Biomedical subjects

A D Rahimtula

Publications and source records attributed to A D Rahimtula.

At least 19 recordsLinked to original sources

PubMed1996

Effect of cytochrome P450 induction on the metabolism and toxicity of ochratoxin A.

Liver microsomes from rats treated with various P450 inducers were examined for their ability to metabolize the mycotoxin ochratoxin A (OTA) to 4(R)-4-hydroxyochratoxin A (4R), the major metabolite, and 4(S)-4-hydroxyochratoxin A (4S), the minor metabolite. Pretreatment of rats with phenobarbital (PB), dexamethasone (DXM), 3-methylcolcanthrene (3MC) and isosafrole (ISF) greatly induced 4R formation. PB, DXM, 3MC, clofibrate (CLF) and ISF treatments also induced 4S formation. Isoniazid (INH) pretreatment primarily induced 4S formation. The pH optimum for 4R formation was found to be 6.0 with 3MC microsomes, and 6.5 with PB and DXM microsomes. For 4S formation, the pH optimum was 7.0. At the optimum pH (compared with pH 7.4), 4R formation increased 40-50% with PB and DXM microsomes but 8.0-fold with 3MC microsomes. Studies using the inhibitors metyrapone and alpha-naphthoflavone as well as monoclonal antibodies against various P450s suggested that at least the P450 isoforms IA1/IA2, IIB1 and IIIA1/IIIA2 are involved in 4R formation. Using urinary excretion of the enzymes alkaline phosphatase and gamma-glutamyl transferase as an index of renal damage, we observed that pretreatment of rats with PB, which induced hepatic P450 (P450II2B1), protected against OTA nephrotoxicity, whereas cobalt-protoporphyrin IX pretreatment, which decreased P450 levels, exacerbated OTA nephrotoxicity. Our results suggest that at least P450IIB1-dependent metabolism of OTA leads to its detoxication and that OTA itself may be toxic in some circumstances or that other pathways are responsible for its activation.

Alkaline Phosphatase↗

PubMed1993

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.

Animals↗

PubMed1992

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.

Adenosine Triphosphate↗

PubMed1991

Role of cytochrome P-450 in ochratoxin A-stimulated lipid peroxidation.

The role of cytochrome P-450 in the stimulation of lipid peroxidation by the nephrotoxic mycotoxin ochratoxin A has been investigated. Ochratoxin A was previously shown to markedly stimulate lipid peroxidation in a reconstituted system consisting of phospholipid vesicles, NADPH-cytochrome P-450 reductase, Fe3+, ethylenediaminetetraacetic acid (EDTA), and reduced nicotinamide adenine dinucleotide phosphate (NADPH). We now show that purified cytochrome P-450IIB1 could effectively replace EDTA in stimulating lipid peroxidation suggesting that it could mediate the transfer of electrons from NADPH to Fe3+. Cobalt protoporphyrin is known to cause an extensive and long-lasting depletion of hepatic cytochrome P-450 in rats, and it has been used to evaluate the role of hepatic cytochrome P-450 in xenobiotic metabolism and toxicity. We have observed that microsomes isolated from livers of cobalt protoporphyrin-pretreated rats underwent ochratoxin A-dependent lipid peroxidation much more slowly than control microsomes. Also, the level of ethane exhaled (an index of in vivo lipid peroxidation) on ochratoxin A administration was much lower in cobalt protoporphyrin-pretreated rats than in control rats. Taken together, these results provide evidence for the stimulatory role of cytochrome P-450 in ochratoxin A-induced lipid peroxidation in a reconstituted system and strongly implicate its role in microsomal and in vivo ochratoxin A-induced lipid peroxidation.

Animals↗

PubMed1991

In vitro inhibition of rat platelet aggregation by ochratoxin A.

The mycotoxin ochratoxin A (OA) consists of 5-chloro-3-methyl-3,4-dihydro-8-hydroxyisocoumarin moiety linked by an amide bond to beta-L-phenylalanine. When added to washed rat platelets in vitro, OA caused a dose-dependent inhibition of aggregation induced by agonists such as adenosine diphosphate (ADP) or thrombin. The aggregatory response induced by prior addition of an agonist was also reversed in a dose-dependent manner by OA. Inhibition of aggregation appeared to be irreversible since exposure of platelets to OA followed by several washings removed most of the mycotoxin associated with the platelets but did not diminish the inhibitory response. Serotonin secretion from dense granules and arachidonic acid release from membrane phospholipid (especially phosphatidylcholine) as well as its further metabolism were also inhibited by OA. These results suggest that a disruption of the platelet plasma membrane structure by OA is probably responsible for inhibition of the primary and secondary phases of aggregation.

