Induction of laurate omega-hydroxylase by di (2-ethylhexyl)phthalate in rat liver microsomes

Effects of HCFC-123 exposure to maternal and infant rhesus monkeys on hepatic biochemistry, lactational parameters and postnatal growth

Peroxisome proliferators are a class of nongenotoxic rodent hepatocarcinogens that cause peroxisome proliferation and liver tumors when administered to rats and mice; but other species, including guinea pigs, dogs, and primates are less sensitive or refractory to the induction of peroxisome proliferation. Therefore, rodent peroxisome proliferators are not believed to pose a hepatocarcinogenic hazard to humans. Some peroxisome proliferators produce developmental toxicity in rats that is expressed as suppressed postnatal growth. To evaluate the relevance of the rat developmental effect to primates, groups of 4 lactating female Rhesus monkeys and their infants were exposed for 6 h/day, 7 days/week for 3 weeks to air or 1000 ppm HCFC-123. Animals were evaluated for clinical signs, body weights, clinical pathology parameters, and biochemical and pathological evaluations of liver biopsy samples. The effect of HCFC-123 exposure on milk quality (protein and fat concentration) was evaluated. The concentrations of HCFC-123 and the major metabolite, trifluoroacetic acid (TFA), were measured in the blood of the mothers and infants and in the milk. Exposure of monkeys to 1000 ppm HCFC-123 did not result in exposure-related clinical observations, or changes in body weight, appetence and behavior. There were no exposure-related effects on serum triglycerides, cholesterol, or glucose levels. HCFC-123 and TFA were present in milk, although maternal HCFC-123 exposure did not affect milk protein and fat content. In general, HCFC-123 was not detected in maternal or infant blood. TFA was detected in the majority of the mothers and TFA levels in infants ranged from 2 to 6 times higher than levels in the corresponding maternal blood. A pharmacokinetic analysis in a maternal animal indicated a peak concentration of TFA at approximately 1 h post-exposure, with a half-life of approximately 20 h. Liver microsomal P450 and peroxisome oxidase activities showed exposure-related decreases in CYP4A1 and CYP2E1 and acyl-CoA oxidase for animals exposed to HCFC-123. Microscopic evaluation of maternal liver from HCFC-123 exposed animals revealed mild to moderate centrilobular hepatocyte vacuolation, trace to mild centrilobular necrosis, and trace to mild subacute inflammation. The histopathological damage and altered hepatic biochemical activities produced by HCFC-123 in monkeys are not consistent with the HCFC-123 peroxisome proliferation response observed in rat livers. These findings demonstrate that HCFC-123 is not a peroxisome proliferator in adult Rhesus monkeys and postnatal exposure to HCFC-123 does not affect body weight of nursing infant monkeys.

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.

Omega-oxidation of monocarboxylic acids in rat brain

The accumulation of dicarboxylic acids is a prominent feature of inborn and toxin induced disorders of fatty acid metabolism which are characterized by impaired mental status. The formation of dicarboxylic acids is also a critical step in liver in the induction of intracellular fatty acid binding proteins and the proliferation of peroxisomes. In order to understand what potential roles dicarboxylic acids have in brain, we examined the extent of omega-oxidation in rat brain. Homogenates of rat brain catalyze the omega-oxidation of monocarboxylic acids with a specific activity of between 0.87 and 5.23 nmol/mg of post-mitochondrial protein/h, depending on the substrate. The activity is remarkably high, between one-fourth and 4 times the activity found in rat liver, depending on the chain length of the substrate. Specific activity increases with increasing chain length of the substrate. The omega-oxidation of palmitic acid is linear over a range of 0.125-3.0 mg of protein and 5-50 microM substrate for up to 45 minutes of incubation. The product of omega-oxidation in brain is almost exclusively dicarboxylic acid. Cultured rat neurons, astrocytes, and oligodendrocytes all contain omega-oxidation activity. Western blots of rat brain homogenate demonstrate a protein that is recognized by antibody to rat liver CYP4A omega-hydroxylase. These results demonstrate that the omega-oxidative pathway is prominent in brain and could play a role in brain fatty acid metabolism.

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.

