Rat steroid 5 alpha-reductase kinetic characteristics: extreme pH-dependency of the type II isozyme in prostate and epididymis homogenates

Aromatase as a novel target of parabens in human and rat placentas: 3D-quantitative structure-activity relationship and docking analysis

Aromatase (CYP19A1), a pivotal enzyme in the biosynthesis of estradiol from testosterone, is predominantly expressed in reproductive tissues including placentas. This study investigated the effects of paraben acid and nine parabens on the activity of human and rat CYP19A1 using microsomes derived from human and rat placentas and on estradiol secretion in human choriocarcinoma BeWo cells. The results showed that propyl, butyl, hexyl, heptyl, and nonyl parabens significantly inhibited human CYP19A1 activity, with IC50 values of 66.37, 61.08, 55.65, 48.26, and 27.24 μM, respectively. In BeWo cells, these parabens notably diminished estradiol secretion at concentrations of 100 μM. Similarly, rat CYP19A1 was inhibited by these parabens, with IC50 values of 98.07, 70.10, 41.30, 27.93, and 6.33 μM for propyl, butyl, hexyl, heptyl, and nonyl parabens, respectively. Kinetic analysis identified these compounds as mixed inhibitors. Bivariate correlation analysis revealed a negative correlation between the partition coefficient value, molecular weight, the number of carbon atoms in the alcohol moiety, as well as heavy atom number and IC50 values. Three-dimensional quantitative structure-activity relationship analysis highlighted the critical role of hydrophobic regions in determining inhibitory potency. Docking studies suggested that parabens interact with the heme-iron binding site of both human and rat CYP19A1. This study elucidates the inhibitory effects of various parabens on CYP19A1 and their binding mechanisms, thereby providing a deeper understanding of their potential impact on estrogen biosynthesis.

Development of a Liquid Chromatography/Mass Spectrometry-Based Inhibition Assay for the Screening of Steroid 5-α Reductase in Human and Fish Cell Lines

Steroid 5-α reductase (5AR) is responsible for the reduction of steroids to 5-α reduced metabolites, such as the reduction of testosterone to 5-α dihydrotestosterone (DHT). A new adverse outcome pathway (AOP) for 5AR inhibition to reduce female reproduction in fish (AOP 289) is under development to clarify the antiestrogenic effects of 5AR inhibitors in female fish. A sensitive method for the DHT analysis using chemical derivatization and liquid chromatography–tandem mass spectrometry was developed. A cell-based 5AR inhibition assay that utilizes human cell lines, a transient overexpression system, and fish cell lines was developed. The measured IC₅₀ values of two well-known 5AR inhibitors, finasteride and dutasteride, were comparable in the different systems. However, the IC₅₀ of dutasteride in the fish cell lines was lower than that in the human cell lines. Finasteride showed a higher IC₅₀ against the RTG-2 cell line. These results demonstrated that 5ARs inhibition could differ in terms of structural characteristics among species. The assay has high sensitivity and reproducibility and is suitable for the application in 5AR inhibition screening for various endocrine disruption chemicals (EDCs). Future studies will continue to evaluate the quantitative inhibition of 5AR by EDCs to compare the endocrine-disrupting pathway in different species.

Derivatives of Z-bisdehydrodoisynolic acid provide a new description of the binding-activity paradox and selective estrogen receptor modulator activity

