Circulating C-terminal propeptide of type I procollagen is cleared mainly via the mannose receptor in liver endothelial cells.

The cytochrome P-450 enzyme which catalyses 25-hydroxylation of vitamin D3 (cytochrome P-450(25] from pig kidney microsomes [Postlind & Wikvall (1988) Biochem. J. 253, 549-552] has been further purified. The specific content of cytochrome P-450 was 15.0 nmol.mg of protein-1, and the protein showed a single spot with an apparent isoelectric point of 7.4 and an Mr of 50,500 upon two-dimensional isoelectric-focusing/SDS/PAGE. The 25-hydroxylase activity towards vitamin D3 was 124 pmol.min-1.nmol of cytochrome P-450-1 and towards 1 alpha-hydroxyvitamin D3 it was 1375 pmol.min-1.nmol-1. The preparation also catalysed the 25-hydroxylation of 5 beta-cholestane-3 alpha,7 alpha-diol at a rate of 1000 pmol.min-1.nmol of cytochrome P-450-1 and omega-1 hydroxylation of lauric acid at a rate of 200 pmol.min-1.nmol of cytochrome P-450-1. A monoclonal antibody raised against the 25-hydroxylating cytochrome P-450, designated mAb 25E5, was prepared. After coupling to Sepharose, the antibody was able to bind to cytochrome P-450(25) from kidney as well as from pig liver microsomes, and to immunoprecipitate the activity for 25-hydroxylation of vitamin D3 and 5 beta-cholestane-3 alpha,7 alpha-diol when assayed in a reconstituted system. The hydroxylase activity towards lauric acid was not inhibited by the antibody. By SDS/PAGE and immunoblotting with mAb 25E5, cytochrome P-450(25) was detected in both pig kidney and pig liver microsomes. These results indicate a similar or the same species of cytochrome P-450 in pig kidney and liver microsomes catalysing 25-hydroxylation of vitamin D3 and C27 steroids. The N-terminal amino acid sequence of the purified cytochrome P-450(25) from pig kidney microsomes differed from those of hitherto isolated mammalian cytochromes P-450.     

Variations in hepatic progesterone 21-hydroxylase activity reflect differences in the microsomal concentration of rabbit cytochrome P-450 1

A monoclonal antibody specific for cytochrome P-450 1 that extensively (greater than 95%) inhibits the hepatic 21-hydroxylation of progesterone was used in a two-site immunoradiometric assay to estimate the concentration of cytochrome P-450 1 in microsomes prepared from 24 individual, untreated New Zealand White rabbits. The progesterone 21-hydroxylase activities of these microsomes ranged from 0.2 to 5.8 nmol min-1 mg microsomal protein-1. Scatchard analysis revealed similar slopes and thus apparent affinities between the antibody and microsome samples that varied greater than 10-fold in 21-hydroxylase activity. The maximal extent of binding of the antibody to different microsomal preparations was greater for microsomes exhibiting high as compared to low 21-hydroxylase activity, suggesting that the level of binding reflects the microsomal content of P-450 1. Quantitation was based on the extent of binding of the 125I-labeled monoclonal antibody to P-450 1 sequestered from a sample by a heterologous monoclonal antibody adsorbed to the wells of a microtiter plate. These results indicate that the microsomal content of P-450 1 varies from less than 0.05 to 0.5 nmol/mg microsomal protein. The microsomal content of this antigen as determined in the two-site immunoradiometric assay was highly correlated (r = 0.97) with progesterone 21-hydroxylase activity. Linear regression analysis was used to estimate the turnover number for progesterone in situ, yielding a value of 11 nmol deoxycorticosterone formed min-1 nmol microsomal P-450 1(-1). This is similar to the value of 14 nmol deoxycorticosterone formed min-1 nmol-1 obtained for the reconstituted, purified P-450 1 used as a standard in the immunoquantitation assay.     

Participation of a cytochrome P450 enzyme from the 2C subfamily in progesterone 21-hydroxylation in sheep liver.

