Anver D. Rahimtula

In vitro metabolism of 3-t-butyl-4-hydroxyanisole and its irreversible binding to proteins

A method for the preparation of methyl-labelled 3-t-butyl-4-hydroxyanisole (BHA) is described. Metabolism of [14C]BHA using four different enzyme systems (liver microsomes + NADPH; liver microsomes + cumene hydroperoxide (CHP); sheep seminal vesicle (SSV) microsomes (as a source of prostaglandin synthetase) + arachidonic acid (AA); horseradish peroxidase (HRP) + hydrogen peroxide) was investigated. In all systems, BHA was oxidized to a variety of products including formaldehyde, a dimer di-BHA, polar and water soluble metabolites as well as a reactive intermediate(s) that binds irreversibly to proteins. With liver microsomes and NADPH, phenobarbital (PB) induction gave increased yields of all products while 3-methylcholanthrene (MC) induction specifically increased protein binding but decreased other metabolite formation. BHA addition effectively discharged the activated oxygen complex of cytochrome P-450 (liver microsomes) as well as Comp. I and Comp. II of HRP suggesting that it is a good one electron peroxidase donor. BHA addition also increased the net rate of NADPH oxidation in the presence of liver microsomes suggesting uncoupling. It is proposed that in all system investigated BHA is oxidized predominantly via a one electron oxidation process to yield first the BHA free radical which then dimerizes, forms more products or binds to proteins.

Hydroperoxide-dependent activation of p-phenetidine catalyzed by prostaglandin synthase and other peroxidases

p-Phenetidine is metabolized by ram seminal vesicle (RSV) microsomes, horseradish peroxidase (HRP) and rat liver microsomes to protein-binding products. These reactions are very rapid and depend on the presence of arachidonic acid (AA) or various hydroperoxidases. The RSV- and HRP-mediated binding was inhibited more than 80% by the addition of reduced glutathione (1 mM) or the antioxidant butylated hydroxyanisole (0.5 mM). Indomethacin (100 microM) and acetylsalicylic acid (1 mM) reduced the AA-dependent reaction in RSV microsomes to less than 5% of control values. When hydrogen peroxide replaced AA, the RSV/H2O2-supported binding in the presence of 50 microM p-phenetidine proceeded at rates similar to that observed with RSV/AA. Unlike the AA-dependent reaction, the H2O2-supported reaction showed no inhibition of protein binding at higher p-phenetidine concns. The data in this report are consistent with a peroxidatic activation of p-phenetidine possibly involving an amine radical catalyzed by prostaglandin synthase (PGS) present in RSV microsomes as well as by other peroxidases. The mechanism for this activation and physiological implications are discussed.

Mechanisms of petroleum hydrocarbon toxicity: Studies on the response of rat liver mitochondria to Prudhoe Bay crude oil and its aliphatic, aromatic and heterocyclic fractions

The effect of Prudhoe Bay crude oil (PBCO) and its different fractions [aliphatic, aromatic, heterocyclic (NOS)] on the bioenergetic functions of isolated rat liver mitochondria were studied. A DMSO extract of PBCO inhibited state 3 respiration (in the presence of ADP) with either succinate or beta-hydroxybutyrate as substrate. The ascorbate-TMPD dependent state 3 respiration was not affected. Succinate dehydrogenase and beta-hydroxybutyrate dehydrogenase activities were also lost in the presence of the PBCO extract suggesting that inhibition of state 3 respiration may be due to blockage of the electron transport chain. Stimulation of state 4 respiration (in the absence of ADP) and of the oligomycin sensitive ATPase activity by the PBCO extract was observed. Fractionation of PBCO indicated that the aromatic fraction was mainly responsible for its inhibitory effects. By comparison, the heterocyclic fraction had weak inhibitory properties while the aliphatic fraction was essentially inactive. It is concluded that the aromatic components of PBCO inhibit mitochondrial respiration and oxidative phosphorylation mainly through impairment of the mitochondrial membrane and inhibition of beta-hydroxybutyrate and succinate dehydrogenase supported electron transfer activities of the respiratory chain.

