Uptake of lipophilic carcinogens by plasma lipoproteins. Structure-activity studies

Ovarian susceptibility to benzo[a]pyrene: tissue burden of metabolites and DNA adducts in F-344 rats

Exposure to environmental toxicants has been implicated as one of the causative factors for infertility in mammals. The objective of this study was to determine the amount of ingested benzo[a]pyrene (BaP), an environmental toxicant that reaches the reproductive tissues (internal dose) subsequent to a single acute exposure. Toward this end, the concentrations of BaP reactive metabolites and BaP-DNA adducts were measured throughout the course of BaP's residence in the body. Ten-week-old female Fischer-344 rats weighing approximately 220 g were administered 5 mg BaP/kg body weight orally. 1, 7, 14, 2,1 and 28 d post BaP exposure, BaP parent compound and metabolites from plasma, ovaries, and liver tissues were extracted using liquid-liquid extraction. The extracts were analyzed by reverse-phase highperformance liquid chromatography (HPLC). DNA was isolated and analyzed for BaP-induced DNA adducts by (32)P-postlabeling method. The BaP total metabolite concentrations in plasma, ovaries, and liver showed a gradual decrease from d 1 to 28 post BaP administration. The BaP-DNA adducts concentrations in ovaries and liver tissues from the treatment group demonstrated a trend similar to that observed for metabolites. Ovaries showed greater concentrations of DNA adducts compared to liver. However, with an increase in time post cessation of exposure, the adduct concentrations in liver tissue started declining rapidly, from d 1 to 28. For ovaries, the adduct concentrations demonstrated a significant decline from d 1 to 7 and a gradual fall thereafter. A concordance between BaP reactive metabolite levels and adduct concentrations indicates that the bioavailability of reactive metabolites determines the binding with DNA and consequently the formation and persistence of adducts in an acute exposure regimen.

Influence of physicochemical properties on the patterns of association of a series of aliphatic esters of halofantrine with plasma lipoproteins

Although the association of lipophilic drugs with plasma lipoproteins has not been fully characterized, there are several reports of lipoprotein association being influential in pharmacokinetic and pharmacodynamic profiles of important therapeutic agents. The current studies utilized a series of aliphatic esters of halofantrine to evaluate the role of several physicochemical properties on the interaction of the different compounds with plasma lipoproteins. Density gradient ultracentrifugation techniques were employed to determine drug association in triglyceride rich (TRL), low-density (LDL) and high-density lipoproteins (HDL), under both fasted and post-prandial conditions. Compound solubility in medium or long chain triglycerides was a useful indicator of the extent of drug-lipoprotein association, particularly for the triglyceride rich lipoproteins (chylomicrons and very low-density lipoproteins). This is likely a function of the compounds being solubilized within the apolar (triglyceride and cholesterol ester) lipid core. However, molecular size also played an important role in determining lipoprotein distribution, particularly for association with the more protein abundant lipoproteins, such as HDL. Lipoprotein association of Hf analogues containing longer unsaturated esters was best correlated with total lipoprotein surface area rather than with lipoprotein core lipid volumes.

In vivo acute effects of carbofuran on protein, lipid, and lipoproteins in rat liver and serum

The objective of this investigation was to determine the changes in proteins, lipids, and lipoproteins in liver and serum of rats acutely intoxicated with carbofuran (1.5 mg/kg sc). Under the influence of carbofuran acute intoxication, analysis of globulin fractions revealed remarkable changes: In liver, the levels of alpha-2, alpha-3, and gamma were significantly elevated while alpha-1 was reduced; in serum, alpha-1 and alpha-3 fractions were elevated while alpha-2, beta, and gamma remained unchanged. A transient increase in total protein and albumin was noted only in liver. Carbofuran produced significant increases in triglycerides and cholesterol in liver that were also seen in serum. In both the liver and serum the levels of low-density-lipoprotein cholesterol (LDL-C) were reduced while the values of very-low-density-lipoprotein cholesterol (VLDL-C) were elevated. The concentration of high-density-lipoprotein cholesterol (HDL-C) was drastically reduced in liver (23% of control) with a proportional rise in serum (176%). In liver, carbofuran caused marked depletion of adenosine triphosphate (ATP) and phosphocreatine (PCr) (38% and 22% of controls, respectively), resulting in increased cell membrane permeability, thereby allowing leakage of cell constituents. It was concluded that carbofuran, directly or indirectly, produced perturbations in lipoprotein metabolism.

