Inhibition of phylloquinone epoxide reductase by BHT quinone methide, salicylic acid and alpha-tocopherolquinone

Effects of PON1 Gene Promoter DNA Methylation and Genetic Variations on the Clinical Outcomes of Dual Antiplatelet Therapy for Patients Undergoing Percutaneous Coronary Intervention

INTRODUCTION AND OBJECTIVE

The relationship between either paraoxonase 1 (PON1) gene promoter DNA methylation or genetic variations and bleeding or major adverse cardiac events after dual antiplatelet therapy has been incompletely characterized. We aimed to systematically investigate the role of genetic variations and DNA methylation of the PON1 CpG island promoter on the clinical outcomes of dual antiplatelet therapy for patients with coronary artery disease (CAD) who underwent percutaneous coronary intervention (PCI).

METHODS

This study included 653 patients with CAD undergoing PCI and receiving dual antiplatelet therapy. Genomic DNAs were isolated from whole blood and were genotyped for the three single nucleotide polymorphisms (SNPs) of the PON1 gene. The DNA methylation levels in the PON1 promoter region were determined by bisulfite sequencing or pyrosequencing at five CpG sites (positions -142, -161, -163, -170, and -184 from the transcription start site). Clopidogrel and its metabolites in plasma were examined using high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS), and platelet function analysis was performed using the VerifyNow assay.

RESULTS

Statistically significant associations between methylation levels at five PON1 CpG sites and bleeding were observed: -184 [odds ratio (OR) 0.98, 95% confidence interval (CI) 0.96-1.00, p = 0.028]; -170 (OR 0.99, 95% CI 0.97-1.00, p = 0.048); -163 (OR 0.98, 95% CI 0.96-1.00, p = 0.029); -161 (OR 0.98, 95% CI 0.97-1.00, p = 0.026); and -142 (OR 0.98, 95% CI 0.97-1.00, p = 0.042) at a false discovery rate of <5%. Statistical analysis also revealed that aspirin reaction units (ARUs) were significantly associated with PON1 methylation level at CpG site -163 (p = 0.0342). The ARUs of patients with the PON1 126 CC genotype was 527 ± 94, which was higher than the ARUs (473 ± 89) of patients with the 126 CG genotype (p = 0.0163). Multivariate logistic regression analysis indicated that the PON1 methylation level at CpG site -161 (OR 0.95, 95% CI 0.92-0.98, p = 0.002) and the use of angiotensin-converting enzyme inhibitors (OR 0.48, 95% CI 0.26-0.89, p = 0.021) were associated with a decreased risk of bleeding events.

CONCLUSIONS

Hypomethylation of CpGs in the PON1 promoter may be a weak, albeit statistically significant, risk factor of bleeding after dual antiplatelet therapy. Further large-scale studies are needed to verify our results.

Final report on the safety assessment of BHT(1)