Progesterone 21-hydroxylation in hepatic microsomes from adult male sheep is a quantitatively important metabolic pathway (0.27 +/- 0.08 nmol deoxycorticosterone formed/min/mg protein; representing 13-25% of total progesterone conversion). This study was undertaken to determine whether the ovine hepatic progesterone 21-hydroxylase may be another member of the P450 2C subfamily, normally associated with progesterone 21-hydroxylation in rodent liver. An IgG preparation raised in rabbits against purified rat liver microsomal cytochrome P450 2C6 was found to recognize a single antigen (MW 52 kDa) in sheep liver microsomes. This protein was present in sheep liver (apparent concentration 16 +/- 4 ng/micrograms microsomal protein) representing approx. 28% of the corresponding content of P450 2C6 in untreated rat liver. Preincubation of the anti-P450 2C6 IgG with hepatic microsomes was found to decrease the rate of progesterone 21-hydroxylation to 50-80% of uninhibited control. Taken together, from these findings it is apparent that a P450 enzyme, most likely from the 2C subfamily, catalyses deoxycorticosterone formation from progesterone in sheep liver and that this is a quantitatively important pathway of progesterone hydroxylation in these fractions.     

Mouse liver phenobarbital-inducible P450 system: purification, characterization, and differential inducibility of four cytochrome P450 isozymes from D2 mouse

Three novel cytochrome P450 isozymes were purified from phenobarbital (PB)-treated D2 mouse liver microsomes and compared to the previously characterized coumarin 7-hydroxylase, P450Coh. The molecular masses were 56.5, 55, 51, and 49.5 kDa, and the peaks of the reduced CO complexes were at 450, 447.5, 451.5, and 449 nm for P450PBI, P450PBII, P450PBIII, and P450Coh, respectively. The NH2-terminal sequences suggest that these isozymes belong to the P450 gene subfamilies 2B, 1A, 2C, and 2A, respectively. On the basis of reconstituted activities and microsomal immunoinhibition studies, P450Coh was the sole catalyst of coumarin 7-hydroxylation. P450PBI was the major isozyme catalyzing the high Km 7-pentoxyresorufin O-dealkylation. This reaction was also mediated at a slower rate by the low Km isozyme, P450PBII. P450PBIII contributed significantly to the microsomal O-deethylation of 7-ethoxyresorufin and N-demethylation of benzphetamine. Western blotting and dot immunobinding analyse of microsomes showed that the induction patterns of the isozymes were different. PB and TCPO-BOP induced all isozymes variably: P450PBI (19- and 31-fold), P450PBII (2- and 3-fold), P450PBIII (9- and 4-fold), and P450Coh (about 2-fold). Pyrazole induced only P450Coh, while all other isozymes were decreased by 30 to 60%. The changes in the microsomal amounts of these isozymes correlated generally well with the variation in the immunoinhibitable enzyme activities. On the basis of the structural and catalytic properties, immunochemical characteristics, and induction profiles, all three isozymes were different from each other and from the previously characterized P450Coh. This mouse PB-inducible P450 model may be valuable in further studies on the induction mechanisms of PB and TCPOBOP.     

In vivo administration of H2 blockers,cimetidine and ranitidine,reduced the contents of the cytochrome P450IID (CYP2D) subfamily and their activities in rat liver microsomes

• 1.1. The effects of in vivo administration of H₂ blockers, cimetidine and ranitidine (0.6 mmol/kg body weight/day, for 5 days), on several P450 isozymes, the P450IID (CYP2D) subfamily, and their monooxygenase activities in rat liver microsomes were investigated.
• 2.2. In vivo administration of cimetidine and ranitidine decreased the contents of P450 isozymes and the activities of P450-linked monooxygenase systems; i.e., benzphetamine N-demethylase, aminopyrine N-demethylase, 7-ethoxycoumarine O-deethylase, debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase.
• 3.3. The inhibitory effect on the enzymatic activities of the P450IID (CYP2D)-linked monooxygenase systems was studied by Western blot analysis with serum containing antiCYP2D6 IgG, i.e., LKM1 autoantibody. The amount of P450IID (CYP2D) in liver microsomes decreased more remarkably in the group administered ranitidine or cimetidine in vivo than in controls.
• 4.4. The effects of cimetidine and ranitidine on the activities of the P450IID (CYP2D)-linked monooxygenase systems were investigated in vitro. The activities of debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase were inhibited in vitro by cimetidine or ranitidine at a higher concentration than that on in vivo administration of either H₂ blocker.
• 5.5. The kinetic parameters for cimetidine or ranitidine as to the activities of debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase in liver microsomes were determined by means of Lineweaver-Burk plots.
• 6.6. The suppressive effects of cimetidine and ranitidine on the activities of the P450IID (CYP2D)-linked monooxygenase systems in vivo were found to be due to a decrease of the content of the P450IID (CYP2D) protein.