Binding of drugs to human plasma proteins, exemplified by Sn(IV)-etiopurpurin dichloride delivered in cremophor and DMSO

1. The mode-delivery-effect upon the binding of Sn(IV)-etiopurpurin dichloride (SnET2) in human plasma has been studied by ultracentrifugation, combined with absorption and fluorescence spectroscopy. SnET2 was delivered to plasma either in Cremophore EL (CRM) or in dimethyl sulfoxide (DMSO). To facilitate interpretation, optical, conductivity and aggregation properties of SnET2 were obtained for various solutions. 2. The second order rate constant for the aggregation of SnET2 monomers seemed to be remarkably small, of the order of 10(3) M-1 min-1. 3. SnET2 was bound as monomeric entities. Such entities had environmental-sensitive fluorescent properties dependent on the type of protein or solvent (DMSO, CRM, H2O) with which they interacted. 4. SnET2 showed saturable binding with high density subfraction(s) of high density lipoproteins and with one or more high density proteins. Complete or substantial saturation was achieved at the SnET2 level of 3.5 micrograms/ml. Such binding might be mediated by apolipoprotein D and alpha 1-acid glycoprotein. 5. There was little effect of SnET2 concentrations (3.5-35 micrograms SnET2/ml) upon the plasma binding of SnET2, irrespective of the mode of delivery. 6. The percentages of SnET2 bound to low density lipoproteins (LDL), high density lipoproteins (HDL), and high density proteins (HDP) were 10, 70 and 20%, respectively, for delivery in DMSO. The value for LDL also includes binding with very low density lipoproteins (VLDL). For delivery in CRM the corresponding values were 20, 50 and 30%. Apparently, CRM interacted with HDL entities and reduced their affinity for SnET2. 7. The distribution pattern of SnET2 among lipoproteins reflects interactions with apoproteins and/or with surface phospholipids rather than with core lipid constituents of lipoproteins. 8. Conductivity measurements showed that SnET2 was partly an ionic entity in water. 9. The plasma binding of SnET2 is compared with the corresponding binding of other drugs, both tetrapyrroles and nontetrapyrroles.

Distribution of toxaphene, DDT, and PCB among lipoprotein fractions in rat and human plasma

The distribution of 14C-toxaphene, 14C-DDT, and 14C-PCB among lipoprotein fractions was studied in vitro and in vivo using rat and human plasma. The association of these substances with rat plasma fractions was similar in both in vitro and in vivo experiments. Thirty-seven to fifty-two per cent of the total radioactivity was associated with the cholesterol-rich high density lipoproteins (HDL2, d = 1.075-1.21 g/ml) and 18-52% was recovered in the albumin-rich bottom fraction (BF, d greater than 1.21 g/ml). A time-dependent redistribution of the radioactivity from the lipoprotein fractions to the BF was also observed in the in vivo studies. In human plasma, the distribution of the three compounds was different and uncorrelated to the cholesterol level of the individual lipoprotein fractions. Toxaphene was almost equally distributed between BF (d greater than 1.21 ml), HDL (d = 1.063-1.21 g/ml) and low density lipoproteins (LDL, d = 1.006-1.063 g/ml) (26%, 27% and 29%, respectively), while only 18% appeared in the very low density lipoprotein (VLDL, d less than 1.006) fraction. In contrast, a large proportion of DDT and PCB radioactivity was recovered in the BF (52% and 62%, respectively) while only 38-48% was present in lipoprotein fractions. The complex nature of the interaction between xenobiotics and plasma lipoproteins is discussed.