Induction of cytochrome P-450 in the Norway rat, Rattus norvegicus, following exposure to potential environmental contaminants

Cytochrome P-450 (CYP) induction (consisting of increases in cellular RNA and protein content and associated catalytic activities) occurs predominantly in the liver, but also in small intestine, lung, kidney, and placenta, of Norway rats (Rattus norvegicus) exposed to certain types of potential environmental contaminants. The specific isoform(s) induced in the rat and the magnitudes of the increases observed depend upon the chemical nature of the xenobiotic. For instance, the predominant isoforms induced by nonhalogenated polycyclic aromatic hydrocarbons, such as petroleum derivatives and coal-tar constituents such as the benzopyrenes and the anthracenes, are those of the CYP1A subfamily. Polyhalogenated aromatic hydrocarbons, such as the halogenated dibenzodioxins, dibenzofurans, and biphenyls, may cause the induction of predominantly the CYP1A subfamily, predominantly the CYP2B subfamily, or mixed CYP1A- and CYP2B-type induction, depending upon the halogen substitution pattern. In contrast, the chlorinated hydrocarbon pesticides, such as DDT, dieldrin, chlordane, and mirex, cause almost exclusively the induction of isoforms of the CYP2B (and to a lesser extent the CYP3A) subfamilies. The commonly employed plasticizing agent di-(2-ethylhexyl)phthalate elicits predominantly induction of the CYP4A subfamily. Those xenobiotics that would be expected to be the most pervasive environmental contaminants are typically those that have also been found to cause the most profound CYP induction responses. Such chemicals are extremely lipophilic and tend to accumulate in animal tissues, especially fatty tissues such as the liver. The hepatic CYP induction response to such potential environmental contaminants is typical of the animals' response to lipophilic xenobiotics in general, and serves as a mechanism by which the excretion of such compounds from the body is facilitated.

Characterization of cytochromes P450 in liver and kidney of rats treated with di-(2-ethylhexyl)phthalate

A polyclonal antibody was made to a liver cytochrome P450 purified from di-(2-ethyl-hexyl)phthalate (DEHP)-treated Sprague-Dawley rats and was used to identify the CYP4A forms in liver and kidney cortex microsomes of control rats and rats treated with this peroxisome proliferator. Three clearly separated major protein bands were recognized on western blots in liver microsomes of control male rats or male rats treated with a single dose of DEHP, which, based on the description of relative mobility, tissue specificity, and sex dependent expression of CYP4A forms (Sundseth and Waxman (1992). J. Biol. Chem., 267, 801-810), correspond to the migration pattern of forms 4A1, 4A2, and 4A3 in clofibrate-treated rats. The administration of DEHP for 2 or 3 days caused a loss of resolution of two of the protein bands. The protein band corresponding to 4A2 was absent in liver or kidney cortex microsomes of DEHP-treated or control female rats and was not always visible in the livers of control male rats. The purified P450DEHP supported the hydroxylation of arachidonic acid at both the 19- and 20-carbon atoms with turnover rates of 1.4 +/- 0.2 and 22.7 +/- 2.5 nmoles per minute per nmol P450, respectively. No measurable amounts of hydroxylated products were obtained when prostaglandin E1, leukotriene B4, or testosterone were used as substrates. Another member of the CYP4 family, 4B1 from rabbit lung microsomes, was also recognized by this antibody on western blot analysis; however, rabbit lung form 4A4 showed only minimal cross-reactivity with this antibody.

Effects of diethyl phthalate and other plasticizers on laurate hydroxylation in rat liver microsomes