     

Variation in hepatic microsomal cytochrome P-450 1 concentration among untreated rabbits alters the efficiency of estradiol hydroxylation

The metabolism of 17 beta-estradiol was examined using both rabbit liver microsomes and highly purified forms of rabbit liver microsomal cytochrome P-450. The predominant microsomal metabolite of 17 beta-estradiol is the 2-hydroxylated product. 2-Hydroxyestradiol is also the principal metabolite in reconstitution experiments in which P-450 1 exhibits the greatest Vmax, ca. 6 mol min-1 mol P-450 1(-1), vs less than 0.6 mol min-1 mol P-450(-1) for forms 2, 3b-, 3b+, 3c, 4, and 6. In addition P-450 1 has the lowest Km, ca. 2 microM. This suggested that microsomes which differ in their content of P-450 1 would also differ in the kinetic parameters characterizing the 2-hydroxylation of 17 beta-estradiol. Microsomes containing low amounts of P-450 1, less than 0.1 nmol/mg protein, exhibit a low-efficiency (Vmax/Km) 2-hydroxylase activity. Microsomes containing elevated concentrations of P-450 1, greater than 0.3 nmol/mg protein, exhibit a substrate dependence suggestive of an additional high-efficiency enzyme. The latter is specifically inhibited by a monoclonal antibody that recognizes P-450 1. These results indicate that the elevated expression of P-450 1 in microsomes leads to a marked increase in the apparent first-order rate constant for the 2-hydroxylation of 17 beta-estradiol, as it does for the 21-hydroxylation of progesterone. This should have a marked effect on the metabolism of these two steroid hormones at concentrations that are likely to occur in vivo.     

Reversible and time-dependent inhibition of the hepatic cytochrome P450 steroidal hydroxylases by the proestrogenic pesticide methoxychlor in rat and human

Methoxychlor, a currently used pesticide, is demethylated and hydroxylated by several hepatic microsomal cytochrome P450 enzymes. Also, methoxychlor undergoes metabolic activation, yielding a reactive intermediate (M*) that binds irreversibly and apparently covalently to microsomal proteins. The study investigated whether methoxychlor could inhibit or inactivate certain liver microsomal P450 enzymes. The regioselective and stereoselective hydrox-ylation of testosterone and the 2-hydroxylation of estradiol (E₂) were utilized as markers of the P450 enzymes inhibited by methoxychlor. Both reversible and time-dependent inhibition were examined. Coincubation of methoxychlor and testosterone with liver microsomes from phenobarbital treated (PB-microsomes) male rats, yielded marked diminution of 2α- and 16α-testosterone hydroxylation, indicating strong inhibition of P4502C11 (P450h). Methoxychlor moderately inhibited 2β-, 7α-, 15α-, 15β-, and 16β-hydroxylation and androstenedi-one formation. There was only a weak inhibition of 6β-ydroxylation of testosterone. The methox-ychlor-mediated inhibition of 6β-hydroxylation was competitive. By contrast, when methoxychlor was permitted to be metabolized by PB-microsomes or by liver microsomes from pregnenolone-16α-car-bonitrile treated rats (PCN-microsomes) prior to addition of testosterone, a pronounced time-dependent inhibition of 6β-hydroxylation was observed, suggesting that methoxychlor inactivates the P450 3A isozyme(s). The di-demethylated methoxychlor (bis-OH-M) and the tris-hydroxy (ca-techol) methoxychlor metabolite (tris-OH-M) inhibited 6β-hydroxylation in PB-microsomes competitively and noncompetitively, respectively; however, these methoxychlor metabolites did not exhibit a time-dependent inhibition. Methoxychlor inhibited competitively the formation of 7α-hydroxytestosterone (7α-OH-T) and 16α-hydroxy-testosterone (16α-OH-T) but exhibited little or no time-dependent inhibition of generation of these metabolites, indicating that P450s 2A1, 2B1/B2, and 2C11 were inhibited but not inactivated. Methoxychlor inhibited in a time-dependent fashion the 2-hydroxylation of E2 in PB-microsomes. However, bis-OH-M exhibited solely reversible inhibition of the 2-hydroxylation, supporting our conclusion that the inactivation of P450s does not involve participation of the demethylated metabolites. Both competitive inhibition and time-dependent inactivation of human liver P450 3A (6β-hydroxylase) by methoxychlor, was observed. As with rat liver microsomes, the human 6β-hydroxylase was inhibited by bis-OH-M and tris-OH-M competitively and noncompetitively, respectively. Testosterone and estradiol strongly inhibited the irreversible binding of methoxychlor to microsomal proteins. This might explain the “clean” competitive inhibition by methoxychlor of the 6β-OH-T formation when the compounds were coin-cubated. Glutathione (GSH) has been shown to interfere with the irreversible binding of methoxychlor to PB-microsomal proteins. The finding that the coincubation of GSH with methoxychlor partially diminishes the time-dependent inhibition of 6β-hydroxylation provides supportive evidence that the inactivation of P450 3A isozymes by methoxychlor is related to the formation of M*.     

