Testosterone oxidation by rat liver microsomes: effects of phenobarbital pretreatment and the detection of seven metabolites by HPLC

The steroid metabolite 16(β)-OH-androstenedione generated by CYP21A2 serves as a substrate for CYP19A1

The 21-hydroxylase (CYP21A2) is a steroidogenic enzyme crucial for the synthesis of mineralo- and glucocorticoids. It is described to convert progesterone as well as 17-OH-progesterone, through a hydroxylation at position C21, into 11-deoxycorticosterone (DOC) and 11-deoxycortisol (RSS), respectively. In this study we unraveled CYP21A2 to have a broader steroid substrate spectrum than assumed. Utilizing a reconstituted in vitro system, consisting of purified human CYP21A2 and human cytochrome P450 reductase (CPR) we demonstrated that CYP21A2 is capable to metabolize DOC, RSS, androstenedione (A4) and testosterone (T). In addition, the conversion of A4 rendered a product whose structure was elucidated through NMR spectroscopy, showing a hydroxylation at position C16-beta. The androgenic properties of this steroid metabolite, 16(β)-OH-androstenedione (16bOHA4), were investigated and compared with A4. Both steroid metabolites were shown to be weak agonists for the human androgen receptor. Moreover, the interaction of 16bOHA4 with the aromatase (CYP19A1) was compared to that of A4, indicating that the C16 hydroxyl group does not influence the binding with CYP19A1. In contrast, the elucidation of the kinetic parameters showed an increased K_m and decreased k_cat value resulting in a 2-fold decreased catalytic efficiency compared to A4. These findings were in accordance with our docking studies, revealing a similar binding conformation and distance to the heme iron of both steroids. Furthermore, the product of 16bOHA4, presumably 16-hydroxy-estrone (16bOHE1), was investigated with regard to its estrogenic activity, which was negligible compared to estradiol and estrone. Finally, 16bOHA4 was found to be present in a patient with 11-hydroxylase deficiency and in a patient with an endocrine tumor. Taken together, this study provides novel information on the steroid hormone biosynthesis and presents a new method to detect further potential relevant novel steroid metabolites.

2β- and 16β-hydroxylase activity of CYP11A1 and direct stimulatory effect of estrogens on pregnenolone formation

The biosynthesis of steroid hormones in vertebrates is initiated by the cytochrome P450 CYP11A1, which performs the side-chain cleavage of cholesterol thereby producing pregnenolone. In this study, we report a direct stimulatory effect of the estrogens estradiol and estrone onto the pregnenolone formation in a reconstituted in vitro system consisting of purified CYP11A1 and its natural redox partners. We demonstrated the formation of new products from 11-deoxycorticosterone (DOC), androstenedione, testosterone and dehydroepiandrosterone (DHEA) during the in vitro reaction catalyzed by CYP11A1. In addition, we also established an Escherichia coli-based whole-cell biocatalytic system consisting of CYP11A1 and its redox partners to obtain sufficient yields of products for NMR-characterization. Our results indicate that CYP11A1, in addition to the previously described 6β-hydroxylase activity, possesses a 2β-hydroxylase activity towards DOC and androstenedione as well as a 16β-hydroxylase activity towards DHEA. We also showed that CYP11A1 is able to perform the 6β-hydroxylation of testosterone, a reaction that has been predominantly attributed to CYP3A4. Our results are the first evidence that sex hormones positively regulate the overall production of steroid hormones suggesting the need to reassess the role of CYP11A1 in steroid hormone biosynthesis and its substrate-dependent mechanistic properties.

A Toxicologist's Guide to the Preclinical Assessment of Hepatic Microsomal Enzyme Induction

The assessment of hepatic microsomal enzyme induction at the completion of preclinical toxicology studies in rodents and large mammals provides a wealth of information to the toxicologist and pharmacokineticist regarding how the drug-metabolizing system of the hepatocyte endoplasmic reticulum responded to high-dose levels of a xenobiotic designed for a specific pharmacological target in any of several target organs. The interpretation of these data can be greatly enhanced by a clear understanding of how this system functions and what the immediate and long-term ramifications are to organs and organ systems. This review focuses on how drugs modify the hepatic cytochrome P450 system, how those modifications are detected, the various consequences of these modifications, and some differences in the induction response among species.

Determination of testosterone metabolites in human hepatocytes. I. Development of an on-line sample preparation liquid chromatography technique and mass spectroscopic detection of 6beta-hydroxytestosterone