Animals↗

PubMed1991

Alterations in calcium homeostasis as a possible cause of ochratoxin A nephrotoxicity.

Disruption of calcium homeostasis, leading to a sustained increase in cytosolic calcium level, has been associated with cytotoxicity in response to a variety of agents in different cell types. We have observed that a single high dose or multiple lower doses of ochratoxin A administered to rats resulted in an increase in renal endoplasmic reticulum calcium pump activity. The increase was very rapid, being evident within 10 min of ochratoxin A administration and remained elevated for at least 6 h thereafter. Ochratoxin A also decreased renal mitochondrial state-3 respiration and calcium uptake. The latter may lead to an increase in cytosolic calcium level, and the increase in microsomal calcium uptake activity may be an attempt to restore calcium homeostasis. Repeated moderate doses of ochratoxin A led to an eventual decrease in microsomal calcium pump activity, and this could lead to even higher cytosolic calcium levels. Changes in the rate of microsomal calcium uptake correlated with changes in the steady-state levels of the phosphorylated Mg2+/Ca(2+)-ATPase intermediate, indicating that this enzyme is responsible for the calcium pump activity.

Animals↗

PubMed1990

NADPH-cytochrome-P-450 reductase promoted hydroxyl radical production by the iron(III)-ochratoxin A complex.

The Fe3+ complex of ochratoxin A has been shown to produce hydroxyl radicals in the presence of NADPH and NADPH-cytochrome-P-450 reductase. ESR spin-trapping experiments carried out in the presence of the hydroxyl radical scavenger ethanol and the spin trap DMPO (5,5-dimethyl-1-pyrroline-1-oxide) produced ESR spectra characteristic of the hydroxyl radial-derived carbon-centered DMPO-alkoxyl radical adduct. Thus hydroxyl radicals produced by the Fe3(+)-ochratoxin A complex in the presence of an enzymatic reductase may be be partly responsible for ochratoxin A toxicity.

Electron Spin Resonance Spectroscopy↗

PubMed1990

Mechanism of ochratoxin A stimulated lipid peroxidation.

Lipid peroxidation, measured as malondialdehyde formation or by oxygen uptake, was stimulated markedly by the mycotoxin ochratoxin A (OTA) in a reconstituted system consisting of phospholipid vesicles, the flavoprotein NADPH-cytochrome P450 reductase, Fe3+, EDTA and NADPH. Deletion of EDTA lowered the extent of lipid peroxidation but did not eliminate it. Fluorometric and spectrophotometric studies demonstrated the formation of a 1:1 Fe3(+)-OTA complex. The rate of reduction of Fe3+ to Fe2+ was enhanced markedly in the presence of OTA, and there was a further increase in the rate when EDTA was also included. The data indicate that OTA stimulates lipid peroxidation by complexing Fe3+ and facilitating its reduction. Subsequent to oxygen binding, an iron-oxygen complex of undetermined nature initiates lipid peroxidation. Free hydroxyl radicals appear not to participate in lipid peroxidation stimulated by Fe3(+)-OTA.

Animals↗

PubMed1989

Perturbation of liver microsomal calcium homeostasis by ochratoxin A.

The effect of ochratoxin A on hepatic microsomal calcium sequestration was studied both in vivo and in vitro. The rate of ATP-dependent calcium uptake was inhibited by 42-45% in ochratoxin A intoxicated rats as compared to controls. In the presence of NADPH, addition of ochratoxin A (2.5 to 100 microM) caused a concentration-dependent inhibition of calcium uptake (28-94%) by untreated rat liver microsomes. The rate of NADPH-dependent lipid peroxidation, measured as malondialdehyde formed, was also greatly enhanced by ochratoxin A. Various agents that inhibited ochratoxin A enhanced lipid peroxidation were also able to block the destruction of calcium uptake activity. Lipid peroxidation enhanced by ochratoxin A was also accompanied by leakage of calcium from calcium-loaded microsomes. These results suggest that ochratoxin A disrupts microsomal calcium homeostasis by an impairment of the endoplasmic reticulum membrane probably via enhanced lipid peroxidation.

Animals↗

PubMed1989

Effect of a Prudhoe Bay crude oil on hepatic and renal peroxisomal beta-oxidation and mixed-function oxidase activities in rats.