Diethyl phthalate (DEP) is used in pharmaceutical coatings, cosmetics, and plastic films to wrap foods. There is a health concern associated with the exposure to certain phthalate esters because they belong to a class of compounds referred to as peroxisome proliferators which have been shown to increase the incidence of liver tumors when administered to rats. In this study, we have compared DEP to four other commonly used plasticizers, 2-diethylhexyl phthalate (DEHP), dibutyl phthalate (DBP), 2-diethylhezyl adipate (DEHA), and acetyltributyl citrate (ATBC), for their ability to induce the cytochrome P450-mediated fatty acid omega-hydroxylation system, which is one of the initial cellular responses when animals are treated with peroxisome proliferators. The administration of DEHP, DBP, and DEHA to rats increased the specific activity of laurate 12-hydroxylase from 2.8 +/- 1.1 in control rats to 30.3 +/- 11.6, 14.5 +/- 4.1, and 9.7 +/- 1.9 nmol 12-hydroxylaurate formed/min/nmol P450, respectively. In contrast, laurate 12-hydroxylase activity in DEP- and ATBC-treated rats were 4.4 +/- 1.2 and 4.4 +/- 1.0 nmol 12-hydroxylaurate formed/min/nmol P450, respectively. In addition, whereas DEHP increased peroxisomal palmitoyl-CoA oxidation 6-fold, DEP increased this activity only 1.3-fold. Two protein bands, at 51 and 52 kDa, were found to increase 6- to 12-fold in microsomes of DEHP-, DBP-, and DEHA-treated rats, but these bands were increased only 2-fold in DEP- or ATBC-treated rats.

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.

Characterization of a cytochrome P450 from di(2-ethylhexyl) phthalate-treated rats which hydroxylates fatty acids

A cytochrome P450 was purified from liver microsomes of rats treated with di(2-ethylhexyl) phthalate (DEHP). DEHP is a member of a group of structurally diverse compounds which have been classified as peroxisome proliferators and are inducers of cytochromes P450 which hydroxylate lauric acid and other fatty acids. The P450 isolated from DEHP-treated rats (P450DEHP) was observed to have a Mr value of 51 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and a maximum absorbance of 452 nm in its reduced carbon monoxide bound state. The amino terminal residue for P450DEHP was alanine and an 18-amino acid segment at the N-terminal region was identified. The N-terminal amino acid for the P450 4A1 from clofibrate-treated rats is methionine and alignment of the N-terminal segment of the P450DEHP with P450 4A1 indicated that the first four amino acids were absent. There were two amino acid differences between the two P450s in this 18-amino acid segment; in P450DEHP an alanine and a phenylalanine were substituted for serines in P450 4A1. The P450DEHP was found to catalyze the hydroxylation of several saturated fatty acids, having the highest turnover activity with laurate (82.1 nmol 12-OH-laurate formed/min/nmol P450). Myristate, palmitate, and stearate were also metabolized but at decreasing rates. Cytochrome b5 stimulated laurate 12-hydroxylation 10-fold in a reconstituted system. Laurate was not metabolized at its 11-carbon atom; however, the longer chain length fatty acids were metabolized at the (omega-1)-carbon atom in addition to the omega-carbon atom. A polyclonal antibody to the P450DEHP recognized three protein bands in liver microsomes from control and DEHP-treated rats on Western blot analysis, but only two protein bands from phenobarbital-treated rats.

Induction of omega-oxidation of monocarboxylic acids in rats by acetylsalicylic acid

The accumulation of dicarboxylic acids, particularly long chain, is a prominent feature of Reye's syndrome and diseases of peroxisomal metabolism. We assessed the omega-oxidation of a spectrum of fatty acids in rats and asked whether pretreatment of rats with aspirin, which is known to predispose children to Reye's syndrome, would affect omega-oxidation of long chain fatty acids. We found that aspirin increased liver free fatty acids and increased the capacity for omega-oxidation three- to sevenfold. Omega-oxidation of long chain substrate was stimulated to a greater degree than medium chain substrate and was apparent within one day of treatment, at serum aspirin concentrations below the therapeutic range in humans. The apparent Km for lauric acid was 0.9 microM and 12 microM for palmitate. We also found a difference in the storage stability of activity toward medium and long chain substrate. Saturating concentrations of palmitate had no effect on the formation of dodecanedioic acid, whereas laurate decreased but never eliminated the omega-oxidation of palmitate. 97% of the total laurate omega-oxidative activity recovered was found in the microsomes, but 32% of palmitate omega-oxidative activity was present in the cytosol. These results demonstrate that aspirin is a potent stimulator of omega-oxidation and suggest that there may be multiple enzymes for omega-oxidation with overlapping substrate specificity.