Inactivation of rat liver microsomal steroid hydroxylations by 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine: evidence for selectivity among steroid-inducible cytochrome P450IIIA forms

Various 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6- trimethylpyridine (DDC) cause mechanism-based inactivation of cytochrome P-450 (P-450) via destruction of the heme prosthetic group. This is an important component of these compounds' porphyrinogenic mechanism. In an attempt to map the P-450 isozyme selectivities of DDC analogues, we have examined the effects of these compounds on the regioselective and stereoselective hydroxylation of androstenedione (AD) and progesterone (PG) in rat liver microsomal systems. In microsomes from phenobarbital-treated male rats, DDC analogues did not cause time-dependent inactivation of AD 7 alpha-hydroxylase, AD 16 beta-hydroxylase, and PG 21-hydroxylase, selective markers for P450IIA 1/2, IIB1, and IIC6, respectively. In contrast, DDC analogues were effective inactivators of PG 2 alpha-hydroxylase and steroid 6 beta-hydroxylases, selective markers for P450IIC11 and IIIA forms, respectively. We conclude that differences in porphyrinogenicity observed with various DDC analogues are not likely to be due to the selective destruction of different P-450 isozymes by different analogues, but rather to properties of the DDC analogues themselves. 4-Ethyl DDC was found to be capable of discriminating between P450IIIA subfamily forms. In microsomes from untreated male rats, which express P450IIIA2 but not IIIA1, 4-ethyl DDC inactivated both AD and PG 6 beta-hydroxylases. However, in microsomes from dexamethasone-treated female rats, which express P450IIIA1 but not IIIA2, no inactivation of the steroid 6 beta-hydroxylases was observed. Thus, 4-ethyl DDC appears to be a potentially valuable tool for differentiating between P450IIIA forms.     

Selective potent restriction of P450b- but not P450e-dependent 7,12-dimethylbenz[a]anthracene metabolism by the microsomal environment