BHT is the recognized name in the cosmetics industry for butylated hydroxytoluene. BHT is used in a wide range of cosmetic formulations as an antioxidant at concentrations from 0.0002% to 0.5%. BHT does penetrate the skin, but the relatively low amount absorbed remains primarily in the skin. Oral studies demonstrate that BHT is metabolized. The major metabolites appear as the carboxylic acid of BHT and its glucuronide in urine. At acute doses of 0.5 to 1.0 g/kg, some renal and hepatic damage was seen in male rats. Short-term repeated exposure to comparable doses produced hepatic toxic effects in male and female rats. Subchronic feeding and intraperitoneal studies in rats with BHT at lower doses produced increased liver weight, and decreased activity of several hepatic enzymes. In addition to liver and kidney effects, BHT applied to the skin was associated with toxic effects in lung tissue. BHT was not a reproductive or developmental toxin in animals. BHT has been found to enhance and to inhibit the humoral immune response in animals. BHT itself was not generally considered genotoxic, although it did modify the genotoxicity of other agents. BHT has been associated with hepatocellular and pulmonary adenomas in animals, but was not considered carcinogenic and actually was associated with a decreased incidence of neoplasms. BHT has been shown to have tumor promotion effects, to be anticarcinogenic, and to have no effect on other carcinogenic agents, depending on the target organ, exposure parameters, the carcinogen, and the animal tested. Various mechanism studies suggested that BHT toxicity is related to an electrophillic metabolite. In a predictive clinical test, 100% BHT was a mild irritant and a moderate sensitizer. In provocative skin tests, BHT (in the 1% to 2% concentration range) produced positive reactions in a small number of patients. Clinical testing did not find any depigmentation associated with dermal exposure to BHT, although a few case reports of depigmentation were found. The Cosmetic Ingredient Review Expert Panel recognized that oral exposure to BHT was associated with toxic effects in some studies and was negative in others. BHT applied to the skin, however, appears to remain in the skin or pass through only slowly and does not produce systemic exposures to BHT or its metabolites seen with oral exposures. Although there were only limited studies that evaluated the effect of BHT on the skin, the available studies, along with the case literature, demonstrate no significant irritation, sensitization, or photosensitization. Recognizing the low concentration at which this ingredient is currently used in cosmetic formulations, it was concluded that BHT is safe as used in cosmetic formulations.

Haemorrhagic toxicity of a large dose of alpha-, beta-, gamma- and delta-tocopherols, ubiquinone, beta-carotene, retinol acetate and L-ascorbic acid in the rat

Antioxidants occasionally have become prooxidants when a large amount was ingested. The haemorrhagic toxicity of butylated hydroxytoluene, a synthetic antioxidant, may involve such a mechanism. This study investigated whether haemorrhage is induced by overdoses of tocopherols, beta-carotene, ubiquinone or L-ascorbic acid, which are representative biological antioxidants. Male Jcl:SD rats (six rats/group) were fed d-alpha, d-beta, d-gamma or d-delta-tocopherols, ubiquinone Q-10, beta-carotene or retinol acetate at a level of 0.5%, or L-ascorbic acid at 5% in the diet for 7 days. Only two rats given retinol acetate died with lung haemorrhages. Haemorrhages were observed in five or six, six, one, one, one or one of six surviving rats given d-alpha, d-beta or d-gamma-tocopherols, ubiquinone Q-10, beta-carotene or retinol acetate, respectively (except for a retinol group in which four rats survived). Major haemorrhages were noted in the epididymis. In the alpha-, beta- and gamma-tocopherol, ubiquinone Q-10, beta-carotene or retinol acetate-treated groups, prothrombin and kaoline-activated partial thromboplastin time indices were 26-28, 37, 59, 42, 63 and 65% or 27-28, 35, 65, 38, 59 and 28%, respectively, of the control values. Only the prothrombin index was significantly decreased to 67% in delta-tocopherol-administered rates, whereas controls and those receiving L-ascorbic acid showed no signs of bleeding or coagulation defect. The same tendency was also seen in the decreasing effect on vitamin K-dependent blood coagulation factors. These results suggest that the four naturally occurring tocopherols have a tendency to cause haemorrhage in the order of alpha > beta > gamma > delta, and ubiquinone Q-10 and beta-carotene als0o have relatively strong and weak haemorrhagic effects, respectively, with regard to prothrombin and partial thromboplastin time indices.

The dose-dependent effect of BHT (butylated hydroxytoluene) on vitamin K-dependent blood coagulation in rats