A rapid and sensitive RP-HPLC assay for determination of 6beta-hydroxytestosterone in human hepatocytes with corticosterone as the internal standard is described. The procedure employs on-line sample enrichment using a BioTrap 500 MS (20x4 mm I.D.) extraction pre-column and subsequent gradient separation on a Prontosil 60-5 C(18)-H (250x2 mm I.D., 5 micrometer particle size) analytical column in the back-flush mode using a ternary eluent system composed of methanol, tetrahydrofuran and water. Signal monitoring was done by measurement of the responses from liquid chromatography coupled to mass spectroscopy (LC-MS/MS) using an atmospheric pressure chemical ionization (APCI) source conducted in the selected reaction monitoring (SRM) mode. Mean recoveries of 6beta-hydroxytestosterone from an estimate of the biological matrix, i.e., Dulbecco's modified Eagle medium "High Glucose", ranged from 101.8-104.4% for samples containing the target analyte at the 250, 500 and 1000 ng/ml level. The limit of quantitation (LOQ) was 20 ng/ml at an injection volume of 100 microliter determined in the same matrix. Linearity of signal responses versus concentration for all three analytes was accomplished in the range of 100-4000 ng/ml. Mean values of the coefficients of variation (C.V.) for the target analyte obtained for the concentrations 250, 500 and 1000 ng/ml at 5 different days in quintuplicate ranged from 1.5-7.7% (within-day) and 4.8-7.3% (between-day). The corresponding values for the accuracy ranged from 87.7-106.1% for the within-day and from 98.8-102.5% for the between-day measurements. The target analyte was sufficiently stable at both storage and sample preparation conditions because no substantial deviations between analyte concentrations measured before and after subsequently performed freeze and thaw cycles were observed.

Characterization of hepatic cytochrome P450 isozyme composition in the transgenic rat expressing low level human growth hormone

1. The present authors have previously developed a transgenic rat carrying a chimeric gene of the mouse whey acidic protein promoter and the structural portion of human growth hormone (GH) gene. Among this (hGH-TG) rat, a line (low GH rat) missing a male-specific pulsatile GH secretary pattern due to suppression of endogenous GH secretion and having a continuous low GH (hGH and rat GH) level in the peripheral circulation was identified. The latter rat was also characterized as having severe obesity with age. This strain (low Gh rat) was used to correlate the sex-specific secretory pattern of GH with the sex-specific expression of cytochrome P450 (CYP) in rat. 2. Comparisons were made between the low GH rat and the non-transgenic rat as to the expression of liver microsomal CYP isozymes. The following enzyme activities were assessed: testosterone (T) hydroxylation and oxidation; ethoxyresorufin O-dealkylation (EROD); bunitrolol (BTL) 4-hydroxylation and T5 alpha-reduction. Protein expression of CYP1A, CYP2C11, CYP2D, CYP2E1, CYP3A2 and CYP4A1 were also assessed by Western blot analysis. 3. Enzyme activities and protein expression of CYP2C11 (T16 alpha and 2alpha-hydroxylase and 17-oxidase activities) and CYP3A2 (T6beta and 2beta-hydroxylase activities) levels, which are known to be higher in the male than in the female rat, were significantly lower in the adult male low GH rat than in the control male rat. In contrast, CYP2A1 (T7 alpha-hydroxylase) and T5-alpha-reductase activities, which are known to be specifically elevated in the female, were significantly higher in the adult male low GH rat than in the control male rat. Thus, the loss of male-specific secretory pattern of GH results in feminization of the pattern of expression of CYP and T5 alpha-reductase activity in the liver. 4. In contrast to other GH-deficient models so far studied, an increase in CYP4A1 and a decrease in CYP2E1 protein expression were observed in the low GH rat. These trends are consistent with the characteristic phenotype of obesity in the transgenic rat because CYP4A1 and CYP2E1 enhance fatty acid excretion and glyconeogenesis from fatty acids respectively.

Involvement of CYP3A1, 2B1, and 2E1 in C-8 hydroxylation and CYP 1A2 and flavin-containing monooxygenase in N-demethylation of caffeine; identified by using inducer treated rat liver microsomes that are characterized with testosterone metabolic patterns

Caffeine (CA) is oxidized by rat liver microsomal enzymes to theobromine (TB), paraxanthine (PX), and theophylline (TP) by N-demethylation and to trimethylurate (TMU) by C-8 hydroxylation, In order to identify the specific enzymes responsible for productions of these primary CA metabolites, liver microsomes enriched with various isoforms of cytochrome P450 (CYP) and flavin-containing monooxygenase (FMO) are prepared by pretreatment of rats with several inducers. The specific increases in various CYP or FMO activities are identified with the diagnostic testosterone metabolic patterns or the thiobenzamide S-oxidation assay. They are then employed to metabolize the CA. Liver microsomes isolated from rats pretreated with phenobarbital (PB-microsomes) did not have increased FMO activity but had increased activities for hydroxylating the testosterone at 6 beta-(CYP3A1), 16 beta-(CYP2B1), and 2 beta-(CYP3A1) positions. This PB-microsomes had increased activity for TMU production from CA (result of C-8 hydroxylation). Liver microsomes isolated from rats pretreated with acetone (AC-microsomes) had a normal level of FMO activity but had enhanced rates of 6 beta-(CYP3A1) and 2 beta-(CYP3A1) hydroxylations of testosterone. The AC-microsomes again had increased activity for production of TMU. Similarly, the liver microsomes isolated from rats pretreated with dexamethasone (DEX-microsomes) had a normal level of FMO activity but had enhanced rates of forming 6 beta-and 2 beta-hydroxytestosterone (Cyp3A1) as well as androstenedione (CYP3A1). The DEX-microsomes again had increased activity for production of TMU only. Liver microsomes isolated from rats pretreated with 3-methylcholanthrene (MC-microsomes), however, had increased FMO activity and also enhanced rates of forming the 7 alpha-(CYP1A1/2, and 2A1), 6 beta-(CYP3A1), and 2 beta-(CYP3A1) hydroxytestosterone. The MC-microsomes had increased activity for producing all of the four primary metabolites of CA, i.e. the N-demethylation metabolites like TB, PX. and TP, as well as the C-8 hydroxylation metabolite TMU. By the process of association of the obtained results, liver microsomes with increased contents of CYP2B1, 3A1, and 2E1 could catalyze the C-8 hydroxylation at an increased rate producing increased amount of TMU. Increased productions of CA N-demethylation metabolites (TB, PX, and TP) are, however, catalyzed by the increased activities of CYP1A2 and FMO which are associated uniquely with the MC-microsomes.