The prototypic members of the rat liver cytochrome P450IIB subfamily, P450b and P450e, differ by only 13 amino acids and yet purified P450b is considerably more active than P450e for all known substrates. A unique regioselectivity difference between cytochromes P450b and P450e for the metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) and a genetic deficiency in P450e expression in the Marshall (M520/N) rat strain have been exploited to determine the microsomal contributions of the respective forms toward the metabolism of DMBA. The total contribution to metabolism by each isozyme has been assessed based on the sensitivity to rabbit anti-P450b/e IgG and comparison with microsomal P450b and P450e content as measured by Western blots. Liver microsomes from untreated M520/N rats do not express detectable levels of P450e but express P450b at a level that is 2-fold higher than that of P450e in liver microsomes from untreated F344 rats (50 pmol/mg). However, only 4% of the constitutive DMBA metabolizing activity of liver microsomes from the M520/N rat strain could be inhibited by anti-P450b/e IgG. A 30-fold induction of hepatic P450b by phenobarbital (PB) was also completely ineffective in increasing P450b-dependent DMBA metabolism. PB treatment had no appreciable effect on either the levels of expression of P450b protein or P450b-dependent DMBA metabolism, in M520/N lung and adrenal microsomes. In contrast, PB treatment of F344 rats considerably increased P450b/e-dependent metabolism by liver, lung, and adrenal microsomes. The regioselectivity of the anti-P450b/e-sensitive metabolism (predominantly 12-methyl hydroxylation), however, indicated a much greater contribution from P450e than P450b in every tissue examined despite a several fold higher expression of P450b than of P450e. P450b was expressed constitutively in lung microsomes from both strains but again failed to exhibit appreciable DMBA metabolizing activity. Based on these activities and microsomal P450b contents, P450b consistently exhibited turnover numbers (0.02-0.15 nmol/nmol P450b/min) that were at least 10-fold lower than those of pure P450b. In contrast, the calculated turnover numbers for microsomal P450e were consistently comparable to those of pure P450e (approximately 1 nmol/nmol P450e/min).     

Occurrence and inducibility of cytochrome P450IIIA in maternal and fetal rats during prenatal development

The purpose of this study was to quantify cytochrome P450IIIA1 in fetal and maternal livers of uninduced and pregnenolone-16 alpha-carbonitrile (PCN) induced rats during the course of prenatal development. The activities and levels of P450IIIA in hepatic microsomes from maternal rats and fetuses at 15-21 days of gestation were measured by triacetyloleandomycin (TAO) inhibited debenzylation of (benzyloxy)phenoxazone and by immunoassay with defined antiserum specific for P450IIIA. P450IIIA was not detectable (less than 10 pmol/mg for maternal microsomes and less than 2 pmol/mg for fetal microsomes) by immunoassay in uninduced maternal or fetal livers. In hepatic microsomes from PCN-induced dams, values ranged from 59.3 to 116 micrograms P450IIIA1/mg of protein during the same gestational period. Changes in debenzylase activity of 15.9-46.5 pmol of resorufin (mg of protein)-1 min-1 were consistent with these findings as were the changes in TAO-inhibitable debenzylase activity. In the transplancentally induced fetal liver, debenzylase activity increased steadily from 0.19 pmol of resorufin mg-1 min-1 at day 15 to 9.34 pmol of resorufin mg-1 min-1 at day 21 and was paralleled by the TAO-inhibitable activity that ranged from 0.09 pmol of resorufin mg-1 min-1 at day 15 to 3.33 pmol of resorufin mg-1 min-1 at day 21. The amount of immunoreactive P450IIIA1 also increased from 0.5 to 28.7 micrograms/mg of microsomal protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Demethylation and denitrosation of nitrosamines by cytochrome P-450 isozymes

Metabolism of nitrosamines was studied in a reconstituted monooxygenase system composed of cytochrome P-450 isozymes purified from liver microsomes of ethanol- and phenobarbital-treated rats. The ethanol-induced isozyme (P-450et) was efficient in catalyzing the demethylation of N-nitrosodimethylamine (NDMA), with a Km of 2.4 mM and Vmax of 7.2 nmol min-1 nmol P-450(-1), but less active with N-nitrosomethylbenzylamine and N-nitrosomethylaniline. The phenobarbital-induced form (P-450b) was ineffective in NDMA metabolism but was active in catalyzing the demethylation of N-nitrosomethylaniline, with an estimated Km of 0.08 mM and a Vmax of 7.2 nmol min-1 nmol-1. P-450et also catalyzed the denitrosation of NDMA with a Km of 13.6 mM and a Vmax of 1.36 nmol min-1 nmol-1. With control liver microsomes, multiple Km values were observed for the demethylation and denitrosation of NDMA. Involvement of superoxide radicals in the metabolism of NDMA was suggested by the action of superoxide dismutase, which inhibited the denitrosation by 43 to 73% and the demethylation by 13 to 22% in different monooxygenase systems. The P-450et-dependent NDMA demethylation was strongly inhibited by 2-phenylethylamine and 3-amino-1,2,4-triazole; these compounds were previously believed not to be inhibitors of P-450-dependent reactions but were found to inhibit microsomal NDMA demethylase. The present results establish the role of P-450 in nitrosamine metabolism and help to clarify some of the previous confusion in this area of research.     