Effectiveness of a Prudhoe Bay crude oil and its aliphatic, aromatic and heterocyclic fractions in inducing mortality and aryl hydrocarbon hydroxylase in chick embryo in ovo

Prudhoe Bay crude oil (PBCO) and its aliphatic, aromatic and heterocyclic fractions were tested on the developing chick embryo for (i) embryotoxicity (ii) their ability to induce hepatic and renal cytochrome P450 levels as well as hepatic, renal and pulmonary aryl hydrocarbon hydroxylase activities. On the basis of its concentration in PBCO, the aromatic fraction was responsible for most of the embryotoxicity as well as for the enzyme inducing ability. The NOS fraction constituted less than 7% (w/v) of PBCO but, on a weight equivalent basis, was roughly as potent as the aromatic fraction in causing embryotoxicity and in inducing cytochrome P450 levels and aryl hydrocarbon hydroxylase. The aliphatic fraction was found to be essentially inactive. The results are consistent with the concept that elevation of aryl hydrocarbon hydroxylase levels by certain components of PBCO may lead to increased embryotoxicity.

Prostaglandin synthase-catalyzed metabolic of some aromatic amines to genotoxic products

Abstract Cultured human fibroblasts were incubated with different aromatic amines in the presence of different activation systems and the induction of strand breaks in fibroblasts DNA was studied. In the presence of ram seminal vesicle microsomes and arachidonic acid, DNA strand breaks were induced by 2-naphthylamine, 2,4-diaminotoluenee and 4-methoxy- m -phenylenediamine. This effect was decreased when the prostaglandin synthase of the ram seminal vesicle microsomes was inhibited. The data suggest that metabolic activation catalyzed by prostaglandin synthase may be of importance in the formation of genotoxic products by certain urinary tract carcinogens.

Alterations in ATP-dependent calcium uptake by rat renal cortex microsomes following ochratoxin A administration in vivo or addition in vitro

A disruption of calcium homeostasis, leading to a sustained increase in cytosolic calcium levels, has been associated with cytotoxicity in response to a variety of agents in different cell types. We have observed that administration of a single high dose or multiple lower doses of the carcinogenic nephrotoxin ochratoxin A (OTA) to rats resulted in an increase of the renal cortex endoplasmic reticulum ATP-dependent calcium pump activity. The increase was very rapid, being evident within 10 min of OTA administration and remained elevated for at least 6 hr thereafter. The increase in calcium pump activity was inconsistent with previous observations that OTA enhances lipid peroxidation (ethane exhalation) in vivo, a condition known to inhibit the calcium pump. However, no evidence of enhanced lipid peroxidation was observed in the renal cortex since levels of malondialdehyde and a variety of antioxidant enzymes including catalase, DT-diaphorase, superoxide dismutase, glutathione peroxidase, glutathione reductase and glutathione S-transferase were either unaltered or reduced. In in vitro studies, addition of OTA to cortex microsomes during calcium uptake inhibited the uptake process although the effect was reversible. Preincubation of microsomes with NADPH had a profound inhibitory effect on calcium uptake but inclusion of OTA was able to reverse the inhibition. Changes in the rates of microsomal calcium uptake correlated with changes in the steady-state levels of the phosphorylated Mg2+/Ca(2+)-ATPase intermediate, suggesting that in vivo/in vitro conditions were affecting the rate of enzyme phosphorylation.

Mechanisms of petroleum hydrocarbon toxicity: Functional changes in rat liver mitochondria after exposure to a prudhoe bay crude oil

Administration of a Prudhoe Bay Crude Oil (PBCO) to rats (5 ml/kg/day for 2 days) resulted in a 30-35% inhibition of liver mitochondrial state 3 respiration with beta-hydroxybutyrate or succinate as substrates. beta-Hydroxybutyrate dehydrogenase, succinate dehydrogenase, NADH oxidase and succinate oxidase activities were also decreased in PBCO-treated rats. A significant increase in latent ATPase activity was also observed. Our results suggest that PBCO exerts its effects on liver mitochondria through the inhibition of beta-hydroxybutyrate- and succinate-supported electron transfer activities and by impairment of the mitochondrial membrane.