Z-Bisdehydrodoisynolic acid [(+/-)-Z-BDDA], an estrogenic carboxylic acid, is highly active in vivo yet binds poorly to estrogen receptors (ERs). Studies of Z-BDDA and its enantiomers demonstrate therapeutic potential as selective ER modulators; however, the activity vs. binding paradox has remained. One possible explanation is that the carboxylic acid group of Z-BDDA may be modified in vivo to an ester or amide. Synthesis of these derivatives showed the relative binding affinity (RBA) of the methyl ester for ERalpha and ERbeta was increased approximately 14- and 20-fold, respectively, relative to the parent compound. Yet, this increased affinity did not result in increased reporter gene expression. In contrast, the amide showed an unexpected approximately 4-fold decrease in RBA to both ERs compared with the parent. The relationship among the RBAs of ester, acid, and amide is consistent with their predicted polarity, suggesting the carboxylic acid, and not the carboxylate of BDDA, binds to ERs. Studies at pH 6.5, 7.4, and 8.0 were consistent with a simple acid-base equilibrium model, with BDDA binding as the undissociated acid and with affinity equal to or exceeding that of estradiol, consistent with high in vivo potency. Furthermore, the alcohol BDD-OH also demonstrated high affinity and increased activity in gene expression assays. In addition to suggesting a resolution to the decades-old binding/activity paradox, these studies may provide a direction for definitive in vivo metabolic and pharmacokinetic studies and provide additional insight into the chemical and metabolic determinants of BBDA's unique tissue selectivity and selective ER modulator activities.

Comparative pharmacophore development for inhibitors of human and rat 5-alpha-reductase

There are a number of diseases where the 5-alpha-reductase (5AR) enzyme is of therapeutic interest as a drug target. Currently the crystal structure for 5-alpha-reductase is unavailable, thus ligand-based pharmacophore techniques are beneficial in the drug development process. We have developed pharmacophores to aid inhibitor design for both human types I (preliminary) and II 5-alpha-reductase isozymes and also the rat type II isozyme. To our knowledge, these are the first published pharmacophores for inhibitors of the human type I and rat type II enzymes. A comparison between isozymes and the previously published human type II isozyme pharmacophore is also presented.

Inhibitory effects of Serenoa repens on the kinetic of pig prostatic microsomal 5alpha-reductase activity

The pathogenesis of benign prostatic hyperplasia is linked to the accumulation of dihydrotestosterone (DHT), the active form of testosterone (T), in prostatic tissue. We have defined characteristics of 5alpha-reductase enzyme which catalyzes the conversion of T into DHT in prostatic microsomes of growing pigs. Peaks for the 5alpha-reductase activity were found at pH 5.5 and 8.0, which indicates the presence of both type 1 and type 2 isozymes. Kinetic parameters of porcine 5alpha-reductase in the presence of Serenoa repens extracts revealed uncompetitive, noncompetitive, and mixed types of inhibitions. Our results show the inhibitory action of S. repens on prostate porcine microsomal 5alpha-reductase activity.

Identification and partial characterization of two steroid 5 alpha-reductase isozymes in the canine prostate

BACKGROUND

The dog has been extensively used as an in vivo model to test the pharmacokinetics and effects on pathological prostatic growth of 5 alpha-reductase inhibitors. However, no information is available on the existence or characteristics of canine 5 alpha-reductase isozymes.

METHODS

The 5 alpha-reduction of testosterone is analyzed in dog prostatic homogenates. Three human-specific inhibitors are tested for their activity against dog 5 alpha-reductase.

RESULTS

Two pH optima of 5 alpha-reductase activity in dog prostatic homogenates are described, comparable to the pH optima of rat and human 5 alpha-reductase isozymes. Kinetic analysis of 5 alpha-reductase enzymatic activity at pH 7.0 revealed isozymes with a low apparent affinity constant (Km = 2.67 nM) and a high apparent affinity constant (Km = 1.23 microM). These apparent affinity constants compare favorably to the human and rat isozymes types II and I, respectively. The human type II inhibitor finasteride selectively inhibited the low Km isozyme, whereas the human type I inhibitor MK386 preferentially inhibited the high Km isozyme. The human type I inhibitor LY306089 was nonspecific for the dog isozymes.

CONCLUSIONS

We postulate that the high and low Km isozymes described here represent the dog type I and type II 5 alpha-reductase isozymes, respectively.

