Purification and characterization of a new form (RLM2) of liver microsomal cytochrome P-450 from untreated rat

A new cytochrome P-450 isozyme (RLM2) has been purified to electrophoretic homogeneity from liver microsomes of the untreated rat. It has an apparent minimum molecular weight on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 49,000. Absolute spectrum of the oxidized form indicates that this isozyme is essentially all in the low spin state. The maximum of the reduced CO complex is at 449 nm. Amino-terminal partial amino acid sequence and amino acid composition are different from those of RLM3 and RLM5, two other native forms of cytochrome P-450 previously reported from this laboratory as well as other forms reported in the literature. RLM2 is capable of oxidizing a variety of drug substrates, like benzphetamine and aminopyrine, and to a lesser extent ethoxycoumarin. With the steroid substrate multiple isomeric products are formed differentially. Progesterone is preferentially hydroxylated at the 15-position (15 beta-hydroxylation (34%) and 15 alpha-hydroxylation (13%) of the total) and at the 6 beta-position (21%). The major metabolite when testosterone was the substrate, 15 alpha-hydroxytestosterone, comprised 43% of the total, while a modest amount of 6 beta-hydroxytestosterone (12%) is formed. Another major metabolite (31%) has yet to be unequivocally identified, but is suggested to be 7 beta-hydroxytestosterone. Examination of the substrate dependence of major and minor isomeric metabolites provides evidence for a single substrate-binding site on RLM2. Regardless of the position hydroxylated, a common Km value was obtained. It is suggested that differences in formation of the isomeric and epimeric products relate to differences in distance from the active oxygen center and the position of attack.     

Further characterization of RLM2 and comparison with a related form of cytochrome P-450, RLM2b

We have extended the characterization of RLM2, a constitutive form of rat liver cytochrome P-450, using immunochemical means to quantitate its presence in microsomes, to follow its development in maturing male and female rats, and to determine its response to prototypical P-450 inducers. In addition, RLM2 is compared to RLM2b, a form of P-450 with similar migration on SDS-PAGE and NH2-terminal amino acid sequence. RLM2b is expressed in both sexes at a level of 0.08 nmol/mg microsomal protein at 2 weeks of age. In female rats, this level is unchanged with maturation. However, in the male, the level declined with maturation to reach 0.02 nmol/mg protein by 12 weeks of age. RLM2 is a male-specific form of cytochrome P-450. Originally absent in the 2-week-old rat, it reached a level of 0.03 nmol/mg protein in the adult male, its appearance and increase coinciding with the onset of puberty. Both phenobarbital and 3-methylcholanthrene induced microsomal levels of RLM2b in the adult male and female rat. RLM2, however, was suppressed in the male rat, 58 and 42%, respectively, by the same treatments. RLM2b and RLM2 each catalyze a unique spectrum of hydroxytestosterone metabolites. RLM2b is highly site specific. In contrast, RLM2 produces several isomeric products in the same region of the testosterone molecule. Substitution of the acetyl group of progesterone for the 17-hydroxy group of testosterone did not alter the site specificity of RLM2b, but did alter it for RLM2, indicating, further, a difference in the active site conformation of the two enzymes. Although RLM2b and RLM2 responded differently to inducers and to a changing physiology during maturation, and were functionally quite distinct, the proteins showed a high degree of immunologic relatedness which is suggestive of significant structural similarities. Structural differences do exist, however, as alpha-chymotryptic digestion formed a number of peptide fragments that differed between the two proteins.     

Responses to insulin by two forms of rat hepatic microsomal cytochrome P-450 that undergo major (RLM6) and minor (RLM5b) elevations in diabetes