Simultaneous variation of temperature and gradient steepness for reversed-phase high-performance liquid chromatography method development. II. The use of further changes in conditions

The preceding paper (Part I) suggests that simply optimizing temperature and gradient steepness will often provide an adequate reversed-phase HPLC separation. In some cases, however, this procedure will prove unsuccessful, and then further method-development experiments (involving change in other separation conditions) will be required. One strategy is to change a variable other than temperature or gradient steepness, followed by re-optimization of the latter two variables. The present paper examines the application of this approach with the aid of computer simulation to several samples.

CDNA cloning, heterologous expression, and characterization of rat intestinal CYP2J4

The small intestine is the major portal of entry of ingested xenobiotics. Previous studies from this and other laboratories indicated that at least 6 of the 33 xenobiotic metabolizing forms of P450 currently identified are expressed in rat small intestinal epithelial cells. In the present study, a previously unidentified rat P450, designated CYP2J4, was identified in rat small intestine using PCR. The full-length CYP2J4 cDNA contains an open reading frame for a protein of 501 residues and is 72.5 and 75.8% identical to rabbit CYP2J1 and human CYP2J2, respectively, in deduced amino acid sequences. The coding region of CYP2J4 cDNA has been cloned into a baculoviral expression vector (pVL1392) and expressed in cultured Spodoptera frugiperta (SF9) cells. The heterologously expressed CYP2J4 protein displayed a typical p450 CO-difference spectrum, with maximum absorbance at 449 nm. When purified to near electrophoretic homogeneity, it was active toward arachidonic acid in a reconstituted system with NADPH-P450 reductase and phospholipid, producing both hydroxyeicosatetraenoic and epoxyeicosatrienoic acids. RNA blot analysis with CYP2J4 cDNA as a probe detected two mRNA species, about 2.0 and 2.4 kb, respectively, in RNA preparations from liver, intestine, olfactory mucosa, kidney, heart, and lung. The 2.0-kb mRNA species was abundant in liver, small intestine, and olfactory mucosa, whereas the 2.4-kb mRNA species was predominant only in the olfactory mucosa. Immunoblot analysis of microsomal fractions from different rat tissues with a polyclonal anti-peptide antibody to CYP2J4 detected a protein with the same electrophoretic mobility as purified CYP2J4 most abundantly in small intestine and to a lesser extent in liver and other immunoreactive proteins with slightly higher electrophoretic mobility than purified CYP2J4 in a number of tissues, including small intestine, liver, kidney, lung, and olfactory mucosa. The predominant distribution of CYP2J4, which has activity toward arachidonic acid, is provocative, but its physiological function is as yet unknown.

Study of coumarin metabolism by Chinese hamster lung fibroblasts expressing a human cytochrome P450 using H-nmr

1. Human cytochrome P450 2A6 is expressed in Chinese hamster lung fibroblasts. The isozyme hydroxylates coumarin at the 7-position. 2. Metabolism of coumarin in these lung fibroblasts was monitored using 1H-nmr. Media samples were taken from cells grown in flasks and also in a fluidized bed bioreactor. In each case 7-hydroxycoumarin was readily observable by nmr in crude extracts of the medium. 3. The rate of formation of 7-hydroxycoumarin observed in the acute bioreactor experiments on a per cell basis was found to be much higher than that obtained under chronic monolayer conditions, and correlated with glucose consumption rates.

Separation of testosterone metabolites in microsomal incubates using a new capillary electrophoresis assay

A capillary electrophoresis method has been developed to separate the products of liver microsomal testosterone metabolism. The microsomal mixture undergoes liquid-liquid extraction and pre-concentration, and then electrophoretic analysis takes less than 25 min including capillary conditioning steps. The development of the complex background electrolyte (Tris-HCl and borate buffers, sodium dodecyl sulfate, beta-cyclodextrin and ethanol) necessary for this separation is described. A z-type capillary flow cell is used to obtain adequate detection sensitivity. The proportion in which the metabolites are produced as determined by this method allows assignment of the relative activity of cytochrome P-450 enzymes in the microsomes. The technique is useful for comparison of activity in normal and abnormal hepatic microsomes.