Binding of spin-labeled clofibrate to lipoproteins

The binding of spin-labeled clofibrate to native and partially delipidated lipoproteins is a rapid, linear and non-saturable process observed up to the critical micellar concentration of the drug. Low-density lipoproteins (LDL) display a lower affinity for the drug than very-low-density lipoproteins (VLDL) and high-density lipoproteins (HDL) relative to their respective specific volume. Unlike various lipophilic drugs, uptake of spin-labeled clofibrate does not correlate with lipoprotein lipid volume. Spin-labeled clofibrate binding to LDL is enhanced when the temperature increases above 25 degrees C. The binding to HDL and VLDL is less temperature-sensitive. The simulation of the ESR spectra has shown that two types of motion should be superimposed for the spin-labeled clofibrate in HDL, in LDL or in partially delipidated LDL. From 40 down to 25 degrees C for HDL and LDL, a fast anisotropic motion is observed. From 25 degrees C down to 5 degrees C, a two-component motion takes place, including a slow isotropic motion of the probe tumbling in a highly hydrophobic environment. Interactions of spin-labeled clofibrate with the apolipoproteins in HDL and LDL are assumed from the emergence of this strongly immobilized component observed when the temperature decreases. In contrast, for spin-labeled clofibrate inserted in the apolar core of VLDL, ESR shows only one component in the whole temperature range (5-40 degrees C). The location of the spin-labeled drug inside the various lipoprotein particles is discussed as a function of temperature.

Effect of preexposure to dietary benzo[a]pyrene (BP) on the first-pass metabolism of BP by the intestine of toadfish (Opsanus tau): in vivo studies using portal vein-catheterized fish

The effect of preexposure of fish to dietary benzo[a]pyrene (BP) on the intestinal metabolism of BP was examined in toadfish (Opsanus tau). The portal veins of toadfish were cannulated following administration of radiolabeled BP to the intestinal lumen. Because these fish lack a lymphatic vessel system, the portal vein is the sole route by which BP and its metabolites enter the circulation. In fish preexposed to dietary BP (10 mg BP/kg food), the radioactivity entering the portal vein was almost entirely (ca. 90%) BP metabolites. In fish fed a laboratory control diet, a smaller percentage (ca. 60%) of the radioactivity entering the portal vein was in the form of BP metabolites. The enhanced efficiency of the intestines of preexposed fish in metabolizing BP appears to be a result of induction of intestinal aryl hydrocarbon hydroxylase (AHH) activity. Intestinal microsomal AHH activities in control and preexposed fish were 0.033 +/- 0.032 and 0.320 +/- 0.060 nmol.min-1.mg-1, respectively. Gel filtration of portal vein plasma indicated differences in the roles of plasma proteins in transporting BP and BP metabolites. Native BP was associated primarily with the high density lipoproteins, whereas organic-soluble BP metabolites were associated primarily with serum albumin fractions. A large percentage of BP metabolites was recovered as water-soluble conjugates. These studies indicate that in fish, the intestine can be an important organ involved in dietary BP metabolism.

Cicletanine binding to human plasma proteins and erythrocytes, a particular HSA-drug interaction

The binding of cicletanine to human serum, isolated proteins and red blood cells was studied in vitro by equilibrium dialysis. Our results show this drug is highly bound to serum (97.3%) at therapeutic levels. No saturation to the binding sites was seen. Human serum albumin was shown to mainly responsible for this binding (93.5%) with a saturable process characterized by one binding site with a moderate affinity (K = 75800 M-1) and a non saturable process with a low total affinity (nK = 6400 M-1). Like many basic lipophilic drugs, cicletanine showed a saturable binding to alpha-1-acid glycoprotein with one site and a moderate affinity (K = 38,800 M-1). Its binding to lipoproteins and red blood cells was weak and non saturable. Over the range of therapeutic concentrations, the unbound fraction in blood remains constant (3.6%). Moreover, interactions were studied using bilirubin and non esterified fatty acids at pathological concentrations and these endogenous compounds did not alter cicletanine binding human serum or to human serum albumin likewise cicletanine shared the diazepam-site on HSA but no inhibition could take place between cicletanine and the drugs sharing the same binding site in serum at therapeutic levels.