Total cytochrome P-450 levels rise in diabetic rats. Two specific forms of cytochrome P-450 that are elevated have been isolated from liver microsomes of streptozotocin-induced idabetic male rats. One enzyme, termed RLM6, metabolizes aniline and acetol, but not testosterone, in a reconstituted system with NADPH-cytochrome P-450 reductase. RLM6 is isolated as a high spin cytochrome with a minimum molecular weight of 53,500. It has a unique amino-terminal amino acid sequence lacking methionine at the amino-terminal position. Polyclonal antibodies to RLM6 recognized most other forms of cytochrome P-450 in Western blots, but could be made monospecific by adsorption to cross-reacting proteins coupled to Sepharose 4B. Using the monospecific antibodies, RLM6 was estimated to be present in microsomes of untreated male rats at 0.04 nmol/mg protein (5% of total P-450). In chronically diabetic rats this level rose to 0.35 nmol/mg protein and 24% of the P-450 content. Immunoreactive protein of molecular weight identical to RLM6 was elevated in microsomes of non-diabetic rats treated with ethanol, acetone, or isoniazid as well as in rats starved for 48 h. Insulin treatment of diabetic rats for 1 week lowered the immunologically detectable levels of RLM6 to levels found in the untreated rat. The other form of cytochrome P-450, RLM5b, does not metabolize aniline and only poorly metabolizes acetol and testosterone. This 52.5-kDa protein is isolated as a predominantly (60%) high spin enzyme. It has a unique NH2-terminal amino acid sequence with methionine as the terminal residue, and is present in untreated male rat liver microsomes at 0.16 nmol/mg protein. It is elevated in diabetes, like RLM6, but treatment with insulin for 1 week does not completely restore the microsomal content to that of the non-diabetic rat.     

Adult male-specific and neonatally programmed rat hepatic P-450 forms RLM2 and 2a are not dependent on pulsatile plasma growth hormone for expression

Rat hepatic cytochrome P-450 form RLM2 is a testosterone 15 alpha-hydroxylase reported to be male-specific on the basis of purification studies (Jansson, I., Mole, J., and Schenkman, J. B. (1985) J. Biol. Chem. 260, 7084-7093). The sex dependence, developmental regulation, xenobiotic induction, and hormonal control of P-450 RLM2 expression were studied using P-450 form-specific immunochemical and catalytic assays. Polyclonal antibodies raised to rat hepatic P-450 3 (P-450 gene IIA1) were found to cross-react strongly with P-450 RLM2, but not with 10 other rat P-450 forms, suggesting that P-450 3 and P-450 RLM2 are highly conserved in primary structure. Western blotting of liver microsomes under conditions where P-450s 3 and RLM2 are resolved electrophoretically revealed that P-450 RLM2 is markedly induced at puberty in male rats, with no protein detected (less than or equal to 5% of adult male levels) in adult females or immature animals of either sex. A similar developmental dependence was observed for hepatic microsomal testosterone 15 alpha-hydroxylase activity, which was found to be catalyzed primarily by P-450 RLM2. P-450 RLM2 was resistant to induction by several xenobiotics and in the case of phenobarbital and beta-naphthoflavone, was suppressed by 50-60%. Studies on the steroid hormonal regulation of P-450 RLM2 revealed that its adult male-specific expression is imprinted (programmed) in response to neonatal testosterone exposure. Ovariectomy studies demonstrated that suppression by estrogen does not contribute significantly to the absence of P-450 RLM2 in adult female rats. Although the male-specific developmental induction of P-450 RLM2 in response to neonatal testosterone is strikingly similar to that of P-450 2c (testosterone 2 alpha/16 alpha-hydroxylase; gene IIC11), P-450 RLM2 expression is not dependent on the pulsatile pituitary growth hormone secretion required for P-450 2c synthesis. Rather, hypophysectomy of adult male rats increased P-450 RLM2 and its associated testosterone 15 alpha-hydroxylase activity by 50-100%.(ABSTRACT TRUNCATED AT 400 WORDS)     

Decrease in the levels of a constitutive cytochrome P-450 (RLM5) in hepatic microsomes of diabetic rats

Cytochrome P-450 dependent hydroxylation of testosterone has been measured in hepatic microsomes of control, diabetic and insulin-treated diabetic rats. The observed decrease in testosterone 16 alpha-hydroxylase activity in diabetes, an activity previously shown to be largely due to RLM5, was accompanied by a dramatic decrease in immunodetectable RLM5. Diabetic rats which received insulin had elevated testosterone 16 alpha-hydroxylase activity relative to the diabetic animals, which was accompanied by a corresponding increase in the levels of RLM5. These results provide evidence that specific constitutive cytochrome P-450 enzymes are altered in the diabetic state and that these changes are not permanent since they can be overcome, at least partially, by insulin replacement therapy.     

