Kinetic studies of inhibition of estradiol 2- and 16 alpha-hydroxylases in rat liver microsomes with various cytochrome P-450 inhibitors

Equine cytochrome P450 aromatase exhibits an estrogen 2-hydroxylase activity in vitro

Aromatase (estrogen synthetase) is a steroidogenic enzyme complex which catalyzes the conversion of androgens to estrogens (termed aromatization). This enzyme was purified from adult equine testis to homogeneity by five chromatographic steps. The ability of purified and reconstituted equine aromatase to exhibit an estrogen 2-hydroxylase activity was tested and compared to testosterone aromatization. Enzymatic activities were assessed by tritiated water release from labelled estradiol and testosterone. Kinetic analysis of estradiol 2-hydroxylation showed an apparent K(m) of 23 microM and a V(max) of 18 nmol/min/mg, whereas the values for testosterone aromatization were a K(m) of 15.7 nM and a V(max) of 34.6 pmol/min/mg. A specific antiserum raised against purified testicular equine P450arom and known to inhibit aromatase activity [1] was also found to inhibit the estrogen hydroxylase activity of equine placental microsomes in a dose-dependent manner with an IC50 value of 15 microl serum: 0.5 ml incubate. The estrogen hydroxylase activity was inhibited in a dose-dependent manner by two classes of aromatase inhibitors, i.e. steroidal-- (4-hydroxyandrostenedione and 7alpha-([4-aminophenyl]thio)-androst-4-ene-3, 17-dione)--and non-steroidal--(fadrozole and miconazole). The IC50 values were approximately 300 and 890 nM for 4-hydroxyandrostenedione and 7alpha-([4-aminophenyl]thio)-androst-4-ene-3, 17-dione, and 92 and 285 nM, for fadrozole and miconazole, respectively. Furthermore, 4-hydroxyandrostenedione caused a time-dependent inactivation of estrogen hydroxylase activity. We conclude that equine aromatase is able to use estradiol as a substrate, and converts it to catechol estradiol in vitro, possibly using the active site of aromatization. This is the first demonstration that equine aromatase functions as an estrogen 2-hydroxylase, in addition to transforming androgens into estrogen.

P450 enzymes of estrogen metabolism

Endogenous and exogenous estrogens undergo extensive oxidative metabolism by specific cytochrome P450 enzymes. Certain drugs and xenobiotics have been found to be potent inducers of estrogen hydroxylating enzymes with C-2 hydroxylase induction being greater than that of C-16 hydroxylase. Oxygenated estrogen metabolites have different biological activities, with C-2 metabolites having limited or no activity and C-4 and C-16 metabolites having similar potency to estradiol. Pathophysiological roles for some of the oxygenated estrogen metabolites have been proposed, e.g. 16 alpha-hydroxyestrone and 4-hydroxyestrone. These reactive estrogens are capable of damaging cellular proteins and DNA and may be carcinogenic in specific cells.

The role of mixed function oxidase (MFO) in the metabolism of the spin trapping agent alpha-phenyl-N-tert-butyl-nitrone (PBN) in rats

It has been previously demonstrated that alpha-phenyl-N-tert-butyl-nitrone (PBN), one of the most widely used free radical trapping agents, is rapidly absorbed, evenly distributed among a wide range of tissues, and metabolized to form one metabolite. The objective of this study was to determine if the distribution and metabolism of PBN can be affected by inducers or inhibitors of the microsomal mixed function oxidase (MFO) system. Rats were pretreated with MFO inducers (phenobarbital and 3-methylcholanthrene) or MFO inhibitors (metyrapone and piperonyl butoxide) before 14C-PBN was injected (i.p.) The concentrations of 14C-PBN and its metabolite were measured in plasma, urine, liver, lung and kidney 2 hours after injection. The results indicated that when MFO was induced, the concentration of 14C-PBN metabolite was significantly reduced in all tissues measured. The maximum concentration of PBN parent compound in the tissues where MFO was induced was 50% of that found in saline controls. Manipulation of tissue MFO levels with inducers and inhibitors altered the ratio of 14C-PBN parent compound to the PBN-metabolite. When 14C-PBN was incubated with rat liver microsomes at 37 degrees C in the presence of NADPH, the rate of metabolism was 1,752 dpm of 14C-PBN-metabolite formed/nmole P-450/min. Inactivation of MFO by heat (80 degrees C for 1 min) or deletion of NADPH diminished the formation of PBN metabolite in vitro. It is concluded that the MFO system may be responsible for the metabolism of PBN. Tissue concentrations of PBN can be affected by drugs or toxins which are inducers or inhibitors of MFO.

Antidepressant-like effects of trazodone on a behavioral screen are mediated by trazodone, not the metabolite m-chlorophenylpiperazine

Trazodone is an atypical antidepressant drug (i.e. blocks neither monoamine uptake nor monoamine oxidase) which tests as an antidepressant drug on the differential-reinforcement-of-low-rate 72-s (DRL 72-s) schedule of reinforcement by increasing the reinforcement rate and decreasing the response rate. m-Chlorophenylpiperazine (m-CPP) is a 5-HT1B and 5-HT1C agonist, weak 5-HT2 antagonist, and trazodone metabolite. It has been suggested that formation of m-CPP is responsible for the antidepressant action of trazodone. Administration of m-CPP (1-10 mg/kg i.p.) 60, 30 or 10 min before the behavioral session did not mimic the reinforcement rate-increasing effects of trazodone (10-20 mg/kg i.p.) on rats performing under the DRL 72-s schedule of water reinforcement. Pretreatment with proadifen (50 mg/kg i.p.), an inhibitor of trazodone metabolism, caused a greater than 30-fold leftward shift in the dose-response curve for both the reinforcement rate and the response rate. These results suggest that the parent compound and not the trazodone metabolite m-CPP, mediates the antidepressant-like effects of trazodone on DRL 72-s behavior.

2-Bromo- and 16-bromo-estrogens and related compounds: potential inhibitors for both estradiol 2- and 16 alpha-hydroxylases in rat liver microsomes

Kinetic studies of inhibition of estradiol 2- and 16 alpha-hydroxylase activities in male rat liver microsomes with 2-bromoestrogens, 4-bromo-estrone (4-BrE1), 16 alpha- and 16 beta-bromoestrones and 16 beta-acetylthioestrone (16-AcSE1) were carried out. 2-Bromoestrogens and 4-BrE1 nonspecifically blocked the two enzyme activities in a competitive manner, and 2-bromo-estradiol (2-BrE2) was the most potent inhibitor for the two hydroxylases among the 2- and 4-bromo steroids. Kinetic data, the apparent Ki's for the inhibitors and the apparent Km's for the substrate E2 in the assay, demonstrate that the A-ring bromo steroids are potent inhibitors for the two enzymes (Ki/Km ranging from 0.28 to 0.48 for the 2-hydroxylation and ranging from 0.26 to 0.49 for the 16 alpha-hydroxylation). In contrast, 16-bromoestrones and 16-AcSE1 non-competitively inhibited the 2-hydroxylation (Ki = ca. 70 microM) while the other was competitively blocked by them (Ki/Km ranging from 0.24 to 0.30). These 16-substituted steroids were very potent inhibitors for the 16 alpha-hydroxylase rather than the 2-hydroxylase and preferentially blocked the former.