Identification of human cytochrome P450 isoforms involved in the 7-hydroxylation of chlorpromazine by human liver microsomes

Studies to identify the cytochrome P450 (CYP) isoform(s) involved in chlorpromazine 7-hydroxylation were performed using human liver microsomes and cDNA-expressed human CYPs. The kinetics of chlorpromazine 7-hydroxylation in human liver microsomes showed a simple Michaelis-Menten behavior. The apparent Km and Vmax values were 3.4+/-1.0 microM and 200.5+/-83.7 pmol/min/mg, respectively. The chlorpromazine 7-hydroxylase activity in human liver microsomes showed good correlations with desipramine 2-hydroxylase activity (r = 0.763, p < 0.05), a marker activity for CYP2D6, and phenacetin O-deethylase activity (r = 0.638, p < 0.05), a marker activity for CYP1A2. Quinidine (an inhibitor of CYP2D6) completely inhibited while alpha-naphthoflavone (an inhibitor of CYP1A2) marginally inhibited the chlorpromazine 7-hydroxylase activity in a human liver microsomal sample showing high CYP2D6 activity. On the other hand, alpha-naphthoflavone inhibited the chlorpromazine 7-hydroxylase activity to 55-65% of control in a human liver microsomal sample showing low CYP2D6 activity. Among eleven cDNA-expressed CYPs studied, CYP2D6 and CYP1A2 exhibited significant activity for the chlorpromazine 7-hydroxylation. The Km values for the chlorpromazine 7-hydroxylation of both cDNA-expressed CYP2D6 and CYP1A2 were in agreement with the Km values of human liver microsomes. These results suggest that chlorpromazine 7-hydroxylation is catalyzed mainly by CYP2D6 and partially by CYP1A2.     

Cytochrome P-450 isozyme/isozyme functional interactions and NADPH-cytochrome P-450 reductase concentrations as factors in microsomal metabolism of warfarin

The basis for our previous observations [Kaminsky, L.S., Guengerich, F.P., Dannan, G.A. & Aust, S.D. (1983) Arch. Biochem. Biophys. 225, 398-404] that rates of microsomal metabolism of warfarin were markedly less than the sum of rates of the reconstituted constituent isozymes of cytochrome P-450 has been investigated. Metabolism of warfarin to 4'-, 6-, 7-, 8-, and 10-hydroxywarfarin and dehydrowarfarin by highly purified rat liver cytochrome P-450 (P-450) isozymes reconstituted with NADPH-cytochrome P-450 reductase and by hepatic microsomes from variously pretreated rats was used to probe functional consequences of P-450 isozyme/isozyme interactions and of the effect of microsomal reductase concentrations. Binary mixtures of P-450 isozymes were reconstituted and the regioselectivity and stereoselectivity were used to probe metabolism by each individual isozyme. The isozymes specifically inhibited each other to variable extents and the order of inhibitory potency was: P-450UT-F greater than P-450PB-D greater than or equal to P-450UT-A greater than or equal to P-450BNF/ISF-G greater than P-450PB/PCN-E greater than P-450PB-B greater than or equal to P-450PB-C greater than or equal to P-450BNF-B. The inhibition, possibly a consequence of aggregation, explains the low rate of microsomal metabolism relative to the metabolic potential of the component P-450 isozymes. When purified reductase was added to microsomes it appeared to bind to microsomes at different sites from endogenous reductase and it enhanced warfarin hydroxylase activity only to a minor extent, thus possibly precluding low reductase concentrations from being a major factor in the relatively low rates of microsomal metabolism. Antibody to the reductase differentially inhibited microsomal metabolism of warfarin by the various P-450 isozymes. The results suggest that the reductase and P-450 isozymes may be located differently relative to one another in the various microsomal preparations.     