Lack of regio- and stereospecificity in oxidation of (+) camphor by Streptomyces griseus enriched in cytochrome P-450soy

Oxidation of (+) camphor by cytochrome P-450soy-enriched intact cells of Streptomyces griseus resulted in the formation of one major and several minor metabolites. The minor metabolites were identified as 3-endo-hydroxycamphor (2%), 5-endo-hydroxycamphor (7%), 5-exo-hydroxycamphor (9%), 2,5-diketobornane (2%), and camphorquinone (3%). The major metabolite was isolated and conclusively identified as 6-endo-hydroxycamphor (60%). When supplemented with NADPH, spinach ferredoxin:NADP oxidoreductase and spinach ferredoxin, homogeneous preparations of cytochrome P-450soy oxidized camphor to a mixture of 3-endo-, 5-endo-, 5-exo-and 6-endo-hydroxycamphor. The data presented indicates that cytochrome P-450soy resembles its mammalian counterparts in its lack of regio- and stereospecificity in camphor oxidation.     

Participation of two structurally related enzymes in rat hepatic microsomal androstenedione 7 alpha-hydroxylation.

Rat hepatic cytochrome P-450 form 3 (testosterone 7 alpha-hydroxylase; P-450 gene IIA1) and P-450 form RLM2 (testosterone 15 alpha-hydroxylase; P-450 gene IIA2) are 88% identical in primary structure, yet they hydroxylate testosterone with distinct and apparently unrelated regioselectivities. In this study, androstenedione and progesterone were used to assess the regioselectivity and stereospecificity of these two P-450 enzymes towards other steroid substrates. Although P-450 RLM2 exhibited low 7 alpha-hydroxylase activity with testosterone or progesterone as substrate (turnover number less than or equal to 1-2 nmol of metabolite/min per nmol of P-450), it did catalyse androstenedione 7 alpha-hydroxylation at a high rate (21 min-1) which exceeded that of P-450 3 (7 min-1). However, whereas P-450 3 exhibited a high specificity for hydroxylation of these steroids at the 7 alpha position (95-97% of total activity), P-450 RLM2 actively metabolized these compounds at four or more major sites including the nearby C-15 position, which dominated in the case of testosterone and progesterone. The observation that androstenedione is actively 7 alpha-hydroxylated by purified P-450 RLM2 suggested that this P-450 enzyme might make significant contributions to microsomal androstenedione 7 alpha-hydroxylation, an activity that was previously reported to be associated with immunoreactive P-450 3. Antibody inhibition experiments were therefore carried out in liver microsomes using polyclonal anti-(P-450 3) antibodies which cross-react with P-450 RLM2, and using a monoclonal antibody that is reactive with and inhibitory towards P-450 3 but not P-450 RLM2. P-450 3 was thus shown to catalyse only around 35% of the total androstenedione 7 alpha-hydroxylase activity in uninduced adult male rat liver microsomes, with the balance attributed to P-450 RLM2. The P-450-3-dependent 7 alpha-hydroxylase activity was increased to approximately 65% of the total in phenobarbital-induced adult male microsomes, and to greater than 90% of the total in untreated adult female rat liver microsomes. These observations are consistent with the inducibility of P-450 3 by phenobarbital and with the absence of P-450 RLM2 from adult female rat liver respectively. These findings establish that P-450 RLM2 and P-450 3 can both contribute significantly to microsomal androstenedione 7 alpha-hydroxylation, thus demonstrating that the 7 alpha-hydroxylation of this androgen does not serve as a specific catalytic monitor for microsomal P-450 3.     

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.     

