Inhibitory effects of flavonoids on the reduction of progesterone to 20alpha-hydroxyprogesterone in rat liver

Bioactivity and structure of biophenols as mediators of chronic diseases

Biophenols and their associated activity have generated intense interest. Current topics of debate are their bioavailability and bioactivity. It is generally assumed that their plasma concentrations are insufficient to produce the health benefits previously attributed to their consumption. However, data on localized in vivo concentrations are not available and many questions remain unanswered. Potential mechanisms by which they may exert significant bioactivity are discussed together with structure activity relationships. Biophenols are highly reactive species and they can react with a range of other compounds. Products of their reaction when functioning as antioxidants are examined.

Several distinct enzymes catalyze 20alpha-hydroxysteroid dehydrogenase activity in mouse liver and kidney

The effects of flavonoids and quinones on NADPH- and NADH-dependent 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD) activities were examined in cytosolic fractions from the liver and kidney of mice. Judging from the data for the inhibition of NADPH- and NADH-dependent 20alpha-HSD activities by flavonoids and quinones, enzyme catalyzing renal NADPH-dependent 20alpha-HSD activity was found to be distinct from enzyme(s) catalyzing hepatic NADPH- and NADH-dependent 20alpha-HSD activities. Sulfobromophthalein (SBP) had little ability to inhibit hepatic NADPH-dependent 20alpha-HSD activity and bromophenol blue (BPB) exhibited a weak activation against the enzyme activity, whereas SBP and BPB were potent and moderate inhibitors, respectively, of hepatic NADH-dependent 20alpha-HSD activity. Thus, enzyme catalyzing hepatic NADPH-dependent 20alpha-HSD activity appeared to be distinct from enzyme catalyzing hepatic NADH-dependent 20alpha-HSD activity. The data for the pH profiles of hepatic NADPH- and NADH-dependent 20alpha-HSD activities also led us to the conclusion. Based on these results, we propose the possibility that several distinct enzymes catalyze NADPH- and NADH-dependent 20alpha-HSD activities in cytosolic fractions from the liver and kidney of mice.

Effects of quinones and flavonoids on the reduction of all-trans retinal to all-trans retinol in pig heart

We have recently purified a tetrameric carbonyl reductase from the cytosolic fraction of pig heart (pig heart carbonyl reductase). Since pig heart carbonyl reductase efficiently reduces all-trans retinal as the endogenous substrate, it probably plays an important role in retinoid metabolism in the heart. The purpose of the present study was to evaluate the inhibitory effects of quinones and flavonoids on the reduction of all-trans retinal to all-trans retinol catalyzed by pig heart carbonyl reductase, using pig heart cytosol. Of quinones tested, 9,10-phenanthrenequinone, a component of diesel exhaust particles, was the most potent inhibitor for the all-trans retinal reduction, and a significant inhibition was also observed for plumbagin and menadione. The order of the inhibitory potencies for flavonoids was kaempferol > quercetin > genistein > myricetin = apigenin = daidzein. However, the inhibitory potencies of flavonoids were much lower than that of 9,10-phenanthrenequinone. 9,10-Phenanthrenequinone competitively inhibited the all-trans retinal reduction, whereas kaempferol exhibited a mixed-type inhibition. It is likely that 9,10-phenanthrenequinone strongly inhibits the reduction of all-trans retinal to all-trans retinol by acting as the substrate inhibitor of pig heart carbonyl reductase present in pig heart cytosol.

Characteristics and inhibition by flavonoids of 20α-hydroxysteroid dehydrogenase activity in mouse tissues

Progesterone was stereoselectively reduced to a metabolite 20alpha-hydroxy-4-pregnen-3-one in the cytosolic fraction from the liver of male mice, indicating that the reduction of progesterone is catalyzed by 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD). The cytosolic 20alpha-HSD activity was observed not only in the liver, but also in the kidney and lung. In liver cytosol, both NADPH and NADH were effective as cofactors for 20alpha-HSD activity, although NADPH was better than NADH for the enzyme activity. On the other hand, 20alpha-HSD activity in kidney cytosol required only NADPH as a cofactor. No significant sex-related difference of 20alpha-HSD activity was observed in liver and kidney cytosols. Flavonoids have been reported to inhibit the biosynthesis and metabolism of steroids. However, little is known about inhibitory effects of flavonoids on 20alpha-HSD activity. Thus, the effects of 16 flavonoids on 20alpha-HSD activity were examined, using liver cytosol of male mice. Among flavonoids tested, fisetin, apigenin, naringenin, luteolin, quercetin and kaempferol exhibited high inhibitory potencies for the 20alpha-HSD activity. We propose the possibility that these flavonoids augment progesterone signaling by inhibiting potently 20alpha-HSD activity in non-reproductive tissues.

Endocrine-disrupting chemicals as modulators of sex steroid synthesis

Endocrine-disrupting chemicals (EDCs) are typically identified as compounds that can interact with oestrogen or androgen receptors and thus act as agonists or antagonists of endogenous hormones. Growing evidence shows that they may also modulate the activity/expression of steroidogenic enzymes. These are expressed not only in the adrenal glands and gonads but also in many tissues that have the ability to convert circulating precursors into active hormones. In this way, EDCs may impact both on sexual differentiation and development and on hormone-dependent cancers. This review summarizes the evidence for EDCs as modulators of steroidogenic enzymes, identifies the structure/activity relationship in terms of inhibiting specific enzyme activity, questions whether experimental observations can equate with natural in vivo exposure or dietary intake of EDCs, and finally looks at the mechanisms through which these chemicals may disrupt normal steroidogenesis. In summarizing the evidence, the question of whether or not the dietary intake of these endocrine disrupters could pose a threat to human sexual development and health will be addressed.