Neonatal exposure to xenobiotics alters adult hepatic protein kinase C alpha levels and testosterone metabolism: differential effects by diethylstilbestrol and phenobarbital

Hepatic enzymes that metabolize endogenous and xenobiotic compounds have been shown to be altered in adult rats that had been exposed to xenobiotics as neonates. Protein kinase C (PKC) is important in intracellular signaling and has been implicated in the regulation of hepatic monooxygenases. Therefore, we examined the effects of neonatal exposure to diethylstilbestrol (DES) and phenobarbital (PB) on hepatic microsomal testosterone metabolism and on the alpha form of protein kinase C (PKC alpha) in adult rats. In adult males, neonatal exposure to DES altered adult testosterone metabolism such that 7 alpha-hydroxylation was increased by 58% but 2 alpha-, 16 alpha-, and 6 beta-hydroxylations and conversion to androstenedione were decreased 31-44%. In contrast, adult males neonatally exposed to PB showed increased (20-27%) testosterone 2 alpha- and 16 alpha-hydroxylations and androstenedione formation, but no effect was observed in the rate of 6 beta- or 7 alpha-hydroxylations. Western blot analyses indicated that cytosolic PKC alpha levels in male rats neonatally exposed to PB were decreased by approximately 63% relative to the vehicle control group but were not significantly altered in the DES males. The PKC alpha levels generally correlated (r = -.75) with 16 alpha-hydroxytestosterone formation in all samples. These results show that neonatal treatment with DES or PB differentially alters hepatic monooxygenase enzyme activities and PKC alpha levels in adult rats.

DNA binding, adduct characterisation and metabolic activation of aflatoxin B1 catalysed by isolated rat liver parenchymal, Kupffer and endothelial cells

In vitro studies with rat liver parenchymal, Kupffer and endothelial cells isolated from male Sprague-Dawley rats were undertaken to investigate cell-specific bioactivation of aflatoxin B1, DNA binding and adduct formation. In the mutagenicity studies, using homogenates of all three separated liver cell populations (co-incubated with NADP+ and glucose-6-phosphate as cofactors for the cytochrome P-450 monooxygenase system) parenchymal, Kupffer and endothelial cells were able to activate aflatoxin B1 to a metabolite mutagenic to Salmonella typhimurium TA 98. In the case of nonparenchymal cells (i.e. Kupffer and endothelial cells) 10-fold higher concentrations of aflatoxin B1 had to be used to obtain a similar number of revertants to that observed with parenchymal cells. Induction studies with Aroclor 1254 led to a striking decrease in the activation of aflatoxin B1 in parenchymal cells, whereas nonparenchymal cells had a slightly enhanced metabolic activation capacity for aflatoxin B1. Metabolism studies with microsomes from induced and noninduced cells using testosterone as substrate revealed comparable results: after induction with Aroclor 1254, parenchymal cells showed a 60% decrease in the formation rate of 2 alpha-hydroxytestosterone, whereas the formation rate of this metabolite remained unchanged in nonparenchymal cells; 2 alpha-hydroxytestosterone is specifically formed by cytochrome P-450 IIC11, which also catalyses the activation of aflatoxin B1 to its epoxide. When freshly isolated, intact cells were incubated with tritiated aflatoxin B1, a dose-dependent aflatoxin B1 binding to DNA in parenchymal and nonparenchymal cells was observed. HPLC analysis of DNA acid hydrolysates of all three cell types showed the major adduct to be 8,9-dihydro-8-(N7-guanyl)-9-hydroxy-aflatoxin B1.

The isoenzyme pattern of cytochrome P450 in rat hepatocytes in primary culture, comparing different enzyme activities in microsomal incubations and in intact monolayers

Changes in the isoenzyme pattern of cytochrome P450 during culture were investigated in primary cultures of rat hepatocytes, measuring specific enzyme activities in microsomes prepared from cultured cells as well as in intact monolayers. Assays of 7-ethoxyresorufin O-deethylation (EROD), 7-pentoxyresorufin O-depentylation (PROD), aniline 4-hydroxylation (AH) and the specific regioselective hydroxylation of testosterone were used as representatives of the activities of seven isoenzymes of cytochrome P450. The isoenzyme profile expressed as catalytic activities was qualitatively and quantitatively similar in microsomes obtained from freshly isolated hepatocytes in comparison with microsomes obtained from whole livers of untreated rats. There was a relatively high activity in EROD, AH and the oxidation of testosterone at the 7 alpha, 2 alpha, 6 beta, 16 alpha and 17 sites (androstenedione). During culture, these microsomal enzyme activities declined at a similar rate to ca. 50% of the activities of microsomes prepared from freshly isolated hepatocytes after 24 hr and to 15% after 96 hr. The overall decline of cytochrome P450-dependent activities during culture was not accompanied with gross changes in catalytic profile. Determining the same drug-metabolizing activities directly in intact hepatocyte monolayers revealed a much higher metabolic rate for all measured P450-dependent activities. The profile of the catalytic activities was essentially the same as measured in microsomes prepared from cultured hepatocytes. The relatively low activity towards the 7 alpha site of testosterone measured in intact hepatocytes, however, remained constant during culture. Determination of enzyme activities directly in intact hepatocytes is a convenient way of studying changes in monooxygenase activities of different P450 isoenzymes in vitro.

Flumecinol, a novel inducer of testosterone 16 alpha-hydroxylation in male rats

1. Flumecinol, a new inducer of the cytochrome P-450 monooxygenases, was studied in rats as a possible effector of liver microsomal testosterone oxidases. The drug enhanced the total content of liver cytochrome P-450 in immature and adult rats. 2. When total testosterone oxidation activity was compared in liver microsomes of treated and untreated rats, no differences in activities were observed in 60-day-old-rats, but a slight decrease was found in 35-day-old treated rats. 3. Several regio- and stereo-specific hydroxylases were modified by flumecinol administration; in 35-day-old rats only 16 alpha-hydroxylation was induced, whereas in 60-day-old rats a slight increase in 2 alpha-hydroxylation was also observed.