Regioselective progesterone hydroxylation catalyzed by eleven rat hepatic cytochrome P-450 isozymes

Quantitative high-pressure liquid chromatographic assays were developed that separate progesterone and 17 authentic monohydroxylated derivatives. The assays were utilized to investigate the hydroxylation of progesterone by 11 purified rat hepatic cytochrome P-450 isozymes and 8 different rat hepatic microsomal preparations. In a reconstituted system, progesterone was most efficiently metabolized by cytochrome P-450h followed by P-450g and P-450b. Seven different monohydroxylated progesterone metabolites were identified. 16 alpha-Hydroxyprogesterone, formed by 8 of the 11 isozymes, was the only detectable metabolite formed by cytochromes P-450b and P-450e. 2 alpha-Hydroxyprogesterone was formed almost exclusively by cytochrome P-450h, and 6 alpha-hydroxyprogesterone and 7 alpha-hydroxyprogesterone were only formed by P-450a. 6 beta-hydroxylation of progesterone was catalyzed by four isozymes with cytochrome P-450g being the most efficient, and 15 alpha-hydroxyprogesterone was formed as a minor metabolite by cytochromes P-450g, P-450h, and P-450i. None of the isozymes catalyzed 17 alpha-hydroxylation of progesterone, and only cytochrome P-450k had detectable 21-hydroxylase activity. 16 alpha-Hydroxylation catalyzed by cytochrome P-450b was inhibited in the presence of dilauroylphosphatidylcholine (1.6-80 microM), while this phospholipid either stimulated (up to 3-fold) or had no effect on the metabolism of progesterone by the other purified isozymes. Results of microsomal metabolism in conjunction with antibody inhibition experiments indicated that cytochromes P-450a and P-450h were the sole 7 alpha- and 2 alpha-hydroxylases, respectively, and that P-450k or an immunochemically related isozyme contributed greater than 80% of the 21-hydroxylase activity observed in microsomes from phenobarbital-induced rats.     

Pronounced and differential effects of ionic strength and pH on testosterone oxidation by membrane-bound and purified forms of rat liver microsomal cytochrome P-450

The aim of this study was to determine the effects of ionic strength and pH on the different pathways of testosterone oxidation catalyzed by rat liver microsomes. The catalytic activity of cytochromes P-450a (IIA1), P-450b (IIB1), P-450h (IIC11) and P-450p (IIIA1) was measured in liver microsomes from mature male rats and phenobarbital-treated rats as testosterone 7 alpha-, 16 beta-, 2 alpha- and 6 beta-hydroxylase activity, respectively. An increase in the concentration of potassium phosphate (from 25 to 250 mM) caused a marked decrease in the catalytic activity of cytochromes P-450a (to 8%), P-450b (to 22%) and P-450h (to 23%), but caused a pronounced increase in the catalytic activity of cytochrome P-450p (up to 4.2-fold). These effects were attributed to changes in ionic strength, because similar but less pronounced effects were observed with Tris-HCl (which has approximately 1/3 the ionic strength of phosphate buffer at pH 7.4). Testosterone oxidation by microsomal cytochromes P-450a, P-450b, P-450h and P-450p was also differentially affected by pH (over the range 6.8-8.0). The pH optima ranged from 7.1 (for P-450a and P-450h) to 8.0 (for P-450p), with an intermediate value of 7.4 for cytochrome P-450b. Increasing the pH from 6.8 to 8.0 unexpectedly altered the relative amounts of the 3 major metabolites produced by cytochrome P-450h. The decline in testosterone oxidation by cytochromes P-450a, P-450b and P-450h that accompanied an increase in ionic strength or pH could be duplicated in reconstitution systems containing purified P-450a, P-450b or P-450h, equimolar amounts of NADPH-cytochrome P-450 reductase and optimal amounts of dilauroylphosphatidylcholine. This result indicated that the decline in testosterone oxidation by cytochromes P-450a, P-450b and P-450h was a direct effect of ionic strength and pH on these enzymes, rather than a secondary effect related to the increase in testosterone oxidation by cytochrome P-450p. Similar studies with purified cytochrome P-450p were complicated by the atypical conditions needed to reconstitute this enzyme. However, studies on the conversion of digitoxin to digitoxigenin bisdigitoxoside by liver microsomes, which is catalyzed specifically by cytochrome P-450p, provided indirect evidence that the increase in catalytic activity of cytochrome P-450p was also a direct effect of ionic strength and pH on this enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inhibition of steroid 5 alpha-reductase and its effects on testosterone hydroxylation by rat liver microsomal cytochrome P-450