Earlier studies have reported a reduction of vitamin K-dependent blood clotting factor activity and incidence of haemorrhagic death in rats fed butylated hydroxytoluene (BHT); however, the vitamin K status of the animals used in these studies was claimed to be inadequate. The aim of the study reported here was to determine the effect of BHT on vitamin K-dependent clotting factors in vitamin K-sufficient and vitamin K-supplemented rats. Rats given BHT (3000 mg/kg body weight) for up to 21 days, in a diet containing a minimum of 3 ppm vitamin K3 (six times the recommended requirement), showed decreased vitamin K-dependent blood clotting factor activities, demonstrated by increases in factor-specific clotting time assays. Clotting times were prolonged within 7 days, significantly increased within 14 days (P < 0.001) and maximally increased 5.5-fold at 21 days (P < 0.05). Supplementation with a further 250 ppm vitamin K3 reversed this effect. BHT did not increase prothrombin time (PT), the usual index of clotting. However, in a similar 14-day investigation, a small but significant increase in PT (up to 151%, P < 0.005) was seen within 7 days. Further vitamin K supplementation was incapable of reversing this effect completely. A similar trend was shown by activated partial thromboplastin time. The 1/51 dilution Thrombotest, a more sensitive indicator of vitamin K-dependent clotting factor activity in the rat, was significantly increased (more than four fold, P < 0.01) within 7 days. This increase was fully reversed by further vitamin K supplementation. Prolongation of Thrombotest time was significant at a BHT dose level of 600 mg/kg body weight per day and this could be reversed by further supplementation of only 3.0 ppm vitamin K. However, at dose levels of 125 mg BHT/kg body weight per day or less, no clotting defects were observed. These studies confirm that chronic administration of more than 600 mg BHT/kg/day to rats supplied with recommended amounts of vitamin K can depress clotting factors and precipitate haemorrhagic deaths. When further vitamin K is provided, these deaths could be prevented even though not all clotting abnormalities may be reversed. This study disapproves the proposal that BHT-related clotting factor defects are confined to rats of inadequate vitamin K status, but shows that such effects do not occur at dose levels lower than 600 mg/kg/day. The results further indicate that rats receiving a high dose of BHT have a higher vitamin K requirement than would otherwise be considered necessary. However, as BHT produces no clotting defects in these animals receiving an intake 1000 times the acceptable daily intake, such clotting effects are most unlikely to indicate a human safety problem for BHT.

Biological and toxicological consequences of quinone methide formation

Quinone methides are a class of reactive, electrophilic compounds which are capable of alkylating cellular macromolecules. They are formed during xenobiotic biotransformation reactions and are hypothesized to mediate the toxicity of a large number of quinone antitumor drugs as well as several alkylphenols. In addition, oxidation of specific endogenous alkylphenols (e.g. coniferyl alcohol) and alkylcatechols (e.g. N-acetyldopamine, dopa) to quinone methides plays an important role in the synthesis of several complex plant and animal polymers, including lignin, cuticle and melanin. The role of quinone methides in these various processes is reviewed.

Vitamin K metabolism and nutriture

Vitamin K functions as a co-factor for the post-translational carboxylation of specific glutamate residues to gamma-carboxyglutamate (Gla) residues in several blood coagulation factors (II, VII, IX and X) and coagulation inhibitors (proteins C and S) in the liver; as well as a variety of extrahepatic proteins such as the bone protein osteocalcin. This review outlines some recent advances in our understanding of the metabolism of vitamin K and its role in human nutriture. The introduction of new methodologies to measure the low endogenous tissue concentrations of K vitamins and circulating plasma levels of des-gamma-carboxyprothrombin (PIVKA-II) have provided correspondingly more refined indices for the assessment of human vitamin K status. The assays for vitamin K have also been used to study the sources, intestinal absorption, plasma transport, storage and transplacental transfer of K vitamins and the importance of phylloquinone (vitamin K1) versus menaquinones (vitamins K2) to human needs. The ability to biochemically monitor subclinical vitamin K deficiency has reaffirmed the precarious vitamin K status of the newborn and led to an increased appreciation of the risk factors leading to haemorrhagic disease of the newborn and how this may be prevented. Biochemical studies are leading to an increased knowledge of the mode of action of traditional coumarin anticoagulants and how some unrelated compounds (e.g. antibiotics) may also antagonize vitamin K and cause bleeding. There is also an awareness of the possible deleterious effects of vitamin K antagonism or deficiency on non-hepatic Gla-proteins which may play some subtle role in calcium homeostasis.