Purification and characterization of the rat liver microsomal cytochrome P-450 involved in the 4-hydroxylation of debrisoquine, a prototype for genetic variation in oxidative drug metabolism

Genetic polymorphism in oxidative drug metabolism is perhaps best exemplified in the case of debrisoquine 4-hydroxylase activity, where the incidence of deficient metabolism ranges from 1% to 30% in various populations and this defect is also linked to an impaired ability to metabolize a number of other drugs effectively. Sprague-Dawley (SD) rats possess this activity, but females of the DA strain do not, although total cytochrome P-450 (P-450) levels are similar. We have purified, by using debrisoquine 4-hydroxylase activity as an assay, a minor P-450 to electrophoretic homogeneity from male SD rats and designate this as P-450UT-H. P-450UT-H differs from eight other purified rat liver P-450s as judged by peptide mapping and immunochemical analysis and thus appears to be isozymic with these other P-450s. P-450UT-H exhibited considerably more debrisoquine 4-hydroxylase activity than any of the other purified P-450s and, on a total P-450 basis, more than total microsomal P-450. Antibodies raised against P-450UT-H specifically recognized P-450UT-H and inhibited more than 90% of the debrisoquine hydroxylase activity present in SD rat liver microsomes. The level of P-450UT-H in SD rat liver microsomes accounted for less than 10% of the total P-450, as judged by immunochemical quantitation. These assays also indicated that the level of P-450UT-H in female DA rat liver microsomes is only about 5% of that in male or female SD rat liver microsomes, consonant with the view that deficiency of this form of P-450 is responsible for the defective debrisoquine 4-hydroxylase activity in the former animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

A convenient assay for mephenytoin 4-hydroxylase activity of human liver microsomal cytochrome P-450

A simple and rapid method for the determination of (S)-mephenytoin 4-hydroxylase activity by human liver microsomal cytochrome P-450 has been developed. [Methyl-14C] mephenytoin was synthesized by alkylation of S-nirvanol with 14CH3I and used as a substrate. After incubation of [methyl-14C]mephenytoin with human liver microsomes or a reconstituted monooxygenase system containing partially purified human liver cytochrome P-450, the 4-hydroxylated metabolite of mephenytoin was separated by thin-layer chromatography and quantified. The formation of the metabolite depended on the incubation time, substrate concentration, and cytochrome P-450 concentration and was found to be optimal at pH 7.4. The Km and Vmax rates obtained with a human liver microsomal preparation were 0.1 mM and 0.23 nmol 4-hydroxymephenytoin formed/min/nmol cytochrome P-450, respectively. The hydroxylation activity showed absolute requirements for cytochrome P-450, NADPH-cytochrome P-450 reductase, and NADPH in a reconstituted monooxygenase system. Activities varied from 5.6 to 156 pmol 4-hydroxymephenytoin formed/min/nmol cytochrome P-450 in 11 human liver microsomal preparations. The basic system utilized for the analysis of mephenytoin 4-hydroxylation can also be applied to the estimation of other enzyme activities in which phenol formation occurs.     

Methoxyphenamine metabolism in rat models of human debrisoquine phenotypes

The metabolism of the beta 2-adrenoceptor agent methoxyphenamine was investigated in rats of the Lewis and Dark Agouti strains, which are proposed models for human extensive and poor metabolizers of debrisoquine, respectively. Following oral ingestion of 20 mg kg-1 of methoxyphenamine, Dark Agouti excreted, on the average, significantly more methoxyphenamine and less O-demethylmethoxyphenamine and 5-hydroxymethoxyphenamine in 0- to 24-h urine than Lewis. In contrast, the N-demethylation of methoxyphenamine showed no interphenotype differences between the two strains. It is possible that in rats, the form of cytochrome P-450, which controls the 4-hydroxylation of debrisoquine, may also control the O-demethylation and aromatic 5-hydroxylation of methoxyphenamine.     

Identification of rabbit microsomal cytochrome P-450 isozyme,form 1, as a hepatic progesterone 21-hydroxylase

Six highly purified forms of rabbit microsomal cytochrome P-450, isolated from hepatic microsomes, exhibit differences in the regiospecific metabolism of progesterone. Only one of the isozymes studied, form 1, catalyzes the formation of deoxycorticosterone from progesterone at an appreciable rate. This cytochrome P-450 isozyme may participate in the conversion of progesterone to deoxycorticosterone during pregnancy. All six forms of cytochrome P-450 catalyze 6β- and 16α-hydroxylation at the two concentrations of progesterone tested. Form 3b exhibits a lower apparent Km for 6β-hydroxylation than the other five.     