It has been shown previously that liver microsomal steroid 5 alpha-reductase activity increases with age in female but not male rats, which coincides with a female-specific, age-dependent decline in the cytochrome P-450-dependent oxidation of testosterone to 1 beta-, 2 alpha-, 2 beta-, 6 alpha-, 6 beta-, 7 alpha-, 15 beta-, 16 alpha-, 16 beta-, and 18-hydroxytestosterone and androstenedione. To determine whether the increase in steroid 5 alpha-reductase activity is responsible for the decrease in testosterone oxidation, we have examined the effects of the steroid 5 alpha-reductase inhibitor, 4-MA (17 beta-N,N-diethylcarbamoyl-4-methyl-4-aza-5 alpha-androstan-3-one), on the pathways of testosterone oxidation catalyzed by rat liver microsomes. We have also determined which hydroxytestosterone metabolites are substrates for steroid 5 alpha-reductase. At concentrations of 0.1 to 10 microM, 4-MA completely inhibited steroid 5 alpha-reductase activity without inhibiting the pathways of testosterone oxidation catalyzed by liver microsomes from rats of different age and sex, and from rats induced with phenobarbital or pregnenolone-16 alpha-carbonitrile. 4-MA (10 microM) had little or no effect on the oxidation of testosterone catalyzed by liver microsomes from mature male rats (which have low steroid 5 alpha-reductase activity). In contrast, the hydroxylated testosterone metabolites formed by liver microsomes from mature female rats (which have high steroid 5 alpha-reductase activity) accumulated to a much greater extent in the presence of 4-MA. Evidence is presented that 4-MA increases the accumulation of hydroxytestosterones by two mechanisms. First, 4-MA inhibited the 5 alpha-reduction of those metabolites (such as 6 beta-hydroxytestosterone) that were found to be excellent substrates for steroid 5 alpha-reductase. In the absence of 4-MA, these metabolites eventually disappeared from incubations containing liver microsomes from mature female rats. Second, 4-MA inhibited the formation of 5 alpha-dihydrotestosterone, which otherwise competed with testosterone for oxidation by cytochrome P-450. This second mechanism explains why 4-MA increased the accumulation of metabolites (such as 7 alpha-hydroxytestosterone) that were found to be poor substrates for steroid 5 alpha-reductase. Despite its marked effect on the accumulation of hydroxylated testosterone metabolites, 4-MA had no effect on their initial rate of formation by liver microsomes from either male or female rats.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of growth hormone and ectopic transplantation of pituitary gland on sex-specific forms of cytochrome P-450 and testosterone and drug oxidations in rat liver

The effects of growth hormone and ectopic transplantation of pituitary gland on the amounts of sex-specific cytochrome P-450, P-450-male and P-450-female, and the activities of testosterone and drug hydroxylases in male rat liver microsomes were studied. Hypophysectomy decreased the content of P-450-male, without changing the total cytochrome P-450 level. The continuous infusion of growth hormone into hypophysectomized rats and the transplantation of pituitary gland under the renal capsule caused a further decrease in P-450-male content and an expression of P-450-female. In contrast, the intermittent injection of growth hormone into hypophysectomized rats increased P-450-male content to the level seen in intact male rats. The activities of testosterone 2 alpha- and 16 alpha-, but not 6 beta-, 7 alpha-, or 15 alpha-hydroxylase, were changed in association with the level of P-450-male by these treatments. Anti-P-450-male immunoglobulin G inhibited testosterone 2 alpha- and 16 alpha-hydroxylations, but not 6 beta-, 7 alpha- or 15 alpha-hydroxylation. These results indicate that growth hormone regulates the expression of P-450-male responsible for testosterone 2 alpha- and 16 alpha-hydroxylations. The metabolism of 7-propoxycoumarin, benzo(a)pyrene and aminopyrine also changed with the content of P-450-male, although the correlation was less than that observed with testosterone 2 alpha- and 16 alpha-hydroxylation.     

