Determination of the cytochrome P-450 IV marker, omega-hydroxylauric acid, by high-performance liquid chromatography and fluorimetric detection

Single and chronic administration of ciprofibrate or of ciprofibrate-glycinate in male Fischer 344 rats: Comparison of the effects on morphological and biochemical parameters in liver and blood

Fibrates lead to a reduction of serum triglycerides and cholesterol in hyperlipidemic patients. Their therapeutic use, however, can be associated with adverse effects like gastrointestinal disorders, myalgia, myositis and hepatotoxicity. Large doses can even cause hepatocellular carcinoma in rodents. Additionally, interactions with the biotransformation of other compounds at the cytochrome P450 (CYP) system have been observed. Thus, the discovery of new derivatives with less of these side effects is of great interest. In the present study a single (10 mg/kg body weight) or a 4-week (1 or 10 mg/kg body weight daily) oral administration of ciprofibrate or of the newly synthesized ciprofibrate-glycinate was investigated in adult male Fischer 344 rats. Serum lipid concentrations were distinctly decreased after single but only slightly after chronic administration of the two fibrates, whereas liver parameters revealed a slight concentration and time dependent hepatotoxicity. Histologically, a hypertrophy, an eosinophilia, a reduced glycogen content and also an apoptosis of the hepatocytes was observed. Effects were more pronounced after chronic treatment and after application of the higher dosage. All CYP enzymes investigated were induced in a time and concentration dependent manner. Resulting CYP mediated monooxygenase and oxidase activities showed a dependency both on enzyme induction and hepatotoxic effects. With no parameter investigated major differences were seen between ciprofibrate and ciprofibrate-glycinate. Thus, the present investigations revealed no noticeable advantages of ciprofibrate-glycinate over its parent compound ciprofibrate.

Fibrates and their newly synthesized glycinate or glycinate-methylester derivatives: comparison of the interactions with liver cytochrome P450 dependent monooxygenase- and oxidase-functions in vitro

Different fibrates (bezafibrate, ciprofibrate, clofibrate, fenofibrate, gemfibrozil) were investigated in comparison with their newly synthesized glycinate and glycinate-methylester derivatives. Interactions with the cytochrome P450 (CYP) system were studied by assessing binding to CYP and effects on CYP mediated monooxygenase functions in rat liver 9000 g supernatants, as measured by six model reactions for different CYP isoforms (ethoxyresorufin O-deethylation, ethoxycoumarin O-deethylation, pentoxyresorufin O-depentylation, p-nitrophenol-hydroxylation, ethylmorphine N-demethylation, lauric acid 11- or 12-hydroxylation). Possible prooxidant or antioxidant properties were investigated by the stimulated lipid peroxidation, hydrogen peroxide production, and lucigenin and luminol amplified chemiluminescence using rat liver microsomes. Additionally, the influence on luminol amplified rat whole blood chemiluminescence was examined. All substances tested displayed binding to CYP. Effects on the monooxygenase model reactions were in general more distinct with the glycinates than with the parent compounds and most pronounced with the glycinate-methylester derivatives. The slightest effects on all model reactions were seen with clofibrate and its derivatives. On the whole, low antioxidative rather than prooxidative effects were observed. In general and with most model reactions, the antioxidative capacity of the glycinate and glycinate-methylester derivatives slightly exceeded that of the respective parent compounds. Summarizing the results it can be concluded that with respect to possible interactions with the CYP system in vivo and thus with the biotransformation of other concomitantly administered compounds no advantages of the glycinate or glycinate methylester derivatives over their parent fibrates are to be expected. Only the antioxidative capacity of the derivatives was somewhat higher than that of the parent substances, though most probably only of minor therapeutical relevance.