3 Alpha-hydroxysteroid oxidoreductase activities in dihydrotestosterone degradation and back-formation in rat prostate and epididymis

The metabolism of dihydrotestosterone (DHT) and 5 alpha-androstane-3 alpha, 17 beta-diol (3 alpha-Adiol) was assessed in full homogenates of rat prostate and epididymis. The major degradational route of DHT was catalysed by the enzyme(s) 3 alpha-hydroxysteroid oxidoreductase (HSOR). Enzyme kinetic characteristics Vmax, Km and Vmax/Km ratio, were obtained for the NADP(H)- and NAD(H)-dependent interconversion of DHT and 3 alpha-Adiol at pH 7.0 and at saturated co-factor concentration. For both the reduction of DHT and the oxidation of 3 alpha-Adiol, NAD(H) was the preferred co-factor when activities were rated by their Vmax and Vmax/Km ratio. Combining the data with the earlier established Vmax/Km ratios for the 5 alpha-reductase isozyme type I and II activities in rat prostate and epididymis indicated that DHT, at saturated co-factor concentrations, would not be sustained in either tissue considering the reported enzyme characteristics. The reported exclusive bioavailability of the co-factors NADPH and NAD+ in vivo, however, will direct the metabolic pathways in these tissues to sustain the formation of DHT.

Kinetic analysis of rat steroid 5alpha-reductase activity in prostate and epididymis homogenates at neutral pH: evidence for type I activity in epididymis

Immunocytochemical studies and mRNA measurements have shown that the rat epididymis--like the rat prostate--expresses both rat steroid 5alpha-reductase isozymes, i.e. type I and II. So far, enzyme activity measurements in rat epididymis homogenates, however, do not support the presence of type I 5alpha-reductase activity. Incubating homogenates of both tissues with a wide range of substrate concentrations, we were able to detect activity of both isozymes in rat prostate and epididymis tissues at neutral pH. In rat prostate the amount of type I activity, as measured by the Vmax at pH 7.0, exceeds that of type II 5alpha-reductase 50-fold. The efficiency ratio, Vmax/Km, of the type I isozyme accounts for 25% of the total in vivo potential activity. A possible anabolic role for the type I isozyme in rat prostate was thus surmised. In rat epididymis the Vmax of type I and type II 5alpha-reductase at pH 7.0 were similar. Comparison of the efficiency ratio Vmax/Km of either isozyme in the rat epididymis, however, suggested that the type II isozyme would play the major role in the 5alpha-reduction of testosterone at physiological concentrations and at neutral pH. The specific localization of the isozymes should be considered to allow for correct quantification of their in vivo contribution to dihydrotestosterone formation.

Kinetic analysis of steroid 5alpha-reductase activity at neutral pH in benign prostatic hyperplastic tissue: evidence for type I isozyme activity in the human prostate

In human benign prostatic hyperplastic (BPH) tissue homogenates 5alpha-reduction of testosterone was examined at neutral pH. As Lineweaver-Burk and Eadie-Scatchard plots of estimated initial velocities against a wide range of substrate concentrations of 2 nM to 3.2 microM were non-linear, the existence of two 5alpha-reductase isozymes in this tissue was surmised. Indeed, enzyme parameters at pH 7.0 suggested the presence of two isozymes with affinity constants of 1995 and 11.8 nM, characteristic of the well established human steroid 5alpha-reductase isozymes type I and II, respectively. The physiological roles of these isozyme activities remain puzzling. The specific activities, Vmax, of these subtypes indicated an approx. 6-fold higher maximum velocity of type I than of type II 5alpha-reductase in the human hyperplastic prostate at this pH. In contrast, the efficiency ratios, Vmax/Km, demonstrated that the type II isozyme had a nearly 27 times higher potential in vivo activity than the type I isozyme, and is therefore most probably quantitatively responsible for dihydrotestosterone formation at physiological testosterone levels in this tissue at neutral pH. This is the first full paper on type I 5alpha-reductase activity in human BPH tissue.