Immunochemical evidence for the catalysis of vitamin D3 25-hydroxylation and testosterone 16 alpha-hydroxylation by homologous forms of cytochrome P-450 in rat liver microsomes

Polyclonal antibody elicited in a rabbit against purified cytochrome P-450cc25, which catalyzes 25-hydroxylation of vitamin D3, inhibited not only 25-hydroxylation of cholecalciferol and 1 alpha-hydroxycholecalciferol, but also 16 alpha- and 2 alpha-hydroxylation of testosterone catalyzed by the purified P-450cc25 preparation. Antibody inhibition experiments with microsomes revealed that most 16 alpha- and 2 alpha-hydroxylation of testosterone and most 25-hydroxylation of cholecalciferol by male rat liver microsomes were catalyzed by P-450cc25. In order to examine the identity of cholecalciferol 25-hydroxylase and testosterone 16 alpha-hydroxylase, monoclonal antibodies recognizing three different epitopes of P-450cc25 were prepared from hybridoma clones produced by fusion of mouse myeloma cells (P3X63Ag8U1) with the spleen cells of immunized BALB/c mouse. All of these monoclonal antibodies inhibited both 25-hydroxylation of 1 alpha-hydroxycholecalciferol and 16 alpha-hydroxylation of testosterone by purified P-450cc25. These observations suggested that immunochemically indistinguishable form(s) of cytochrome P-450 catalyzed both reactions.     

25-hydroxylation of C27-steroids and vitamin D3 by a constitutive cytochrome P-450 from rat liver microsomes

A constitutive cytochrome P-450 catalyzing 25-hydroxylation of C27-steroids and vitamin D3 was purified from rat liver microsomes. The enzyme fraction contained 16 nmol of cytochrome P-450/mg of protein and showed only one protein band with a minimum molecular weight of 51,000 upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified cytochrome P-450 catalyzed 25-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha-diol, 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol, and 1 alpha-hydroxyvitamin D3 up to 50 times more efficiently, and 25-hydroxylation of vitamin D3 about 150 times more efficiently than the microsomes. The cytochrome P-450 showed no detectable 25-hydroxylase activity towards vitamin D2 and was inactive in cholesterol 7 alpha-hydroxylation as well as in 12 alpha- and 26-hydroxylations of C27-steroids. It catalyzed hydroxylations of testosterone and demethylation of ethylmorphine at the same rates as, or lower rates than, microsomes. The 25-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol and vitamin D3 with the purified cytochrome P-450 was not stimulated by addition of phospholipid or cytochrome b5 to the reconstituted system. Emulgen inhibited 25-hydroxylase activity towards both substrates. The possibility that 25-hydroxylation of C27-steroids and vitamin D3 is catalyzed by the same species of cytochrome P-450 is discussed.     

Hydroxylation of prostaglandin A1 by the microsomes of rabbit intestinal mucosa

Microsomes from rabbit small intestine mucosa were found to catalyze the hydroxylation of PGA1 in the presence of NADPH. The major product was identified as 20-hydroxy PGA1 by using high performance liquid chromatography and gas chromatography-mass spectrometry, and the minor product was assumed to be 19-hydroxy PGA1. The ratio of the former product to the latter was about 24.1. The specific PGA1 omega-hydroxylase activity of small intestine microsomes was comparable to that of liver microsomes, and was significantly higher than those of microsomes from other tissues such as kidney cortex and lung. Microsomes from rabbit colon mucosa also catalyzed the hydroxylation of PGA1 in the presence of NADPH, with the ratio of omega- to (omega-1)-hydroxy PGA1 formed being 33.0. The PGA1 hydroxylase activities of the microsomes from both small intestine and colon were inhibited markedly by carbon monoxide, indicating the participation of cytochrome P-450. A cytochrome P-450 was solubilized from small intestine microsomes, and purified to a specific content of 10.5 nmol of cytochrome P-450/mg of protein. This cytochrome hydroxylated PGA1 at the omega-position with a turnover rate of 38.2 nmol/min/nmol of cytochrome P-450 in the reconstituted system containing cytochrome P-450, NADPH-cytochrome P-450 reductase, cytochrome b5 and phosphatidylcholine. It is suggested that this cytochrome P-450 is specialized for the omega-hydroxylation of PGA1 in small intestine microsomes.     