Genetically engineered V79 Chinese hamster cell expression of purified cytochrome P-450IIB1 monooxygenase activity

Chinese hamster V79 fibroblasts, frequently used as target cells in short-term tests for mutagenicity, do not possess measurable monooxygenase activity; in particular, enzymatic oxidation of testosterone (T) cannot be demonstrated. If these V79 cells, however, had been transfected with the cDNA-encoding rat liver cytochrome P-450IIB1 under control of the SV40 early promoter, they stably expressed monooxygenase activity. These so-called SD1 cells then oxidatively metabolized T at a rate of 27 pmol/mg protein/min, converting it to 16 alpha- and 16 beta-hydroxy-T as well as 4-androsten-3,17-dione as sole metabolites in a ratio of 1.1:1.0:1.6. The regio- and stereoselective conversion of T by SD1 cells, as well as the quantitative distribution of the metabolites, corresponds well with the results reported for pure cytochrome P-450IIB1 in a reconstituted system.

Nutrition-related alterations in liver microsomal testosterone hydroxylases

The oxidation products of testosterone formed by liver microsomes from normal-fed and protein-energy malnourished male rats have been analysed by HPLC. Microsomes from normal-fed rats oxidized testosterone at a rate of 4.52 nmol/min/mg protein. The major products formed were: 6 beta-, 7 alpha- and 16-alpha-hydroxytestosterone; these three metabolites represented 65% of the total testosterone metabolism. Microsomes from protein-energy malnourished rats oxidized testosterone at a reduced rate of 2.03 nmol/min/mg protein. The major product formed was 7 alpha-hydroxytestosterone, which accounted for 43% of total testosterone oxidation. Microsomes from protein-energy malnourished rats showed a CO-reduced cytochrome P-450 spectra with a maxima at 452 nm, and a 38% decrease in the total content of cytochrome P-450. Some testosterone hydroxylases were drastically affected by protein-energy malnutrition but others, such as 7 alpha-hydroxylase, remained unchanged. The present results suggest that nutritional status can modify the relative amounts of individual cytochrome P-450 isozymes, thus explaining the observed changes in several testosterone hydroxylases. Protein-energy malnutrition seems to be an excellent tool with which to obtain a microsomal fraction containing predominantly P-450 isozymes, which are probably involved in key mono-oxygenations of physiological substrates.

Human liver microsomal steroid metabolism: identification of the major microsomal steroid hormone 6 beta-hydroxylase cytochrome P-450 enzyme

Cytochrome P-450-dependent steroid hormone metabolism was studied in isolated human liver microsomal fractions. 6 beta hydroxylation was shown to be the major route of NADPH-dependent oxidative metabolism (greater than or equal to 75% of total hydroxylated metabolites) with each of three steroid substrates, testosterone, androstenedione, and progesterone. With testosterone, 2 beta and 15 beta hydroxylation also occurred, proceeding at approximately 10% and 3-4% the rate of microsomal 6 beta hydroxylation, respectively, in each of the liver samples examined. Rates for the three steroid 6 beta-hydroxylase activities were highly correlated with each other (r = 0.95-0.97 for 25 individual microsomal preparations), suggesting that a single human liver P-450 enzyme is the principal microsomal 6 beta-hydroxylase catalyst with all three steroid substrates. Steroid 6 beta-hydroxylase rates correlated well with the specific content of human P-450NF (r = 0.69-0.83) and with its associated nifedipine oxidase activity (r = 0.80), but not with the rates for debrisoquine 4-hydroxylase, phenacetin O-deethylase, or S-mephenytoin 4-hydroxylase activities or the specific contents of their respective associated P-450 forms in these same liver microsomes (r less than 0.2). These correlative observations were supported by the selective inhibition of human liver microsomal 6 beta hydroxylation by antibody raised to either human P-450NF or a rat homolog, P-450 PB-2a. Anti-P-450NF also inhibited human microsomal testosterone 2 beta and 15 beta hydroxylation in parallel to the 6 beta-hydroxylation reaction. This antibody also inhibited rat P-450 2a-dependent steroid hormone 6 beta hydroxylation in uninduced adult male rat liver microsomes but not the steroid 2 alpha, 16 alpha, or 7 alpha hydroxylation reactions catalyzed by other rat P-450 forms. Finally, steroid 6 beta hydroxylation catalyzed by either human or rat liver microsomes was selectively inhibited by NADPH-dependent complexation of the macrolide antibiotic triacetyloleandomycin, a reaction that is characteristic of members of the P-450NF gene subfamily (P-450 IIIA subfamily). These observations establish that P-450NF or a closely related enzyme is the major catalyst of steroid hormone 6 beta hydroxylation in human liver microsomes, and furthermore suggest that steroid 6 beta hydroxylation may provide a useful, noninvasive monitor for the monooxygenase activity of this hepatic P-450 form.

