Metabolism of the synthetic progestogen norethynodrel by human ketosteroid reductases of the aldo–keto reductase superfamily

Abstract

Human ketosteroid reductases of the aldo-keto reductase (AKR) superfamily, i.e. AKR1C1-4, are implicated in the biotransformation of synthetic steroid hormones. Norethynodrel (NOR, 17α-ethynyl-17β-hydroxy-estra-5(10)-en-3-one), the progestin component of the first marketed oral contraceptive, is known to undergo rapid and extensive metabolism to 3α- and 3β-hydroxy metabolites. The ability of the four human AKR1C enzymes to catalyze the metabolism of NOR has now been characterized. AKR1C1 and AKR1C2 almost exclusively converted NOR to 3β-hydroxy NOR, while AKR1C3 gave 3β-hydroxy NOR as the main product and AKR1C4 predominantly formed 3α-hydroxy NOR. Individual AKR1C enzymes also displayed distinct kinetic properties in the reaction of NOR. In contrast, norethindrone (NET), the Δ4-isomer of NOR and the most commonly used synthetic progestin, was not a substrate for the AKR1C enzymes. NOR is also structurally identical to the hormone replacement therapeutic tibolone (TIB), except TIB has a methyl group at the 7α-position. Product profiles and kinetic parameters for the reduction of NOR catalyzed by each individual AKR1C isoform were identical to those for the reduction of TIB catalyzed by the respective isoform. These data suggest that the presence of the 7α-methyl group has a minimal effect on the stereochemical outcome of the reaction and kinetic behavior of each enzyme. Results indicate a role of AKR1C in the hepatic and peripheral metabolism of NOR to 3α- and 3β-hydroxy NOR and provide insights into the differential pharmacological properties of NOR, NET and TIB.

Promiscuity and diversity in 3-ketosteroid reductases

“…The exception was that AKR1C2 inverted its stereochemistry so that it performed 3β-reduction with 5α-DHT-17β-glucuronide while performing 3α-reduction with 5α-DHT and 5α-DHT-17β-sulfate. This inversion of stereochemistry observed with AKR1C2 is reminiscent of what was observed during the 3-keto reduction of 5β-DHT, tibolone and norethynodrel [21,35]. The catalytic efficiency of the enzymes was significantly impaired by the presence of the 17β-glucuronide but not by the 17β-sulfate group.…”

“…Using human liver autopsy samples the formation of the 3β- and 3α-hydroxytibolone metabolites was confirmed and the formation of 3α-hydroxytibolone was blocked by the AKR1C4 selective inhibitor phenolphthalein indicating that AKR1C4 was responsible for forming this isomer [34]. Metabolism of norethynodrel which differs from tibolone in that it lacks the 7α-methyl group was found to follow the same stereochemical outcome as observed for tibolone, indicating that the presence of the 7α-methyl group does not influence the stereochemical outcome of the reduction of the 3-ketosteroid by each AKR1C isoform [35]. …”

“…Thus it is predicted that these synthetic steroids bind upside down relative to 5α-DHT i.e. , angular methyl groups inverted in the AKR1C2 active site [35,41]. …”

Aldo-Keto Reductase Regulation by the Nrf2 System: Implications for Stress Response, Chemotherapy Drug Resistance, and Carcinogenesis

“…22–25 The human AKR genes are involved in the metabolism of prostaglandins 26–29 and natural and synthetic steroid hormones, 30–34 all of which contain carbonyl functionalities (Table 1). …”

The other side of progestins: effects in the brain

“…The enzymatic pathway for this latter conversion, however, has not been clearly characterized (Bodine et al 2002, Wiegratz et al 2002, Dröge et al 2007, Zacharia et al 2007, Rabe et al 2012. Norethynodrel which is identical to TIB except for lacking a methyl group at carbon 7, follows the same stereochemical outcome as observed for TIB suggesting that the reduction of the 3-ketosteroid by AKR1C isoforms is not influenced by the 7-methyl group (Jin et al 2012).…”