Tibolone Metabolism in Human Liver Is Catalyzed by 3α/3β-Hydroxysteroid Dehydrogenase Activities of the Four Isoforms of the Aldo-Keto Reductase (AKR)1C Subfamily

Elucidation of a complete kinetic mechanism for a mammalian hydroxysteroid dehydrogenase (HSD) and identification of all enzyme forms on the reaction coordinate: the example of rat liver 3alpha-HSD (AKR1C9)

Hydroxysteroid dehydrogenases (HSDs) are essential for the biosynthesis and mechanism of action of all steroid hormones. We report the complete kinetic mechanism of a mammalian HSD using rat 3alpha-HSD of the aldo-keto reductase superfamily (AKR1C9) with the substrate pairs androstane-3,17-dione and NADPH (reduction) and androsterone and NADP(+) (oxidation). Steady-state, transient state kinetics, and kinetic isotope effects reconciled the ordered bi-bi mechanism, which contained 9 enzyme forms and permitted the estimation of 16 kinetic constants. In both reactions, loose association of the NADP(H) was followed by two conformational changes, which increased cofactor affinity by >86-fold. For androstane-3,17-dione reduction, the release of NADP(+) controlled k(cat), whereas the chemical event also contributed to this term. k(cat) was insensitive to [(2)H]NADPH, whereas (D)k(cat)/K(m) and the (D)k(lim) (ratio of the maximum rates of single turnover) were 1.06 and 2.06, respectively. Under multiple turnover conditions partial burst kinetics were observed. For androsterone oxidation, the rate of NADPH release dominated k(cat), whereas the rates of the chemical event and the release of androstane-3,17-dione were 50-fold greater. Under multiple turnover conditions full burst kinetics were observed. Although the internal equilibrium constant favored oxidation, the overall K(eq) favored reduction. The kinetic Haldane and free energy diagram confirmed that K(eq) was governed by ligand binding terms that favored the reduction reactants. Thus, HSDs in the aldo-keto reductase superfamily thermodynamically favor ketosteroid reduction.

Role of microsomal retinol/sterol dehydrogenase-like short-chain dehydrogenases/reductases in the oxidation and epimerization of 3alpha-hydroxysteroids in human tissues

Allopregnanolone (ALLO) and androsterone (ADT) are naturally occurring 3alpha-hydroxysteroids that act as positive allosteric regulators of gamma-aminobutyric acid type A receptors. In addition, ADT activates nuclear farnesoid X receptor and ALLO activates pregnane X receptor. At least with respect to gamma-aminobutyric acid type A receptors, the biological activity of ALLO and ADT depends on the 3alpha-hydroxyl group and is lost upon its conversion to either 3-ketosteroid or 3beta-hydroxyl epimer. Such strict structure-activity relationships suggest that the oxidation or epimerization of 3alpha-hydroxysteroids may serve as physiologically relevant mechanisms for the control of the local concentrations of bioactive 3alpha-hydroxysteroids. The exact enzymes responsible for the oxidation and epimerization of 3alpha-hydroxysteroids in vivo have not yet been identified, but our previous studies showed that microsomal nicotinamide adenine dinucleotide-dependent short-chain dehydrogenases/reductases (SDRs) with dual retinol/sterol dehydrogenase substrate specificity (RoDH-like group of SDRs) can oxidize and epimerize 3alpha-hydroxysteroids in vitro. Here, we present the first evidence that microsomal nicotinamide adenine dinucleotide-dependent 3alpha-hydroxysteroid dehydrogenase/epimerase activities are widely distributed in human tissues with the highest activity levels found in liver and testis and lower levels in lung, spleen, brain, kidney, and ovary. We demonstrate that RoDH-like SDRs contribute to the oxidation and epimerization of ALLO and ADT in living cells, and show that RoDH enzymes are expressed in tissues that have microsomal 3alpha-hydroxysteroid dehydrogenase/epimerase activities. Together, these results provide further support for the role of RoDH-like SDRs in human metabolism of 3alpha-hydroxysteroids and offer a new insight into the enzymology of ALLO and ADT inactivation.

Aldo-keto reductases and bioactivation/detoxication

Aldo-keto reductases (AKRs) are soluble NAD(P)(H) oxidoreductases that primarily reduce aldehydes and ketones to primary and secondary alcohols, respectively. The ten known human AKR enzymes can turnover a vast range of substrates, including drugs, carcinogens, and reactive aldehydes. They play central roles in the metabolism of these agents, and this can lead to either their bioactivation or detoxication. AKRs are Phase I drug metabolizing enzymes for a variety of carbonyl-containing drugs and are implicated in cancer chemotherapeutic drug resistance. They are involved in tobacco-carcinogenesis because they activate polycyclic aromatic trans-dihydrodiols to yield reactive and redox active o-quinones, but they also catalyze the detoxication of nicotine derived nitrosamino ketones. They also detoxify reactive aldehydes formed from exogenous toxicants, e.g., aflatoxin, endogenous toxicants, and those formed from the breakdown of lipid peroxides. AKRs are stress-regulated genes and play a central role in the cellular response to osmotic, electrophilic, and oxidative stress.

