9alpha,11beta-PGF2 and that of PGH2 to PGF2alpha on the same molecule. PGF synthase has at least two isoforms, the lung-type enzyme (Km value of 120 microM for PGD2 (Watanabe, K., Yoshida, R., Shimizu, T., and Hayaishi, O. (1985) J. Biol. Chem. 260, 7035-7041) and the liver-type one (Km value of 10 microM for PGD2 (Chen, L. -Y., Watanabe, K., and Hayaishi, O. (1992) Arch. Biochem. Biophys. 296, 17-26)). The liver-type enzyme was presently found to consist of a 969-base pair open reading frame coding for a 323-amino acid polypeptide with a Mr of 36,742. Sequence analysis indicated that the bovine liver PGF synthase had 87, 79, 77, and 76% identity with the bovine lung PGF synthase and human liver dihydrodiol dehydrogenase (DD) isozymes DD1, DD2, and DD4, respectively. Moreover, the amino acid sequence of the liver-type PGF synthase was identical with that of bovine liver DD3. The liver-type PGF synthase was expressed in COS-7 cells, and its recombinant enzyme had almost the same properties as the native enzyme. Furthermore, to investigate the nature of catalysis and/or substrate binding of PGF synthase, we constructed and characterized various mutant enzymes as follows: R27E, R91Q, H170C, R223L, K225S, S301R, and N306Y. Although the reductase activities toward PGH2 and phenanthrenequinone (PQ) of almost all mutants were not inactivated, the Km values of R27E, R91Q, H170C, R223L, and N306Y for PGD2 were increased from 15 to 110, 145, 75, 180, and 100 microM, respectively, indicating that Arg27, Arg91, His170, Arg223, and Asn306 are essential to give a low Km value for PGD2 of the liver-type PGF synthase and that these amino acid residues serve in the binding of PGD2. Moreover, the R223L mutant among these seven mutants especially has a profound effect on kcat for PGD2 reduction. The Km values of R223L, K225S, and S301R for PQ were about 2-10-fold lower than the wild-type value, indicating that the amino acid residues at 223, 225 and 301 serve in the binding of PQ to the enzyme. On the other hand, the Km value of H170C for PGH2 was 8-fold lower than that of the wild type, indicating that the amino acid residue at 170 is related to the binding of PGH2 to the enzyme and that Cys170 confer high affinity for PGH2. Additionally, the 5-fold increase in kcat/Km value of the N306Y mutant for PGH2 compared with the wild-type value suggests that the amino acid at 306 plays an important role in catalytic efficiency for PGH2.

Purification and characterization of 17beta-hydroxysteroid dehydrogenase from the filamentous fungus Cochliobolus lunatus

17beta-Hydroxysteroid dehydrogenase (17beta-HSD) from the filamentous fungus Cochliobolus lunatus was purified in three steps, yielding a protein of an apparent molecular mass of 28 kDa. According to the obtained experimental data, the native form of the enzyme could be a dimer (60 kDa) and/or a tetramer (120 kDa). The enzyme was found to catalyse preferentially the reduction of steroid substrates using NADPH as an electron donor. Both androgens and estrogens are substrates for 17beta-HSD. Kinetic studies revealed the equilibrium ordered kinetic mechanism with NADPH as the first ligand to be bound to the enzyme followed by the addition of the substrate androstenedione. The purification and characterization of 17beta-HSD from Cochliobolus lunatus represents a step towards the elucidation of the role of this enzyme in fungal metabolism.