Changes in hepatic lipid metabolism associated with lipid accumulation and its reversal in rats given the peroxisome proliferator LY171883

Dietary administration of 0.05, 0.1, and 0.3% LY171883 to rats for 1 day caused a dose-related increase in hepatic triglycerides. When added to rat liver mitochondria in vitro, LY171883 caused competitive inhibition of carnitine palmitoyltransferase 1 (CPT-1), the rate-limiting enzyme for mitochondrial fatty acid oxidation. This effect appears to be involved in the lipid accumulation. The hepatic triglycerides in rats given 0.1% LY171883 increased progressively through 3 months of treatment. In contrast, hepatic triglycerides in high-dose rats returned to control levels by Day 3 and remained there throughout the study. The regression of the lipid corresponded with increases in hepatic peroxisomal beta-oxidation, mitochondrial beta-oxidation, and CPT-1 activity of up to 13-, 7-, and 3.2-fold, respectively. The 0.1% dose increased these parameters modestly compared to those of high-dose rats (2-, 3-, and 1.6-fold, respectively). Addition of LY171883 to mitochondria from rats given dietary treatment for 2 weeks inhibited CPT-I by the same percentage as in control mitochondria. In mid-dose rats, the induction of CPT-I was largely negated by LY171883 in vitro. Even with the inhibition, CPT-I activity in mitochondria from high-dose rats remained 2-fold higher than that in untreated controls. The data suggest that the induction of CPT-I in high-dose rats was sufficient to overcome the inhibitory action of LY171883. The increased oxidative capacity in peroxisomes and mitochondria led to the regression of the lipid in high-dose rats. The more modest increases in fatty acid oxidation in rats given 0.1% LY171883 were not sufficient to reverse the lipid accumulation.

Cloning and expression of three rabbit kidney cDNAs encoding lauric acid omega-hydroxylases

cDNAs encoding three cytochrome P-450 enzymes were cloned from a rabbit kidney cDNA library. These three cDNAs exhibit greater than 90% nucleotide sequence identity across the coding region. This degree of sequence identity is also seen with P450IVA4, an enzyme that catalyzes the omega-hydroxylation of prostaglandins and that is elevated during pregnancy and induced by progesterone in rabbit lung. The 3' untranslated regions of the three cDNAs display very little sequence identity, suggesting that they are the products of distinct genes. The predicted amino acid sequences derived from each cDNA and for P450IVA4 exhibit about 85% identity. Each cDNA was inserted into an expression vector for transient transfection of COS-1 cells. The transfected cells each expressed a protein recognized by antibodies to P450IVA4. Microsomes isolated from the cells transfected with each cDNA efficiently catalyzed the omega-hydroxylation of lauric acid with rates that greatly exceed that catalyzed by microsomes isolated from the host cell line. One of the cDNAs encodes an enzyme that omega-hydroxylates prostaglandin A1; however, the specific activity was 2 orders of magnitude lower than that for lauric acid. Our results indicate that the substrate selectivity of the kidney P-450s encoded by these cDNAs is distinct from that of the lung P450IVA4 and that multiple enzymes comprise P-450 class IVA in the rabbit.

Inhibition of hepatic fatty acid oxidation by bezafibrate and bezafibroyl CoA

The acute effect of the hypolipidemic agent bezafibrate on fatty acid oxidation was studied in rat hepatocytes and mitochondria. Bezafibrate caused a concentration-related inhibition of oleate oxidation in liver cells. In mitochondria bezafibrate inhibited the oxidation of palmitoyl CoA but had no effect on palmitoylcarnitine oxidation, suggesting the site of inhibition was the formation of the carnitine derivative. Bezafibrate and bezafibroyl CoA inhibited the overt carnitine palmitoyltransferase (I) in rat liver mitochondria with comparable potency but with distinct kinetics. The inhibition caused by bezafibrate was not prevented by omission of Mg++-ATP from the assay mixture, indicating activation of bezafibrate to bezafibroyl CoA was not required for inhibition. The data demonstrate that bezafibrate, like several other peroxisome proliferating agents, inhibits mitochondrial fatty acid oxidation in rat liver. The inhibition may be relevant to the mechanism of peroxisome proliferation.