Haemorrhages due to defective blood coagulation do not occur in mice and guinea-pigs fed butylated hydroxytoluene, but nephrotoxicity is found in mice

Groups of ten male Slc:ddY mice were fed a purified diet containing butylated hydroxytoluene (BHT) at levels of 0, 1.35, 1.75, 2.28, 2.96, 3.85 or 5.00%. They were kept in cages with soft-wood chips as bedding for 30 days. Groups of five Slc:ddY male mice were kept in cages with stainless-steel wire-mesh bottoms (without wood-chip bedding) and fed BHT at 0, 0.5, 1.0 or 2.0% in the diet for 21 days. Male Crj:Hartley guinea-pigs were given a purified ration containing BHT at levels of 0, 0.125 or 0.25% (five animals per group) for 14 days, or at 0, 0.5, 1.0 or 2.0% for 17 days (six animals per group). When BHT was given to mice housed in the mesh-bottomed cages there were one, one and two deaths during the experiment in the 0.5, 1.0 and 2.0% dose groups, respectively. Lung haemorrhages were observed in these dead mice, but in all other mice and guinea-pigs no haemorrhages were found. Indices of prothrombin time and kaolin-activated partial thromboplastin time were significantly decreased by up to 30 and 40%, respectively, in the mice kept on wood-chip bedding, and by up to 40 and 60% in the mice kept in cages with wire bottoms. In guinea-pigs, the prothrombin index was significantly reduced only in the 1.0% BHT group. We conclude that the BHT-induced lung haemorrhages in mice are not caused by a severe reduction in the coagulation process, as they are in rats, and that BHT does not cause bleeding like that observed in rats. However, dose-related toxic nephrosis was found in mice given 1.35-5.0% BHT in the diet. The nephrotoxic ED50(1 month) was 2.3 g/kg body weight/day. The results suggest that an extremely large dose of BHT can cause renal toxicity in mice.

Relationship of membrane fluidity, chemoprotection, and the intrinsic toxicity of butylated hydroxytoluene

In isolated rat hepatocytes, many chemicals elicit toxicity which is inhibitable by antioxidants such as butylated hydroxytoluene (BHT). Although BHT protection is evident at concentrations of less than about 50 nmol/mg protein, higher concentrations exhibit intrinsic concentration-dependent toxicity, which involves mitochondrial dysfunction. We evaluated the possibility that both chemoprotection and intrinsic toxicity could be explained by a common mechanism involving alterations in the physical properties of cellular membranes. In the red blood cell (RBC) osmotic fragility assay, BHT at less than 60 nmol/mg protein protected against osmotic fragility; however, BHT at higher concentrations enhanced osmotic fragility such that total osmolysis occurred at 135 nmol/mg. The BHT-mediated alterations in osmotic fragility correlated with changes in membrane fluidity, determined by fluorescence polarization of the hydrophobic probe 1,6-diphenyl-1,3,5-hexatriene. Protection from osmolysis correlated with decreased fluidity, while enhanced RBC fragility correlated with increased fluidity. In rat hepatocyte suspensions, high BHT concentrations also permeabilized the plasma and mitochondrial membranes to enzyme leakage, and these effects were accompanied by enhanced membrane fluidity. Although other mechanisms may be operative, alterations in membrane fluidity appear to be, in part, responsible for the observed chemoprotective effects at low concentrations, and intrinsic toxicity at higher concentrations of BHT.

The site specificity and sensitivity of the rat liver to butylated hydroxytoluene-induced damage