High-performance liquid chromatography of steroid hormones

Although a considerable amount of work has been carried out in the last ten years in developing methods for the separation of steroids by HPLC, it is still not widespread for the reasons discussed above. There is however no doubt that further developments in HPLC technology, in increasing sensitivity and/or specificity of detection systems, perhaps with microbore columns, may lead to an increase in the use of this powerful analytical procedure as an additional separation method to improve specificity of assay. Solution of the problem of simple interfacing of HPLC systems with mass spectrometers (discussed in another chapter by Games) should further increase the application of HPLC. HPLC is of particular value in providing a means of separating unstable compounds prior to assay by relatively nonspecific quantitation methods. Most steroids do not fall into this category, but the steroid vitamin D and its metabolites do and HPLC has proved in this area to be invaluable (see chapter by Jones & DeLuca). There are a multiplicity of different HPLC systems for the separation of steroids, varying in column type (and manufacturer), solvent composition and method of elution, temperature of elution, etc., and only a few attempts have been made to rationalise these data. It would therefore seem that a fruitful area of future study would be the investigation of computerised systems for the selection and optimisation of HPLC systems for particular steroid separations.

Regulation of testosterone hydroxylation by rat liver microsomal cytochrome P-450

The pathways of testosterone oxidation catalyzed by purified and membrane-bound forms of rat liver microsomal cytochrome P-450 were examined with an HPLC system capable of resolving 14 potential hydroxylated metabolites of testosterone and androstenedione. Seven pathways of testosterone oxidation, namely the 2 alpha-, 2 beta-, 6 beta-, 15 beta-, 16 alpha-, and 18-hydroxylation of testosterone and 17-oxidation to androstenedione, were sexually differentiated in mature rats (male/female = 7-200 fold) but not in immature rats. Developmental changes in two cytochrome P-450 isozymes largely accounted for this sexual differentiation. The selective expression of cytochrome P-450h in mature male rats largely accounted for the male-specific, postpubertal increase in the rate of testosterone 2 alpha-, 16 alpha, and 17-oxidation, whereas the selective repression of cytochrome P-450p in female rats accounted for the female-specific, postpubertal decline in testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylase activity. A variety of cytochrome P-450p inducers, when administered to mature female rats, markedly increased (up to 130-fold) the rate of testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylation. These four pathways of testosterone hydroxylation were catalyzed by partially purified cytochrome P-450p, and were selectively stimulated when liver microsomes from troleandomycin- or erythromycin estolate-induced rats were treated with potassium ferricyanide, which dissociates the complex between cytochrome P-450p and these macrolide antibiotics. Just as the testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylase activity reflected the levels of cytochrome P-450p in rat liver microsomes, so testosterone 7 alpha-hydroxylase activity reflected the levels of cytochrome P-450a; 16 beta-hydroxylase activity the levels of cytochrome P-450b; and 2 alpha-hydroxylase activity the levels of cytochrome P-450h. It is concluded that the regio- and stereoselective hydroxylation of testosterone provides a functional basis to study simultaneously the regulation of several distinct isozymes of rat liver microsomal cytochrome P-450.

Effects of hypophysectomy and growth hormone treatment on sex-specific forms of cytochrome P-450 in relation to drug and steroid metabolisms in rat liver microsomes

Hypophysectomy decreased the content of a male specific cytochrome P-450, P-450-male, in male rats, while it expressed P-450-male and completely depressed a female specific cytochrome P-450, P-450-female, in female rats. Intermittent injections of human growth hormone (GH), which mimic the secretion in males, restored P-450-male in hypophysectomized (Hypox) male rats and partially restored P-450-female in Hypox female rats. Continuous infusion of GH, which mimics the female secretion pattern, into Hypox male rats caused a further decrease in P-450-male content, and it caused the expression of P-450-female. In Hypox female rats, the same treatment depressed P-450-male and expressed P-450-female to the level of an intact female. These results indicate that the diurnal changes in the pattern of serum growth hormone level regulate the expression of P-450-male and P-450-female. The activities of testosterone 2 alpha- and 16 alpha-hydroxylases were closely correlated to P-450-male content with the correlation coefficients (r) of 0.955 and 0.929, respectively. Benzo(a)pyrene hydroxylation was also correlated to P-450-male content (r = 0.850). Aminopyrine N-demethylation and propoxycoumarin O-depropylation were correlated to less extents (r = 0.692 and r = 0.720), while aniline hydroxylation and O-ethylresorufin O-deethylation were not correlated to P-450-male content. These results indicate that testosterone 2 alpha- and 16 alpha-hydroxylations are closely dependent, but the metabolism of a variety of drugs are dependent to different extents on P-450-male in rat liver microsomes.

Evidence that isoniazid and ethanol induce the same microsomal cytochrome P-450 in rat liver, an isozyme homologous to rabbit liver cytochrome P-450 isozyme 3a