Induction of lauric acid hydroxylase activity in catfish and bluegill by peroxisome proliferating agents

In mammals, sensitivity to peroxisome proliferation by peroxisome proliferating agents (PPAs) appears to be correlated with inducibility of lauric acid hydroxylase activity. Bluegill and catfish have been shown to respond to PPAs by induction of lauric acid hydroxylase immunoreactive proteins (Haasch, 1996). In this investigation, induction of lauric acid hydroxylase activity was confirmed by HPLC and mass spectral analysis of specific hydroxylation products and possible species-specific differences in metabolism were investigated. Male bluegill, channel catfish and rat, were administered the model PPAs, clofibrate (200 mg kg-1, i.p.), ciprofibrate (100 mg kg-1, i.p.), or olive oil as vehicle control (both sexes of catfish), 48 h prior to hepatic, trunk kidney (catfish only) or kidney (rat) microsome preparation. In general, total metabolism of lauric acid was similar in all species, but female catfish metabolize lauric acid to a greater extent than males. Ciprofibrate treatment produced significant induction of omega- and omega-6 hydroxylation in male catfish kidney. In male bluegill liver, omega-, omega-4 and omega-5 hydroxylations were significantly induced by clofibrate treatment. The data indicate that induction of lauric acid hydroxylase cytochrome(s) P450 occurs in PPA-exposed fish which may be a consideration for environmentally-exposed responsive species.

Liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry of omega- and (omega-1)-hydroxylated metabolites of elaidic and oleic acids in human and rat liver microsomes

In order to characterize the nature of the active site of cytochrome P450 2E1, the metabolism of various fatty acids with cis/trans geometric configurations has been investigated. A system coupling atmospheric pressure chemical ionization-mass spectrometry detection with HPLC separation was developed as an alternative method for the characterization of hydroxylated metabolites of oleic and elaidic acids in rat and human liver microsomes. Oxidation of oleic and elaidic acids led to the formation of two main metabolites which were identified by LC-MS and GC-MS as omega and (omega-1)-hydroxylated (or 17-OH and 18-OH) fatty acids, on the basis of their pseudo-molecular mass and their fragmentation. The assay was accurate and reproducible, with a detection limit of 25 ng per injection, a linear range from 25 to 1128 ng per injection, no recorded interference, intra-day and inter-day precision with variation coefficients <14%. This LC-MS method was validated with oleic acid by using both radiometric and mass spectrometric detections. A significant correlation was found between the two methods in human (r=0.86 and 0.94 with P<0.05 and 0.01) and rat liver microsomes (r=0.90 and 0.85 with P<0.01 and 0.05) for 17-OH and 18-OH metabolites, respectively. HPLC coupled to mass spectrometry for the analysis of hydroxylated metabolites of elaidic acid offers considerable advantages since the method does not require use of a radioactive molecule, completely separates the two hydroxymetabolites, confirms the identification of each metabolite, and is as sensitive as the radiometric analysis method. This method allowed the comparative study of oleic and elaidic acid hydroxylations by both human and rat liver microsomal preparations.

Regioselective and stereoselective metabolism of ibuprofen by human cytochrome P450 2C