The food additive butylated hydroxytoluene (BHT) is capable of damaging centrilobular or periportal cells in the liver according to the dose and duration of treatment. The effect of two hepatotoxicity potentiating agents on the site specificity of acute cell damage was investigated in Sprague-Dawley rats. A 500 mg/kg oral dose of BHT did not cause overt hepatic necrosis or alter the cytochrome P450 concentration, but increased ethoxycoumarin-O-deethylation, implying an alteration in the ratio of P450 isoenzymes. Pretreatment with either phenobarbitone (3 X 80 mg/kg, ip) or the glutathione depleting agent buthionine sulfoximine (900 mg/kg, ip) produced liver necrosis in approximately 50% of animals: mainly in centrilobular areas, but with some necrosis in midzonal or periportal areas. Phenobarbitone and BHT did not significantly change the cytochrome P450 concentration, but did alter the ratio of P450 isoenzymes. In phenobarbitone-pretreated rats centrilobular hepatocyte damage was clearly localized in cells with high immunocytochemical staining for the cytochrome P450IIB subfamily. Buthionine sulfoximine and BHT reduced the cytochrome P450 concentration without reducing ethoxycoumarin-O-deethylase activity, implying a different alteration in the ratio of P450 isoenzymes. These results indicate that phenobarbitone-inducible enzymes are capable of activating high doses of BHT to reactive oxidizing intermediates, which in the absence of adequate glutathione can cause cell death. Enzymes of the P450IIB subfamily are implicated in this mechanism.

Some properties of rat platelet aggregation and effects of butylated hydroxytoluene, warfarin and aspirin

The platelet aggregation characteristics of male Sprague-Dawley (Jcl:SD) rats were investigated. Epinephrine, ristocetin, serotonin and platelet-activating factor were ineffective in rat platelets. Heparinized platelet-rich plasma (PRP) was more sensitive than citrated PRP to three aggregating agents, ADP, collagen and arachidonic acid. Butylated hydroxytoluene (BHT) and BHT quinone methide (2,6-di-tert-butyl-4-methylene-2,5-cyclohexadienone) inhibited ADP- and collagen-induced aggregation at concentrations over 10(-3) M in vitro. The ADP-, collagen- and arachidonic acid (0.5-2.0 mM)-induced aggregations of PRP obtained from rats given 1.20% BHT in the diet for 7 days were normal, while arachidonic acid (3.9 mM)-induced aggregation of PRP from BHT-fed rats was significantly lower than control. PRP from rats given aspirin and warfarin also aggregated normally with ADP or collagen addition. These results suggest that heparinized PRP may be preferable in platelet aggregation analyses in rats and reaffirmed that effects on platelet aggregation may not play a key role in BHT-induced bleeding. Platelet aggregation capacity also does not necessarily reduce in haemorrhages induced by aspirin or warfarin.

A fibroblast cell culture model to study vitamin K metabolism and the inhibition of vitamin K epoxide reductase by known and suspected antagonists

The metabolism and antagonism of vitamin K has been studied in cultured fibroblasts. Monolayers of 3T3 mouse fibroblasts (grown in the absence or presence of warfarin or other putative antagonists) were incubated for 24 h with [1',2'-3H2]phylloquinone (K1) or [1',2'-3H2]phylloquinone epoxide (K1O), the cells harvested and lipid extracts fractionated by high performance liquid chromatography. [3H]K1 was converted to [3H]K1O (about 20% of [3H] lipids) and to unidentified polar metabolites (30%). [3H]K1O was converted to [3H]K1 (3%) and to polar metabolites (50%). Cells grown with warfarin showed a marked increase in the [3H]K1O:K1 ratio and in the proportion of polar metabolites. The metabolic interconversion of K1 and K1O and inhibitory response to warfarin provide evidence for a fibroblast pathway analogous to the vitamin K-epoxide cycle in the liver. From the K1O:K1 ratios it was possible to grade the antagonism of vitamin K epoxide reductase activity by known and suspected inhibitors. Inhibitory ratios were seen for racemic warfarin down to 10(-8) M. S-warfarin was a more potent antagonist than the R-enantiomer. Consistently low K1O:K1 ratios were observed for N-methyl-thiotetrazole and antibiotics with (moxalactam) or without (cefotaxime) this side chain suggesting that none of these compounds are direct inhibitors of vitamin K epoxide reductase. Fibroblasts grown in cell culture provide a useful model to study the extrahepatic role of vitamin K and the mode of action of vitamin K antagonists.