Aflatoxin B1 transport in rat blood plasma. Binding to albumin in vivo and in vitro and spectrofluorimetric studies into the nature of the interaction

Binding of [3H]aflatoxin B1 to rat plasma was investigated in vivo and in vitro. Column chromatographic and polyacrylamide gel electrophoretic analyses clearly demonstrated that aflatoxin B1 bound primarily plasma albumin. Very little binding activity was shown by other plasma proteins. Spectrofluorimetric studies were undertaken to gain some insight into the nature of the aflatoxin-albumin interaction. Quenching of the lone tryptophan fluorescence intensity upon aflatoxin binding was due, at least in part, to a ligand-induced conformational change in the albumin molecule. Aflatoxin B1 binds an apolar site with an association constant of 30 mM-1 at pH 7.4 and 20 degrees C. Neither charcoal treatment of rat albumin nor the presence of 0.15 M NaCl had any significant effect on the interaction. The association constant was pH-dependent, increasing about 1.7-fold as the pH increased from 6.1 to 8.4. This pH dependence is ascribed to a pH-induced conformational change in the albumin molecule. Thermodynamic studies indicated that the aflatoxin-albumin interaction was exothermic (delta H = -29.3 kJ X mol-1), with a delta S value of -13.8 J X mol-1 X K-1.

Uptake of 2,3,7,8-tetrachlorodibenzo-p-dioxin from plasma lipoproteins by cultured human fibroblasts

Tritiated 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) added to human plasma in vitro associated with the plasma lipoproteins. The effects of plasma and lipoproteins on cellular uptake of dioxin were studied using normal human skin fibroblasts and mutant fibroblasts from a patient with homozygous familial hypercholesterolemia. The latter cells lack the normal cell membrane receptor for low density lipoprotein (LDL). The time- and temperature-dependent cellular uptake of [3H]dioxin was greatest from LDL, intermediate from high density lipoprotein (HDL) and least from serum. A significantly greater uptake from LDL by the normal cells compared to the mutant cells indicated the involvement of the LDL receptor-mediated pathway. Concentration-dependent studies indicated that the cellular uptake at 37 degrees C of [3H]dioxin varied linearly with dioxin concentration at constant LDL concentration. Thin-layer chromatography (TLC) showed that conversion to more polar compounds may have occurred after 24-h incubation with cells. [3H]Dioxin could be removed from cells efficiently by incubation with 20% serum greater than HDL greater than LDL. Since the vehicle of delivery may influence subsequent location and metabolism of this compound in cells, it is concluded that the physiologic vehicles (either serum- or LDL-associated dioxin), rather than organic solvents, should be used in experiments with cultured cells or perfused organs.

High-density lipoproteins decrease both DNA binding and mutagenicity of r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene in V79 Chinese hamster cells

The effects of separate lipoproteins or of serum with high or low lipoprotein concentrations on formation of lipophilic carcinogen adducts with DNA and on mutagenicity of the carcinogen was investigated using V79 Chinese hamster lung cells. Binding of r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) to DNA and BPDE induction of 6-thioguanine (6-TG)-resistant mutants in V79 cells was significantly lower after 1 or 4 h when the medium was supplemented with purified HDL, and was lower after 1 h but not 4 h when the medium was supplemented with serum containing a high concentration of mixed lipoproteins (LP). Cells grown in medium without serum or LP supplementation exhibited the highest levels of both BPDE-DNA adduct formation and mutagenesis after 1 h. At 1 h, cells exposed to BPDE in LDL-supplemented medium showed decreased adduct formation and mutagenesis when compared to cells treated with BPDE in PBS-supplemented medium. After 4 h, cells treated with BPDE in LDL-supplemented medium gave the highest levels of adduct formation and the highest mutation frequency. These results suggest that both LDL and HDL effectively decrease the concentration of BPDE available to V79 cells exposed to the mutagen for short periods of time, resulting in decreased interaction of BPDE with DNA and decreased BPDE-associated mutagenesis, but that both BPDE-DNA adduct formation and mutagenesis increased as a function of increased exposure time in the presence of LDL. The results suggest that LDL, but not HDL, uptake by adsorptive endocytosis may be associated with potentiated entry of BPDE into V79 cells as a function of time.