The cytochrome P450s responsible for the regio- and stereoselectivity in the 2- and 3-hydroxylation of the chiral non-steroidal antiinflammatory drug ibuprofen were characterized in human liver microsomes. The rates of formation of both the 2- and 3-hydroxy metabolites exhibited monophasic (N = 2; N is the number of microsomal preparations) and biphasic (N = 2) substrate concentration dependence for both enantiomers of ibuprofen. The high affinity enzyme class parameters for S-ibuprofen (N = 4) were: 2-hydroxylation, Vmax = 566 +/- 213 pmol/min/mg, Km = 38 +/- 13 microM; 3-hydroxylation, Vmax = 892 +/- 630 pmol/min/mg, Km = 21 +/- 6 microM. For R-ibuprofen, the corresponding parameters were: 2-hydroxylation, Vmax = 510 +/- 117 pmol/min/mg, Km = 47 +/- 20 microM; 3-hydroxylation, Vmax = 593 +/- 113 pmol/min/mg, Km = 29 +/- 8 microM. cDNA-expressed CYP2C9 (Arg 144 and Cys 144) favored S-2- and S-3-hydroxyibuprofen formation, but CYP2C8 favored R-2-hydroxyibuprofen formation. Sulfaphenazole, retinol, and arachidonic acid competitively inhibited the rate of formation of all hydroxyibuprofens; Ki values (N = 3) for sulfaphenazole on the 2- and 3-hydroxylations of S-ibuprofen were 0.12 +/- 0.05 and 0.07 +/- 0.04 and of R-ibuprofen were 0.11 +/- 0.07 and 0.06 +/- 0.03 microM, respectively. Sulfaphenazole also competitively inhibited ibuprofen hydroxylation by cDNA-expressed CYP2C9 (Arg 144 and Cys 144) with Ki values in the range of 0.05 to 0.18 microM and CYP2C8 in the range of 0.36 to 0.55 microM. In a bank of 14 human liver microsome samples, significant correlations (r = 0.72 to 0.90; P < 0.01) were observed between the rates of formation of all four hydroxyibuprofens, and for each hydroxyibuprofen and prototypical CYP2C8/9 biotransformations. The regio- and stereoselectivities observed in vitro were consistent with those noted in vivo. The relative levels of both CYP2C8 and CYP2C9 and the expression of the corresponding variants may influence the disposition of ibuprofen in vivo.

Simultaneous radiometric and fluorimetric detection of lauric acid metabolites using high-performance liquid chromatography following esterification with 4-bromomethyl-6,7-dimethoxycoumarin in human and rat liver microsomes

The formation of (omega-1)-hydroxylauric acid from lauric acid (LA) can be used as an indicator of the activity of cytochrome P450 2E1 (CYP2E1) in rat and human liver microsomes. A high-performance liquid chromatographic (HPLC) method that is capable of identifying and measuring the two main metabolites of lauric acid, (omega-1)- and omega-OH-LA, has been developed and used in the study of rat and human liver microsomes. Measurement of the enzymatic activities, based on the esterification of the metabolites and substrate with the fluorescent agent, 4-bromomethyl-6, 7-dimethoxycoumarin, is described using both radiometric and fluorimetric detection methods. Extraction efficiencies of metabolites and residual substrate were calculated using radioactivity and were greater than 85%. The assay is accurate and reproducible and has a detection limit of 75 pg (0.37 pmol). Additionally, a strong correlation between the two techniques was found in both human (r = 0.945, n = 15, p < 0.01) and rat (r = 0.949, n = 18, p < 0.01) livers, for the (omega-1)-hydroxylauric acid.

Hepatocarcinogenic potential of di(2-ethylhexyl)phthalate in rodents and its implications on human risk