Dietary administration of 2(3)-t-butyl-4-hydroxyanisole elevates mouse liver microsome-mediated DNA binding and mutagenicity of aflatoxin B1

Administration of the phenolic antioxidant 2(3)-t-butyl-4-hydroxyanisole (BHA) to mice resulted in a 2-3-fold increase in the liver microsome catalyzed irreversible binding of aflatoxin B1 (AFB1) to calf thymus DNA and up to a 5-fold increase in the ability to induce mutations in Salmonella typhimurium TA98. Maximum induction of AFB1 binding to DNA occurred after 2 days of BHA administration whereas cytosolic glutathione S-transferase was maximally induced (6-fold) only after 10 days of BHA feeding. The induction of a new cytochrome P-450 species was indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and an enhanced sensitivity to inhibition by metyrapone and alpha-naphthoflavone. Addition of control cytosol (containing glutathione S-transferase) + glutathione to control microsomes decreased AFB1 binding to DNA by 26%. However, replacement of control cytosol by BHA cytosol which contained 6 times more glutathione S-transferase only marginally enhanced the inhibition to 38%. These data suggest that BHA may exert its effect in the liver primarily through an alteration of the cytochrome P-450 dependent activation process although an increase in the conjugation of reactive metabolite may play a contributory role.

Possible role of an iron-oxygen complex in 4(S)-4-hydroxyochratoxin a formation by rat liver microsomes

Rat liver microsomes were examined for their ability to oxidize the mycotoxin ochratoxin A (OTA) to 4(R)-4-hydroxyochratoxin A [(R)-4-OH-OTA] and 4(S)-4-hydroxyochratoxin A [(S)-4-OH-OTA] and to induce OTA-dependent lipid peroxidation. Microsomes isolated from rats pretreated with pregnenolone-16 alpha-carbonitrile greatly induced both (R)-4-OH-OTA and (S)-4-OH-OTA formation whereas isoniazid pretreatment primarily induced (S)-4-OH-OTA. (R)-4-OH-OTA and (S)-4-OH-OTA formation showed significant differences with respect to pH optima, effect of antioxidants, and iron chelators. (R)-4-OH-OTA showed a pH optimum of 6.5 and was not inhibited by the antioxidants butylated hydroxyanisole or N,N-diphenyl-1,4-phenylenediamine or the iron chelators. Desferal or bathophenanthrolinedisulfonic acid. In contrast, both (S)-4-OH-OTA and lipid peroxidation showed a pH optimum of 7.0 and both activities were sensitive to inhibition by the above antioxidants and iron chelators. Lipid peroxidation was not involved in (S)-4-OH-OTA formation since addition of linoleic acid hydroperoxide to microsomes did not give rise to (S)-4-OH-OTA. Cytochrome P450 appeared to be essential since other hemoproteins like horseradish peroxidase and hemoglobin were ineffective in metabolizing OTA in the presence of hydroperoxides. The results suggest that (R)-4-OH-OTA is formed by normal mixed-function oxidation but that (S)-4-OH-OTA formation may involve free iron. It is likely that an active Fe2(+)-oxygen complex, formed via NADPH-cytochrome P450 reductase and cytochrome P450-dependent reduction of free Fe3+ followed by oxygen binding, serves as the species inducing lipid peroxidation and at least part of (S)-4-OH-OTA formation.

THE NATURE OF THE ACTIVATED OXYGEN INVOLVED IN THE HYDROXYLATION REACTIONS CATALYZED BY CYTOCHROME P450 FROM A STUDY OF ITS PEROXIDASE PROPERTIES

Publisher Summary This chapter discusses the nature of the activated oxygen involved in the hydroxylation reactions catalyzed by cytochrome P-450 from a study of its reroxidase properties. It has been shown that liver cytochrome P-450 is responsible for the massive lipid peroxidation that ensues when hydroperoxides are injected into rats and results in extensive tissue damage and death. It is now believed that the activated oxygen of peroxidase compound I of cytochrome P-450 catalyzes rapid lipid peroxidation, that is, unsaturated lipid can act as a peroxidase donor. Different cytochrome P-450 and P-450s have different peroxidase activities and affinity for cumene hydroperoxide. Thus, cytochrome P-450 from liver catalyzes the hydroperoxide supported hydroxylations far more readily than other tissues. While enhancing NADPH-supported hydroxylation of benzopyrene or acetanilide, 3-methylcholanthrene induction actually decreases the hydroperoxide-supported hydroxylation. The chapter discusses the mechanism of cytochrome P-450 and the nature of the 440 nm complex. It also describes the implication of the peroxidase activity in cytochrome P-450 function.