Reconstitution of testosterone oxidation by purified rat cytochrome P450p (IIIA1)

Cytochrome P450p (IIIA1) has been purified from rat liver microsomes by several investigators, but in all cases the purified protein, in contrast to other P450 enzymes, has not been catalytically active when reconstituted with NADPH-cytochrome P450 reductase and dilauroylphosphatidylcholine. We now report the successful reconstitution of testosterone oxidation by cytochrome P450p, which was purified from liver microsomes from troleandomycin-treated rats. The rate of testosterone oxidation was greatest when purified cytochrome P450p (50 pmol/ml) was reconstituted with a fivefold molar excess of NADPH-cytochrome P450 reductase, an equimolar amount of cytochrome b5, 200 micrograms/ml of a chloroform/methanol extract of microsomal lipid (which could not be substituted with dilauroylphosphatidylcholine), and the nonionic detergent, Emulgen 911 (50 micrograms/ml). Testosterone oxidation by cytochrome P450p was optimal at 200 mM potassium phosphate, pH 7.25. In addition to their final concentration, the order of addition of these components was found to influence the catalytic activity of cytochrome P450p. Under these experimental conditions, purified cytochrome P450p converted testosterone to four major and four minor metabolites at an overall rate of 18 nmol/nmol P450p/min (which is comparable to the rate of testosterone oxidation catalyzed by other purified forms of rat liver cytochrome P450). The four major metabolites were 6 beta-hydroxytestosterone (51%), 2 beta-hydroxytestosterone (18%), 15 beta-hydroxytestosterone (11%) and 6-dehydrotestosterone (10%). The four minor metabolites were 18-hydroxytestosterone (3%), 1 beta-hydroxytestosterone (3%), 16 beta-hydroxytestosterone (2%), and androstenedione (2%). With the exception of 16 beta-hydroxytestosterone and androstenedione, the conversion of testosterone to each of these metabolites was inhibited greater than 85% when liver microsomes from various sources were incubated with rabbit polyclonal antibody against cytochrome P450p. This antibody, which recognized two electrophoretically distinct proteins in liver microsomes from troleandomycin-treated rats, did not inhibit testosterone oxidation by cytochromes P450a, P450b, P450h, or P450m. The catalytic turnover of microsomal cytochrome P450p was estimated from the increase in testosterone oxidation and the apparent increase in cytochrome P450 concentration following treatment of liver microsomes from troleandomycin- or erythromycin-induced rats with potassium ferricyanide (which dissociates the cytochrome P450p-inducer complex). Based on this estimate, the catalytic turnover values for purified, reconstituted cytochrome P450p were 4.2 to 4.6 times greater than the rate catalyzed by microsomal cytochrome P450p.     

Aflatoxin B1 metabolism by 3-methylcholanthrene-induced hamster hepatic cytochrome P-450s

We have studied the activation of aflatoxin B1 by hamster liver microsomes and purified hamster cytochrome P-450 isozymes using a umu mutagen test. The hamster liver microsomes or S-9 fractions were much more active than rat liver microsomes or S-9 fractions in the activation of umu gene expression by aflatoxin B1 metabolites. 3-Methyl-cholanthrene treatment increased aflatoxin B1 activation by hamster liver microsomes. Two major 3-methylcholanthrene-inducible cytochrome P-450 isozymes, P-450 MC1 (IIA) and P-450 MC4 (IA2), were purified from 3-methylcholanthrene-treated hamster liver microsomes, and the metabolism of aflatoxin B1 by these two cytochromes was studied. In the reconstituted enzyme system, both P-450 MC1 and P-450 MC4 were highly active in the activation of aflatoxin B1, and antibodies against these P-450s specifically inhibited these activities. Antibody against P-450 MC1 inhibited the activation of aflatoxin B1 by 20% in the presence of 3-methyl-cholanthrene-treated hamster liver microsomes. In contrast, antibody against P-450 MC4 stimulated the activity by 175%. These results indicated that hamster P-450 MC1 might convert aflatoxin B1 to more toxic metabolite(s), whereas P-450 MC4 might convert aflatoxin B1 to less toxic metabolite(s), than aflatoxin B1 in liver microsomes. The metabolite(s) produced by both hamster cytochrome P-450 MC1 and MC4 were genotoxic in the umu mutagen test.     