The effect of oral administration of a Prudhoe Bay crude oil (PBCO) to male rats (PBCO, 2.6 g/kg body weight, daily) for 5-12 days on hepatic and renal microsomal monooxygenase activities and peroxisomal beta-oxidation has been investigated. PBCO administration leads to liver enlargement. This is associated with induction of microsomal cytochrome P-450 levels (1.6- to 2.0-fold) and dependent mixed-function oxidase activities (7-ethoxyresorufin-O-deethylase and 7-pentoxyresorufin-O-depentylase, representing cytochrome P-450I and cytochrome P-450IIB isoenzymes respectively, 9- to 15-fold; omega-oxidation of lauric acid representing the cytochrome P-450IVA1 isoenzyme, 1.4- to 1.5-fold) along with peroxisomal beta-oxidation (palmitoyl CoA oxidation, 2- to 5-fold). It was observed that rats exposed to PBCO showed an increase in renal microsomal cytochrome P-450 content (1.6- to 2.3-fold), cytochrome P-450I activity (5- to 8-fold) and omega-oxidation activity (1.3- to 1.4-fold). However, renal peroxisomal beta-oxidation was unaltered. Serum total triglycerides were lowered by 41-46% after PBCO exposure. These results suggest that induction of peroxisomal beta-oxidation and possibly mono-oxygenases may be related to the carcinogenic/tumorigenic potential of crude oil.

Animals↗

PubMed1988

Lipid peroxidation as a possible cause of ochratoxin A toxicity.

Addition of the mycotoxin ochratoxin A (OA), a nephrotoxic carcinogen, to rat liver microsomes greatly enhanced the rate of NADPH or ascorbate-dependent lipid peroxidation as measured by malondialdehyde formation. NADPH-dependent lipid peroxidation in kidney microsomes was similarly enhanced by OA. The process required the presence of trace amounts of iron but cytochrome P-450 and free active oxygen species appeared not to be involved. The efficiency of several ochratoxins (ochratoxins A, B, C, alpha and O-methyl-ochratoxin C) to enhance lipid peroxidation was related to the presence and reactivity of the phenolic hydroxyl group. Furthermore, the ability of these ochratoxins to enhance lipid peroxidation in microsomes correlated precisely with their known toxicities in chicks. Administration of ochratoxin A to rats also resulted in enhanced lipid peroxidation in vivo as evidenced by a seven-fold increase in the rate of ethane exhalation. These results suggest that lipid peroxidation may play a role in the observed toxicity of ochratoxin A in animals; a mechanism is proposed. (Formula: see text). Ochratoxin A: X = Cl; R1 = R2 = R3 = R4 = H Ochratoxin B: X = H; R1 = R2 = R3 = R4 = H Ochratoxin C: X = Cl; R1 = R2 = R3 = H; = R4 = CH3 O-Methyl-ochratoxin C: X = Cl; R2 = R3 = H; R1 = R4 = CH3 (4R)-4-hydroxyochratoxin A: X = Cl; R1 = R3 = R4 = H; R2 = OH (4S)-4-hydroxyochratoxin A: X = Cl; R1 = R2 = R4 = H; R3 = OH Fig. 1. Chemical structures of the various ochratoxins.

Animals↗

PubMed1988

Comparison of the inhibitory effects of some compounds present in crude oils on rat platelet aggregation: role of intra- and extra- cellular calcium.

In vitro addition of some representative aliphatic, aromatic or heterocyclic compounds present in petroleum crude oils to washed rat platelets resulted in a concentration-dependent inhibition of aggregation induced by ADP or thrombin. Increasing concentration of extracellular Ca2+ did not alter the pattern of inhibition. ADP-induced intracellular Ca2+ mobilization was unaffected by most of the compounds tested. However, Ca2+ uptake was significantly inhibited when platelets were preincubated with these agents. This suggests that some components of crude oil may inhibit platelet aggregation by bringing about alterations in the platelet plasma membrane.

Adenosine Diphosphate↗

PubMed1987

The hepatotoxic potential of a Prudhoe Bay crude oil: effect on mouse liver weight and composition.

The hepatotoxic properties of a Prudhoe Bay Crude Oil (PBCO) were evaluated in mice. Administration of PBCO (5.0 ml/kg body wt, daily for 2 days) to mice resulted in an increase in (i) liver wet and dry weight, (ii) hepatic total proteins, RNA, glycogen and total lipids, and (iii) individual lipids such as cholesterol, triglycerides and phospholipids. Hepatic protein biosynthesis, determined in vivo by administration of L-[14C]leucine was increased in PBCO exposed mice. The rate of 3H incorporation from 3H2O was significantly enhanced in liver fatty acids, cholesterol, triglycerides and thus ultimately in total lipids. Also, an increase in 3H incorporation was noticed in hepatic glycogen after PBCO administration. The results suggest that PBCO may induce hepatotoxicity by altering the intermediary metabolism of biochemical constituents.