Cytochrome P-450j has been purified to electrophoretic homogeneity from hepatic microsomes of adult male rats administered ethanol and compared to the corresponding enzyme from isoniazid-treated rats. The enzymes isolated from ethanol- and isoniazid-treated rats have identical chromatographic properties, minimum molecular weights, spectral properties, peptide maps, NH2-terminal sequences, immunochemical reactivities, and substrate selectivities. Both preparations of cytochrome P-450j have high catalytic activity in aniline hydroxylation, butanol oxidation, and N-nitrosodimethylamine demethylation with turnover numbers of 17-18, 37-46, and 15 nmol product/min/nmol of P-450, respectively. A single immunoprecipitin band exhibiting complete identity was observed when the two preparations were tested by double diffusion analysis with antibody to isoniazid-inducible cytochrome P-450j. Ethanol- and isoniazid-inducible rat liver cytochrome P-450j preparations have also been compared and contrasted with cytochrome P-450 isozyme 3a, the major ethanol-inducible isozyme from rabbit liver. The rat and rabbit liver enzymes have slightly different minimum molecular weights and somewhat different peptide maps but similar spectral, catalytic, and immunological properties, as well as significant homology in their NH2-terminal sequences. Antibody to either the rat or rabbit isozyme cross-reacts with the heterologous enzyme, showing a strong reaction of partial identity. Antibody against isozyme 3a specifically recognizes cytochrome P-450j in immunoblots of induced rat liver microsomes. Aniline hydroxylation catalyzed by the reconstituted system containing cytochrome P-450j is markedly inhibited (greater than 90%) by antibody to the rabbit protein. Furthermore, greater than 85% of butanol or aniline metabolism catalyzed by hepatic microsomes from ethanol- or isoniazid-treated rats is inhibited by antibody against isozyme 3a. Results of antibody inhibition studies suggest that cytochrome P-450j is induced four- to sixfold by ethanol or isoniazid treatment of rats. All of the evidence presented in this study indicates that the identical cytochrome P-450, P-450j, is induced in rat liver by either isoniazid or ethanol, and that this isozyme is closely related to rabbit cytochrome P-450 isozyme 3a.

Rates of progesterone oxidation by rabbit liver microsomes before and after phenobarbitone treatment

Liver sections, removed from outbred NZW rabbits under anesthesia, were used to determine the rates of progesterone oxidation prior to assessment of the effects of phenobarbitone(PB)-treatment. Progesterone 21-hydroxylase exhibited the same high affinity, low Km kinetics before and after PB-treatment, whereas the Vmax was significantly reduced. PB-treatment did not affect progesterone 21-hydroxylation by the adrenal microsomes of two groups of PB-treated and untreated rabbits. 21-Hydroxylase activity was detected for the first time with spleen microsomes but the majority of spleens examined lacked activity. 6 beta-Hydroxylation by control liver microsomes showed consistently low affinity, high Km-values and was inhibited to a lesser, inconsistent degree than 21-hydroxylation by PB-treatment.

Increased activity of testosterone hydroxylases in liver microsomes of diabetic rats treated with insulin

Liver microsomes from alloxan diabetic rats displayed decreased activity to hydroxylate testosterone only at the 2-alpha and 6-beta positions. Diabetic insulin-treated rats showed higher hydroxylase activities than diabetic and control rats in the formation of all testosterone metabolites analyzed. The sodium dodecylsulfate electrophoretic profile of liver microsomal proteins from each group of rats exhibited distinct increases as well as decreases in the cytochrome P-450 region. Stimulation of testosterone metabolism by insulin may be associated with a higher synthesis of certain cytochrome P-450 isozymes.

Use of a direct high performance liquid chromatography method for multiple testosterone hydroxylations in studies of microsomal monooxygenase activities

A high performance liquid chromatography method is described for separation of the products of testosterone monooxygenation . The method involves direct injection onto a reverse phase octadecylsilane column of the supernatant from incubations containing as little as 25 micrograms microsomal protein. Isocratic elution with formate buffer/acetonitrile and detection at 254 nm permits the separation and simultaneous quantitation of multiple discrete testosterone-derived peaks on the chromatogram. The production of the eight major oxygenated testosterone metabolites in hamster liver microsomes was determined. This profile was altered uniquely after pretreatment with either phenobarbitone, beta-naphthoflavone, rifampicin or pregnenolone 16 alpha-carbonitrile. These changes reflected analogous dissimilar effects of the enzyme inducers on the metabolism of the exogenous cytochrome P-450 substrates aldrin, acetanilide, benzo[alpha]pyrene and ethylmorphine. Therefore, the testosterone assay provides a sensitive and effective method for separating and quantitating testosterone monooxygenation products and may offer a single alternative to the use of multiple exogenous substrates for categorizing and defining the metabolic activity of cytochromes P-450.

Assay of hepatic microsomal testosterone hydroxylases by high-performance liquid chromatography

A high-performance liquid chromatographic (HPLC) method for the assay of the hepatic microsomal polysubstrate monooxygenase catalyzed hydroxylation of testosterone is described. The metabolites are extracted from the incubation mixture with dichloromethane and the extract is washed with dilute alkali and water, dried over anhydrous sodium sulfate, and evaporated to dryness. The residue is dissolved in methanol and an aliquot analyzed. The products are separated by reverse-phase chromatography with a methanol/water/tetrahydrofuran gradient and quantitated at 240 nm by the internal standard technique. The assay does not use radioactively labeled testosterone and can measure hydroxylase activity in microsomal samples containing less than 1.0 mg protein. At least seven products, 2 alpha-, 2 beta-, 6 beta-, 7 alpha-, 16 alpha-, and 16 beta-hydroxytestosterone and androstenedione, are resolved by HPLC. The major products formed by microsomes from untreated adult male rats are 2 alpha- (not 2 beta-) and 16 alpha-hydroxytestosterone and androstenedione which constituted 60% of the total products, followed by 6 beta-, 7 alpha-, and smaller quantities of 2 beta- and 16 beta-hydroxytestosterone. The carrier of the substrate in the incubation mixture was found to affect significantly the metabolite pattern and total activity, and of the several solvents studied methanol yielded the highest total activity. Since the 6 beta-, 7 alpha-, and 16 alpha-hydroxylation of testosterone is catalyzed by distinct forms of cytochrome P-450, this assay which measures seven products may serve as a useful qualitative probe of the cytochrome P-450 population of the monooxygenase.