Correlation between nortriptyline and debrisoquine hydroxylation in the human liver

The benzylic hydroxylation of nortriptyline (NT) and debrisoquine (D) by isolated human liver microsomes from eight subjects was studied. There was a strong correlation between the 10-hydroxylation of NT and the 4-hydroxylation of D (r = 0.96). The ability to hydroxylate D was also measured in vivo as the ratio between D and 4-OH-D in urine after oral administration of the drug to four subjects. This estimate of hydroxylation capacity agreed with the in vitro measurements. Liver microsomes from a subject defined as a poor in vivo oxidizer of D hydroxylated NT and D unusually slowly. Separation of microsomal proteins by SDS-gel electrophoresis indicated a relative lack of a cytochrome P-450 isozyme with a molecular weight of 54,500 in the liver from the poor oxidizer.     

Purification of a cytochrome P-450 from pig kidney microsomes catalysing the 25-hydroxylation of vitamin D3.

Cytochrome P-450 catalysing 25-hydroxylation of vitamin D3 was purified from pig kidney microsomes. The enzyme fraction contained 7 nmol of cytochrome P-450/mg of protein and showed only one protein band with an apparent Mr of 50,500 upon SDS/polyacrylamide-gel electrophoresis. The purified cytochrome P-450 catalysed 25-hydroxylation of vitamin D3 up to 1,000 times more efficiently, and 25-hydroxylation of 1 alpha-hydroxyvitamin D3 up to 4000 times more efficiently, than the microsomes. The cytochrome P-450 required microsomal NADPH-cytochrome P-450 reductase for catalytic activity. Mitochondrial ferredoxin and ferredoxin reductase could not replace microsomal NADPH-cytochrome P-450 reductase. The enzyme preparation showed no detectable 25-hydroxylase activity towards vitamin D2 or 1 alpha-hydroxylase activity towards 25-hydroxyvitamin D3. CO inhibited the 25-hydroxylation by more than 85%. Mannitol, hydroquinone, catalase and superoxide dismutase did not affect the 25-hydroxylation. The possible role of the kidney microsomal cytochrome P-450 in the metabolism of vitamin D3 is discussed.     

Comparisons of warfarin metabolism by liver microsomes of rats treated with a series of polybrominated biphenyl congeners and by the component-purified cytochrome P-450 isozymes

R- and S-warfarin metabolite profiles (regio- and stereoselectivity) have been determined with hepatic microsomes from untreated rats and rats treated with nine individual polybrominated biphenyl (PBB) congeners, a commercial mixture of PBBs, and, for comparison with phenobarbital and 3-methylcholanthrene. The metabolic rates have been correlated with cytochrome P-450 (P-450) isozyme concentrations in the microsomes determined by immunochemical quantitation techniques (G. A. Dannan, F. P. Guengerich, L. S. Kaminsky, and S. D. Aust, (1983) J. Biol. Chem., 258, 1282–1288). The warfarin hydroxylase activities of the P-450 isozyme components of the various microsomal preparations (F. P. Guengerich, G. A. Dannan, S. T. Wright, M. V. Martin, and L. S. Kaminsky (1982) Biochemistry, 21, 6019–6030) were multiplied by the corresponding isozyme concentrations to obtain an assessment of the potential warfarin hydroxylase capacity of the microsomes, and the results were compared with actual activities. The results of these studies and comparisons indicate that substrate regio- and stereoselectivities of microsomal-bound P-450s are essentially retained on purification of the isozymes to homogeneity and reconstitution, that warfarin metabolism by microsomal preparations can be used to predict microsomal P-450 isozyme compositions, and that microsomal warfarin hydroxylase activity is greater than would be predicted based on the approx 20:1 ratio of P-450 to NADPH-P-450 reductase in the microsomes and on the known activities of constituent isozymes. Two P-450 isozymes which are induced by treatment of rats with phenobarbital appear to be more tightly linked to NADPH-P-450 reductase than does an isozyme induced by β-naphthoflavone.