Gastric retention and delayed absorption of a large dose of butylated hydroxytoluene in the rat

1. After a single oral dose of 800 mg/kg of butylated hydroxytoluene to rats, the plasma concentration of 2,6-di-tert-butyl-4-methylene-2,5-cyclohexadienone (BHT quinone methide), an active metabolite of BHT, reached a maximum 18 h after dosing. 2. The gastrointestinal content of BHT remained constant from 0.5 to 12 h, and began to decline 18 h after dosing. 3. BHT concentrations in epididymal and subcutaneous adipose tissues also attained maxima 18 h after dosing. 4. Volumes and weights of stomachs and contents, and volumes of gastric contents of rats given 800 mg/kg BHT, were 2-3 times larger than those of controls 4-7 h after dosing while there were no significant differences at 24-27 h after dosing. 5. Retention of ingested material in the stomachs was also observed when 18-h starved rats were given 800 mg/kg BHT. 6. Thirty minutes after intraduodenal administration of 800 mg/kg BHT to anaesthetized rats. BHT, but not BHT quinone methide, was 0.4-1.4 micrograms/ml in portal vein plasma. No BHT was detected in plasma from the aorta descendens. BHT and/or BHT radical, and BHT alone, were also found in liver and epididymal adipose tissue at 7-20 and 25-40 micrograms/g wet weight, respectively. 7. These results indicate that delay in attainment of maximum concentration of BHT or its metabolites in internal organs may be due to the long retention in the stomach, caused by high dosage and the inhibitory effect of BHT on gastric function. Moreover, most BHT may not enter lymphatic fluid but it can be absorbed into portal blood.

Hemorrhagic toxicity of d-alpha-tocopherol in the rat

When male Sprague-Dawley rats were administered d-alpha-tocopherol and butylated hydroxytoluene in the diet or intraperitoneally for 7 days, prolongations of prothrombin time and partial thromboplastin time were observed in those given both chemicals by both routes in a dose-dependent manner. However, intraperitoneal d-alpha-tocopherol was less toxic and the prothrombin and partial thromboplastin time indices were approx. 70% and approx. 60% in rats given 2.91 mmol/kg body weight daily. Rats given d-alpha-tocopherol in the diet at a daily dose of 2.31 mmol/kg body weight were approx. 13% and approx. 16% of the control, and in those dietary groups severe hemorrhages in epididymis and other organs were also observed. Plasma concentrations of total tocopherols were more increased by the dietary than the intraperitoneal route. These results suggest that the great difference in the hemorrhagic effect of d-alpha-tocopherol between dietary and intraperitoneal route administration may largely be due to the differing absorption rates of the drug by these two means.

Enhancement of the peroxidase-mediated oxidation of butylated hydroxytoluene to a quinone methide by phenolic and amine compounds

We have recently demonstrated that butylated hydroxyanisole (BHA) markedly stimulates the peroxidase-dependent oxidation of butylated hydroxytoluene (BHT) to the potentially toxic BHT-quinone methide. Using both horseradish peroxidase and prostaglandin H synthase we now report the ability of a wide variety of compounds to stimulate peroxidase-dependent activation of BHT. These compounds include several phenolic compounds commonly present in pharmacologic preparations or occurring naturally in foods. The ability of a given compound to stimulate BHT oxidation was found to depend on the type of radical it forms upon peroxidase oxidation. Compounds which have been shown to form phenoxy radicals or nitrogen-centered cation radicals were observed to enhance BHT oxidation. Conversely, compounds which are known to form peroxy radicals or semiquinone radicals either inhibited or had no effect on BHT oxidation. Compounds which enhanced BHT oxidation (monitored by covalent binding of [14C]BHT to protein) were also observed to stimulate the formation of BHT-quinone methide and stilbenequinone. This suggested a common mechanism of interaction of these compounds with BHT. The stimulation of BHT covalent binding by BHA was also seen in various human and animal tissues using either arachidonic acid or hydrogen peroxide as substrate. The possible toxicologic implications of the enhancement of peroxidase-catalyzed BHT oxidation to BHT-quinone methide are discussed.