Expression of a rat liver microsomal cytochrome P-450 catalyzing testosterone 16 alpha-hydroxylation in Saccharomyces cerevisiae: vitamin D3 25-hydroxylase and testosterone 16 alpha-hydroxylase are distinct forms of cytochrome P-450

Rat cytochrome P-450(M-1) cDNA was expressed in Saccharomyces cerevisiae TD1 cells by using a yeast-Escherichia coli shuttle vector consisting of P-450(M-1) cDNA, yeast alcohol dehydrogenase promoter and yeast cytochrome c terminator. The yeast cells synthesized up to 2 X 10(5) molecules of P-450(M-1) per cell. The microsomal fraction prepared from the transformed cells contained 0.1 nmol of cytochrome P-450 per mg of protein. The expressed cytochrome P-450 catalyzed 16 alpha- and 2 alpha-hydroxylations of testosterone in accordance with the catalytic activity of P-450(M-1), but did not hydroxylate vitamin D3 or 1 alpha-hydroxycholecalciferol at the 25 position. The expressed cytochrome P-450 also catalyzed the oxidation of several drugs and did not show 25-hydroxylation activity toward 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol. However, it cross-reacted with the polyclonal and monoclonal antibodies elicited against purified P-450cc25 which catalyzed the 25-hydroxylation of vitamin D3. These results indicated that P-450(M-1) cDNA coded the 2 alpha- and 16 alpha-hydroxylase of testosterone, and that these two positions of testosterone are hydroxylated by a single form of cytochrome P-450. Vitamin D3 25-hydroxylase and testosterone 16 alpha- and 2 alpha-hydroxylase are different gene products, although these two hydroxylase activities are immunochemically indistinguishable.     

Mouse liver testosterone 16 alpha-hydroxylase (cytochrome P-450(16) alpha). Purification, regioselectivity, stereospecificity, and immunochemical characterization

Microsomal testosterone 16 alpha-hydroxylase (cytochrome P-450(16) alpha) was purified from the livers of male 129/J mice based on enzyme activity in the eluates from columns of DEAE Bio-Gel A, hydroxylapatite, and isobutyl-Sepharose 4B. The specific cytochrome P-450 content of the purified P-450(16) alpha fraction was 9.5 nmol/mg of protein. The specific testosterone 16 alpha-hydroxylation activity of the purified P-450(16) alpha fraction was 80 nmol/min/nmol of cytochrome P-450 or 764 nmol/min/mg of protein, and these values were about 40- and 400-fold higher, respectively, than the activity of solubilized microsomes. The purified P-450(16) alpha showed extremely high regioselectivity and stereospecificity for testosterone hydroxylation; more than 90% of the testosterone metabolites formed by the purified P-450(16) alpha fraction was 16 alpha-hydroxytestosterone. The purified anti-P-450(16) alpha antibody exhibited absolute specificity for inhibition of testosterone 16 alpha-hydroxytestosterone was inhibited by the anti-P-450(16) alpha. Anti-P-450(16) alpha inhibited the 16 alpha-hydroxylation activity of intact microsomes prepared from livers of male or female 129/J mice more than 90%, indicating that P-450(16) alpha is the major cytochrome P-450 isozyme catalyzing 16 alpha-hydroxylation activity of testosterone in these microsomal preparations. The purified P-450(16) alpha fraction also possessed high benzphetamine N-demethylation activity relative to the rates found with other xenobiotic substrates tested in this report.     

Comparative metabolism of benzo[f]quinoline by liver microsomes from brown bullheads and rats

1. Liver microsomes from rats were considerably more active in metabolizing benzo[f]quinoline (B f Q) than those from brown bullheads (Ictalurus nebulosus). 2. The main B f Q metabolites formed by both rat and brown bullhead liver microsomes were qualitatively similar and included B f Q-7,8-dihydrodiol, B f Q-9,10-dihydrodiol, B f Q-N-oxide, 7-hydroxy B f Q, and 9-hydroxy B f Q. 3. The liver microsomes from control brown bullheads and rats metabolized B f Q primarily at the 7,8-and 9,10-positions, respectively, whereas in the case of microsomes from 3-methylcholanthrene (3-MC)-treated rats or brown bullheads, the major site of metabolic attack was the 7,8-position. 4. A 3-MC-type of cytochrome P-450 appears to be primarily responsible for the oxidation of B f Q by control brown bullhead liver microsomes, whereas a phenobarbital-inducible type of cytochrome P-450 seems to be involved in the metabolism of B f Q by control rat liver microsomes.