Differential time course of induction of rat liver microsomal cytochrome P-450 isozymes and epoxide hydrolase by Aroclor 1254

The time course of induction of rat liver microsomal cytochromes P-450a, P-450b + P-450e, P-450c, and P-450d and epoxide hydrolase has been determined in immature male rats administered a single large dose [1500 mumol (500 mg)/kg body wt] of the polychlorinated biphenyl mixture Aroclor 1254. Differential regulation of these xenobiotic-metabolizing enzymes was indicated by their characteristic patterns of induction. The rate of induction of cytochrome P-450a and epoxide hydrolase was relatively slow, and steady-state levels of these enzymes were maintained from approximately Days 9 to 15 after Aroclor 1254 treatment. In contrast, cytochrome P-450c was maximally induced 2 days after Aroclor 1254 treatment and remained at a constant level through Day 15. Steady-state levels of cytochrome P-450d, beginning 1 week after Aroclor 1254 treatment, were preceded by a fairly rapid rate of induction and possibly by a small decline from maximal levels observed around Days 4 to 5. Like those of the other cytochrome P-450 isozymes and epoxide hydrolase, the levels of cytochromes P-450b + P-450e were constant from Day 9 to 15 after Aroclor 1254 treatment. However, an unexpected but reproducible decline (approximately 25%) in total cytochrome P-450 content observed between Days 4 and 9 after Aroclor 1254 treatment principally reflected a dramatic and totally unanticipated decrease (approximately 45%) in the level of cytochromes P-450b + P-450e. This transient decline in the level of cytochromes P-450b + P-450e was not due to an unusual effect of a mixture of polychlorinated biphenyls, since identical results were obtained with two individual congeners, namely 2,3,4,5,4'-penta- and 2,3,4,5,3',4'-hexachlorobiphenyl, that induced the same isozymes as Aroclor 1254. In contrast, when rats were treated with 2,4,5,2',4',5'-hexachlorobiphenyl, which induces cytochromes P-450a and P-450b + P-450e and epoxide hydrolase but not cytochromes P-450c or P-450d, maximal levels of cytochromes P-450b + P-450e were attained on Day 4 and no decrease was observed over the next 11 days. These results suggest that there may be an interaction in the regulation of induction of certain individual cytochrome P-450 isozymes.

Comparative properties of purified cytochrome P-448 from beta-naphthoflavone treated rats and rainbow trout

1. Cytochrome P-448 from beta-naphthoflavone treated rainbow trout (Salmo gairdnerii) liver was purified and compared to purified P-448 from beta-naphthoflavone treated rats (Rattus rattus) and purified P-450 from phenobarbital induced rats. 2. The two P-448 forms had similar spectral properties, substrate specificity, sensitivity to inhibitors and regioselectivity in the metabolism of benzo(a)pyrene and testosterone. 3. Rat and trout P-448 differed in apparent monomeric mol. wt (Mr) by at least 2000 daltons, and did not share identical antigenic determinants. Both rat and trout P-448 were shown to be quite different from rat P-450 using all of the above criteria for distinguishing multiple forms.

Comparisons of highly purified hepatic microsomal cytochromes P-450 from Holtzman and Long-Evans rats

The present study describes the purification and characterization of strain variant forms of a major phenobarbital-inducible microsomal hemoprotein, cytochrome P-450b, from Holtzman and Long-Evans rats. The strain variant hemoproteins cannot be resolved by sodium dodecyl sulfate gel electrophoresis, but can be partially separated in two-dimensional isoelectric focusing SDS gels. If, however, sodium tetradecyl sulfate is incorporated into the one-dimensional gel system, separation of the cytochromes P-450b is achieved. Minor structural differences are detected in the peptides of the cytochromes P-450b following limited proteolysis by Staphylococcus aureus V8 protease, cleavage by cyanogen bromide, or reverse-phase high-pressure liquid chromatography of tryptic peptides. The strain variant cytochromes P-450b are immunochemically and spectrally indistinguishable. The optical spectra of the ferric and ferrous hemoproteins are identical, as are the CO- and ethylisocyanide-reduced difference spectra. Ferrous cytochromes P-450b from both rat strains effectively bind metyrapone with equivalent affinities. In addition, the cytochromes P-450b do not differ in their catalytic activities toward benzphetamine, hexobarbital, benzo [a]pyrene, zoxazolamine, 7-ethoxycoumarin, estradiol-17 beta and testosterone. Cytochrome P-450c, the predominant isozyme inducible in rat liver by 3-methylcholanthrene, was purified from Holtzman and Long-Evans rats. Cytochromes P-450c from both rat strains are indistinguishable based on electrophoretic, immunological, spectral and catalytic properties. Minor structural differences in the cytochromes P-450c were revealed in the reverse-phase high-pressure liquid chromatographic profiles of the tryptic peptides of these hemoproteins, but not in the peptides generated by limited proteolysis or cleavage with cyanogen bromide.