High pressure liquid chromatography methods for separation of omega- and (omega-1)-hydroxy fatty acids: their applications to microsomal fatty acid omega-oxidation

Fatty acids (C12-C18) and their omega- and (omega-1)-hydroxy derivatives, when converted to p-bromophenacyl (PBP) esters, can be completely separated from one another by high pressure liquid chromatography (HPLC) on a silicic acid column using 0.5% (v/v) isopropanol in n-hexane. In this system, fatty acid PBP esters are eluted at the solvent front, whereas the retention times of the omega- and (omega-1)-hydroxy derivatives are 14-20 and 24-29 min, respectively. The PBP esters can also be separated by reverse phase HPLC on a muBondapak C18 column, a method which has been developed by Fan et al. (Fan, L. L., Masters, B. S. S., and Prough, R. A. (1976) Anal. Biochem. 71, 265-272) for separation of methyl esters of fatty acids and their omega- and (omega-1)-hydroxy derivatives. In the latter method, however, the retention times of omega- and (omega-1)-hydroxy derivatives are only about 2 min apart and an increase in the solvent polarity is needed for elution of the esters of unmodified fatty acids. Fatty acid PBP esters, however, can be obtained as independent peaks which are not disturbed by the solvent front. An application of the former method to measure fatty acid omega oxidation by liver microsomes and by a reconstituted monooxygenase system containing purified cytochrome P-450 is described.

Alteration of hepatic phospholipids in rats and mice by feeding di-(2-ethylhexyl)adipate and di-(2-ethylhexyl)phthalate

Effects of di-(2-ethylhexyl)adipate (DOA) and di-(2-ethylhexyl)phthalate (DEHP), plasticizers for polyvinylchloride products, on concentrations and compositions of hepatic phospholipids were studied in rats. When administered to rats at a 2% level for 2 wk, both DOA and DEHP caused a hepatomegaly, an increase in hepatic phospholipids and a decrease an increase in hepatic phospholipids and a decrease in the ratio of phosphatidylcholine (PC) to phosphatidylethanolamine (PE). In the comparable study with mice, the alkyl moiety of DOA was found to be responsible for these alterations. DOA and DEHP specifically altered fatty acid compositions of PC and PE: there was an increase in oleic and palmitic acids and a decrease in stearic and docosahexaenoic acids in PC and an increase in arachidonic acid at the expense of docosahexaenoic acid in PE. In addition, DOA caused an increase in the trienoic and tetraenoic molecular species in PC and an increase in the 1-palmitoyl-2-arachidonyl (16:0@20:4) species in PE. Thus, the effects of DOA on the lipid dynamics resembled those observed with DEHP, although the magnitude was slightly moderated.

Differential induction profile of drug-metabolizing enzymes after treatment with hypolipidaemic agents

Various hypolipidaemic agents differentially induced microsomal drug-metabolizing enzymes. Clofibrate, clofibric acid, fenofibric acid and dulofibrate, which are mainly hypotriglyceridaemic, increased the content in cytochromes P-450 (77-185% over control), and especially cytochrome P-452-dependent lauric acid 12-hydroxylation (5.6- to 8.4-fold increase). Bilirubin glucuronidation was 2.1- to 2.8-fold stimulated; epoxide hydrolase activity (benzo(a)pyrene-oxide) was only slightly increased by the drugs. By contrast, F1379, which lowers plasma cholesterol only, did not change cytochromes P-450 content and slightly affected the 12-hydroxylation of lauric acid. It dramatically enhanced the epoxide hydrolase activity (7.6-fold), and increased (200%) the glucuronidation of planar group I substrates (4-nitrophenol, 4-methylumbelliferone, 1-naphthol). These effects were accompanied by a highly positive staining of gamma-glutamyltransferase in the liver characterized by a great number of intensively coloured foci in the periportal and perilobular area of the tissue. Treatment of rats for three weeks with F1379 did not modify this typical profile in enzyme induction. Such continuous effect could reveal some biochemical changes of hepatocytes with important toxicological relevance. Compared to the parent compound, treatment of rats with two metabolites of F1379 led to a decrease in the induction potency on epoxide hydrolase and on the forms I of UDP-glucuronosyltransferase; by contrast, the content in cytochromes P-450 was increased.