The plasticizer di(2-ethylhexyl) phthalate (DEHP), to which humans are extensively exposed, was found to be hepatocarcinogenic in rats and mice. DEHP is potentially set free from objects made of synthetic materials (e.g., those used in medicine). Chronically, the greatest amounts are transferred to persons undergoing hemodialysis (up to 3.1 mg/kg b.w. per day) who would thus be considered the individuals most endangered by tumorigenesis. Although toxicokinetics seem to play a certain unclear role in the course of DEHP-related toxicity, toxicodynamic factors appear more decisive. DEHP is a representative of "peroxisome proliferators" (PP), a distinct group of substances that, in rodents, do not only induce peroxisomes but also specific enzymes in other organelles, organ growth, and DNA synthesis. The cluster of the characteristic effects of PP is generally, although perhaps not quite appropriately summarized as "peroxisome proliferation," and is strongest in the liver. The lowest observed effect level (LOEL) and the no observed effect level (NOEL) of peroxisome proliferation in the rat, as determined by the induction of specific enzymes (peroxisomal beta-oxidation, carnitine-acetyl-transferase, cytochrome P-452), DNA synthesis, and hepatomegaly, may be assumed as 50 and 25 mg/kg b.w. per day, respectively. DEHP and other carcinogenic PP are neither genotoxic nor tumor initiators, but they appear to be tumor promoters, also implicating a threshold level for the carcinogenic effect. Although a causal relationship between a particular effect of peroxisome proliferation and hepatocarcinogenesis is as yet unknown, peroxisome proliferation as a whole phenomenon appears to be associated with the potential of tumor induction, as shown by comparison of the relative strength of individual PP and by comparison of species and organ specificities. Likewise, LOEL and NOEL of rodent carcinogenesis, that is, 300 and 50 to 100 mg/kg b.w. per day, respectively, are above but not too far from the corresponding values for the investigated parameters of peroxisome proliferation. Thus, with respect to dose alone, worst-case exposure in hemodialysis patients is at least 16-fold below the LOEL of any characterized PP-specific effect of DEHP and approximately 100-fold below that of DEHP-related tumorigenesis. Also, primates are less responsive to PP than rats with respect to the investigated biochemical and morphological parameters. If this lower primate responsiveness is extrapolated to estimate carcinogenicity in humans, we might thus arrive at an even larger safety margin than when based on exposure alone. Doses of PP hypolipidemics that had clearly induced several indicators of peroxisome proliferation in rats did not cause any clear-cut enhancements in the peroxisomes of patients, even though most of these hypolipidemics were considerably stronger PP than DEHP. Thus, an actual threat to humans by DEHP seems rather unlikely. Accordingly, hepatocarcinogenesis was neither enhanced in workers exposed to DEHP nor in patients treated with hypolipidemics.

Effects of the peroxisome proliferator mono(2-ethylhexyl)phthalate in primary hepatocyte cultures derived from rat, guinea pig, rabbit and monkey. Relationship between interspecies differences in biotransformation and peroxisome proliferating potencies

Primary hepatocyte cultures derived from rat, rabbit, guinea pig and monkey have been treated in vitro with metabolites of di(2-ethylhexyl)phthalate, i.e. mono(2-ethylhexyl)phthalate (MEHP), mono(5-carboxy-2-ethylpentyl)phthalate (metabolite V) and mono(2-ethyl-5-oxohexyl)phthalate (metabolite VI). In rat hepatocyte cultures MEHP and metabolite VI were equally potent in inducing peroxisome proliferation, while metabolite V was much less potent. In rat hepatocytes a 50% increase in both peroxisomal palmitoyl-CoA oxidase activity and microsomal lauric acid omega-hydroxylation activity was found after treatment with 5-15 microM MEHP. In guinea pig, rabbit and monkey hepatocyte cultures, a 50% increase in peroxisomal palmitoyl-CoA oxidase activity was found after treatment with 408-485 microM MEHP. No induction of lauric acid omega-hydroxylation activity was found. These results indicate that peroxisome proliferation can be induced by MEHP in rabbit, guinea pig and monkey hepatocytes, but that these species are at least 30-fold less sensitive to peroxisome proliferation induction than rats. The proposed mechanistic inter-relationship between induction of lauric acid omega-hydroxylation activity and peroxisome proliferation is found in rat hepatocytes, but not in hepatocytes of the other three species. Treatment of guinea pig hepatocyte cultures with MEHP resulted in an increase in triglyceride concentrations in the hepatocytes. In rat and rabbit hepatocyte cultures, triglyceride concentrations were much less altered by MEHP. In monkey hepatocytes a decrease in hepatic triglyceride concentration was found after treatment with MEHP. These effects are in agreement with in vivo effects observed before. After treatment of primary hepatocyte cultures with MEHP, high concentrations of omega- and (omega-1)-hydroxylated metabolites of MEHP were found in media from rat, rabbit and guinea pig cultures. The formation of these metabolites did not decline in time. During treatment the metabolite profile in media from rat hepatocyte cultures moved towards omega-hydroxy metabolites of MEHP. In media from monkey hepatocyte cultures the lowest concentrations of hydroxylated metabolites were determined. No major species differences were found in the potency to form oxidized MEHP metabolites, and thus no unique metabolite differences were found, which could explain the species differences in sensitivity for peroxisome proliferation.