Selective tissue distribution of tibolone metabolites in mature ovariectomized female cynomolgus monkeys after multiple doses of tibolone

Tibolone is a selective tissue estrogenic activity regulator (STEAR). In postmenopausal women, it acts as an estrogen on brain, vagina, and bone, but not on endometrium and breast. Despite ample supporting in vitro data for tissue-selective actions, confirmative tissue levels of tibolone metabolites are not available. Therefore, we analyzed tibolone and metabolites in plasma and tissues from six ovariectomized cynomolgus monkeys that received tibolone (0.5 mg/kg/day by gavage) for 36 days and were necropsied at 1, 1.25, 2.25, 4, 6, and 24 h after the final dose. The plasma and tissue levels of active, nonsulfated (tibolone, 3alpha-hydroxytibolone, 3beta-hydroxytibolone, and Delta(4)-tibolone), monosulfated (3alpha-sulfate,17beta-hydroxytibolone and 3beta-sulfate,17beta-hydroxytibolone), and disulfated (3alpha,17beta-disulfated-tibolone and 3beta,17betaS-disulfated-tibolone) metabolites were measured by validated gas chromatography with mass spectrometry and liquid chromatography with tandem mass spectrometry. Detection limits were 0.1 to 0.5 ng/ml (plasma) and 0.5 to 2 ng/g (tissues). In brain tissues, estrogenic 3alpha-hydroxytibolone was predominant with 3 to 8 times higher levels than in plasma; levels of sulfated metabolites were low. In vaginal tissues, major nonsulfated metabolites were 3alpha-hydroxytibolone and the androgenic/progestagenic Delta(4)-tibolone; disulfated metabolites were predominant. Remarkably high levels of monosulfated metabolites were found in the proximal vagina. In endometrium, myometrium, and mammary glands, levels of 3-hydroxymetabolites were low and those of sulfated metabolites were high (about 98% disulfated). Delta(4)-Tibolone/3-hydroxytibolone ratios were 2 to 3 in endometrium, about equal in breast and proximal vagina, and 0.1 in plasma and brain. It is concluded that tibolone metabolites show a unique tissue-specific distribution pattern explaining the tissue effects in monkeys and the clinical effects in postmenopausal women.

Pharmacokinetic Parameters of Sulfated Tibolone Metabolites in Postmenopausal Women after Single and Multiple Doses of Tibolone

The objective of this study was to determine pharmacokinetic parameters of sulfated tibolone metabolites after single dose and their accumulation after multiple doses of tibolone. Blood samples from postmenopausal women in a single-dose (2.5 mg tibolone), open-label study (n=8) and multiple-dose (placebo, 0.3, 0.625, 1.25, or 2.5 mg/day tibolone for twenty-six cycles of 28 days), randomized, double-blind study (n=15) were analyzed for non-sulfated and sulfated tibolone metabolites by validated gas chromatography-mass spectrometry (GC-MS) and liquid chromatography with tanolam mass spectrometry (LC-MS/MS), respectively. The predominant non-sulfated and sulfated metabolites after a single dose were 3alpha-hydroxy-tibolone and 3alpha,17beta-di-sulfated (di-S)-tibolone. At 3 h, >90% of metabolites were sulfated. Tibolone and Delta(4)-tibolone were detectable for about 6 h. After multiple treatment cycles with different doses, metabolite levels at 10 h were dose-related and levels of di-S metabolites were three- to fivefold higher than after a single dose. Tibolone metabolite levels did not differ between cycles. Inactive di-S tibolone metabolites predominated in blood. No accumulation occurred between cycles 7 and 26.

Metabolism of exogenous sex steroids and effect on brain functions with a focus on tibolone

Around the menopause, changes in ovarian secretion of steroids result in changes in brain function: hot flushes and sweating later followed by changes in mood, libido and cognition. The relationship between sex steroids and brain functions are reviewed, with focus on hormonal treatments, in particular tibolone, on the postmenopausal brain and on associations between tissue levels and brain functions. Data on steroid levels in human brain are limited. Exogenous oestrogens alone or combined with progestagens reduce hot flushes and sweating, and may favourably affect anxiety, depression and mood. Testosterone alone or combined with E(2) improves libido and mood. Tibolone reduces hot flushes and sweating, and improves mood and libido, but does not stimulate endometrium or breast, like oestrogens. Tibolone is an ideal compound for studying steroid levels and metabolism in brain in view of its structural differences from endogenous steroids and its extensive metabolism required to express its endocrine effects. Brain levels of tibolone metabolites were measured in ovariectomized cynomolgus monkeys receiving tibolone for 36 days. Compared to serum, higher levels of the oestrogenic 3alpha/beta-hydroxytibolone and the androgenic/progestagenic Delta(4)-tibolone, and lower levels of sulphated metabolites are found in various brain regions. The high levels of oestrogenic metabolites in the hypothalamus explain hot flush reduction. Combined with the presence of Delta(4)-tibolone, the tibolone-induced increase in free testosterone through SHBG reduction explains androgenic effects of tibolone on mood and libido. The levels of tibolone metabolites in the monkey brain support tibolone's effects on brain functions.