Animals↗

PubMed1987

Inhibition of rat platelet aggregation by a Prudhoe Bay crude oil and its aliphatic, aromatic, and heterocyclic fractions.

Washed platelets isolated from rats 24 hr after oral treatment with a Prudhoe Bay crude oil (PBCO) showed a substantial inhibition of aggregation induced by ADP, arachidonic acid, or epinephrine. In vitro addition of a dimethyl sulfoxide extract of PBCO or its aliphatic, aromatic, or heterocyclic fractions to washed platelets also resulted in an inhibition of aggregation. ADP release was inhibited in platelets to which an extract of PBCO or its fractions were added in vitro or in platelets isolated from rats treated in vivo with PBCO. Thromboxane B2 release was increased in platelets isolated from rats intubated with PBCO or in platelets to which a dimethyl sulfoxide extract of the aromatic or heterocyclic fraction was added. However, thromboxane B2 release was inhibited in platelets to which PBCO or the aliphatic fraction extracts were added. The results indicate that PBCO inhibits platelet aggregation presumably by bringing about alterations in the platelet plasma membrane. Inhibition of ADP release could contribute to the inhibition of aggregation but thromboxane B2 is believed not to play a significant role.

Adenosine Diphosphate↗

PubMed1987

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.

Animals↗

PubMed1987

Induction of epidermal and hepatic ornithine decarboxylase by a Prudhoe Bay crude oil.

Application of a Prudhoe Bay crude oil (PBCO) to the backs of mice caused a rapid induction of epidermal ornithine decarboxylase (ODC). A maximum induction of over 60-fold was seen at 6 hr after application of 50 microliters of PBCO. Concurrently, epidermal putrescine levels were elevated 4.7-fold over controls. Intraperitoneal administration of PBCO led to a 15-20-fold increase in hepatic ODC activity but to a 45% decrease in the renal enzyme activity. Maximum induction of ODC was seen at 12 hr following the administration of 4 ml/kg body wt of PBCO. Hepatic putrescine levels were elevated 34-fold over controls. Pretreatment of mice with the protein synthesis inhibitor cycloheximide abolished both epidermal and hepatic ODC induction.

Animals↗

PubMed1987

Embryotoxic evaluation of a Prudhoe Bay crude oil in rats.

The embryotoxic potential of a Prudhoe Bay crude oil (PBCO) was investigated in rats. PBCO was administered orally to pregnant rats as (i) a single dose on various gestation days, (ii) a single variable dose on gestation day 6, or (iii) as daily doses from day 6 to day 17 of pregnancy. PBCO administered during the earlier stages of pregnancy (day 3, 6 or 11) but not during the later stages, affected the reproductive performance of pregnant rats by significantly increasing the number of resorptions including fetal death and by decreasing the fetal weight. A dose-dependent increase in fetomortality was also observed. Multiple exposure to low levels of crude oil also caused a significant reduction in maternal body weight besides other embryotoxic changes.

Administration, Oral↗

PubMed1987

Review and perspective on the use of mixed-function oxygenase enzymes in biological monitoring.

It is often suggested that changes in simple biochemical/physiological responses may be useful for predicting the impacts of pollutants at population and community levels of biological organization. There are serious conceptual constraints to such a thesis and its seems likely that such simple responses can go no further than serving as early warning systems for delineating potential areas of pollutant impact--areas which (if shown to be significant in size) can then be subjected to more detailed population and community type studies. Environmental testing is a prerequisite for any response suggested to have value as a biological monitoring index and the induction of mixed-function oxygenase (MFO) enzymes has now been validated in a large number of field studies worldwide. Investigations have progressed from documenting induction near localized sources of hydrocarbon contamination to more diffuse sources of mixed organic pollution originating from industrial and domestic sources. Studies in the Great Lakes and Europe have demonstrated that the induction of MFO enzymes is a biological response of sufficient sensitivity to discriminate water quality differences over broad geographical areas. We suggest that as an early warning system, the induction of these enzymes can fulfill the requirement of "most sensitive biological response" for assessing a variety of organic pollution conditions. Given the high level of sensitivity of the MFO enzyme response, negative as well as positive field trials can be of value in addressing concerns about the toxicological significance of "high-profile" chemicals (and potent inducers) such as polycyclic aromatic hydrocarbons and organochlorines. MFO enzyme induction can also be an economical tool for environmental managers for reacting to real or perceived concerns about pollution such as effects on commercial fish stocks at sites of petroleum hydrocarbon development in the oceans.

Animals↗