Detection of the enzymatic activity of cytochrome P-450 enzymes by high-performance liquid chromatography

The reactions catalysed by the various cytochrome P-450 enzymes are reviewed with respect to the analysis of products by high-performance liquid chromatography (HPLC). Especially biotransformation reactions of purified cytochrome P-450 enzymes in a reconstituted system and in microsomes mainly of rat liver origin are considered. Emphasis is put on the specificity of product formation due to the individual isozymes of cytochrome P-450. It is shown that the presence of eight cytochrome P-450 isozymes can be monitored and determined by specific product formation after HPLC analysis, which is an important parameter in toxicological studies.

Microsomal lauric acid hydroxylase activities after treatment of rats with three classical cytochrome P450 inducers and peroxisome proliferating compounds

In order to investigate a proposed relationship between induction of hepatic microsomal lauric acid hydroxylase activity and peroxisome proliferation in the liver, male Wistar rats were treated with peroxisome proliferating compounds, and the lauric acid hydroxylase activity, the immunochemical detectable levels of cytochrome P450 4A1 and the activities of peroxisomal enzymes were determined. In addition, the levels of cytochrome P450 4A1 and lauric acid hydroxylase activities were studied after treatment of rats with three cytochrome P450 inducers. After treatment with aroclor-1254, phenobarbital or 3-methylcholanthrene total cytochrome P450 was 1.7-2.7 times induced. However, no induction of lauric acid omega-hydroxylase activities or P450 4A1 levels were found. After treatment of rats with di(2-ethylhexyl)phthalate (DEHP) a dose-dependent induction of lauric acid omega-hydroxylase activities, levels of cytochrome P450 4A1 and peroxisomal fatty acid beta-oxidation was found. Even at a dose-level of 100 mg DEPH/kg body weight per day a significant induction of these activities was observed. The main metabolites of DEHP, mono(2-ethylhexyl)phthalate and 2-ethyl-1-hexanol, also caused an induction of levels of P450 4A1, lauric acid omega-hydroxylase activities and the activity of peroxisomal palmitoyl-CoA oxidase. 2-Ethyl-1-hexanoic acid did not influence lauric acid omega-hydroxylase activities, but did induce levels of P450 4A1 and palmitoyl-CoA oxidase activities. Three other compounds (perfluoro-octanoic acid, valproate and nafenopin) induced both lauric acid omega-hydroxylase activity and peroxisomal palmitoyl-CoA oxidase activity. The plasticizer, di(2-ethylhexyl)adipate, did not induce levels of P450 4A1, lauric acid omega-hydroxylase activities or palmitoyl-CoA oxidase activities. With the compounds tested a close association between the induction of lauric acid omega-hydroxylase activities and peroxisomal palmitoyl-CoA oxidase activity was found. These data support the theory that peroxisome proliferating compounds do induce lauric acid omega-hydroxylase activities and that there might be a mechanistic inter-relationship between peroxisome proliferation and induction of lauric acid omega-hydroxylase activities.

Assay of fatty acid omega-hydroxylation using high-performance liquid chromatography with fluorescence labeling reagent, 3-bromomethyl-7-methoxy-1,4-benzoxazin-2-one (BrMB)

A sensitive nonradioisotopic method is reported for measuring microsomal lauric acid omega-hydroxylation activity. The assay is based upon separation and detection of 12-hydroxylauric acid formed by means of high-performance liquid chromatography following fluorescence labeling of the carboxyl group with 3-bromomethyl-7-methoxy-1,4-benzoxazin-2-one (BrMB). The use of 10-hydroxycapric acid as an internal standard affords the accurate and reproducible assay. The differential effect of dehydroepiandrosterone, a peroxisome proliferator, on the omega-hydroxylation activity in the liver and kidney of rats is also reported.