Diclofenac binding to albumin and lipoproteins in human serum

The binding of diclofenac to human serum albumin (HSA) and to lipoproteins was studied in vitro by equilibrium dialysis. Binding to HSA is characterized by two classes of sites with one site each (K1 = 5 X 10(5) M-1 and K2 = 0.6 X 10(5) M-1). The binding to lipoproteins was shown to be saturable with a larger number of binding sites and low association constants. The evidence of two specific binding sites on HSA was confirmed by circular dichroism data. In addition, an identification of those sites was performed by displacement of fluorescent probes. The data show that the high affinity site (K1 = 5 X 10(5) M-1) is likely to be shared by benzodiazepines while the second one (K2 = 0.6 X 10(5) M-1) is common with the warfarin site.

The role of high density lipoproteins in the biodistribution of two radioiodinated probes in the rat

Two radioiodinated probes, 125I-cholesteryl oleate (125I-CO), a derivative of a natural constituent of lipoproteins, and 1-(2-chlorophenyl)-1-(4[125I]iodophenyl)-2,2-dichlorethane (125I-DDD), an analog of the adrenolytic drug o,p'-DDD (mitotane), were selected to study the role of lipoproteins in drug disposition and to examine the ability of these vehicles to direct foreign molecules to specific tissues. In vivo and in vitro techniques were utilized to associate these probes with rat high density lipoproteins (HDL). Tissue distribution studies indicated that prior incorporation of 125I-CO into rat HDL increased the uptake of 125I-CO by rat adrenal, which was dramatically enhanced when this preparation was administered to animals made hypolipidemic with 4-aminopyrazolo(3,4-d)-pyrimidine (4-APP). Acetylation of HDL labeled with 125I-CO provided evidence that the observed uptake into the adrenal was via a receptor-mediated process. In contrast with these results, prior association of 125I-DDD with rat HDL failed to alter the ability of this compound to accumulate in adrenal tissue of normal or hypolipidemic animals. Polyacrylamide gel electrophoresis (PAGE) was utilized to examine the stability of the association of 125I-CO and 125I-DDD with rat HDL. These results suggested that 125I-CO was associated with the lipophilic core of HDL, whereas 125I-DDD appeared to be partially associated with the surface components of HDL. Saturation of surface components with stable o,p'-DDD offered data to suggest that this binding to apoproteins may disrupt the normal receptor-mediated uptake process. These studies indicate that lipoproteins may effect the distribution and tissue uptake of lipophilic compounds and, conversely, lipophilic molecules can effect the metabolic fate of lipoproteins. The overall result is dependent upon the nature of the association of these lipophilic compounds with lipoproteins which is difficult to predict on the basis of molecular structure alone.

Influence of red blood cells, serum albumin, and serum lipoproteins on the clearance of benzo[alpha]pyrene by isolated livers of 3-methylcholanthrene-treated rats

Red blood cells, serum albumin, and serum lipoproteins transport benzo[alpha]pyrene and other xenobiotic compounds in the circulation. The distribution of benzo[alpha]pyrene and its metabolites among these blood components was examined, and the effect of their presence in the perfusion medium on the ability of isolated livers from 3-methylcholanthrene-pretreated rats to clear circulating benzo[alpha]pyrene was determined. A large fraction (45%) of the benzo[alpha]pyrene in rat blood was associated with the serum lipoproteins. However, only 8% of the benzo[alpha]pyrene metabolites was associated with this component. Forty to forty-five percent of each was associated with red blood cells. Benzo[alpha]pyrene clearance by isolated rat livers was 1.8 +/- 0.2 ml/min when the medium contained only red blood cells and buffer. Addition of serum lipoproteins or serum albumin increased benzo[alpha]pyrene clearance to 5.1 +/- 0.5 or 8.5 +/- 0.9 ml/min respectively. Appearance of benzo[alpha]pyrene metabolites in perfusions medium and bile was similarly altered by the changes in medium composition. These results indicate that the clearance of benzo[alpha]pyrene by rat liver depends on the composition of the medium perfusing the organ and suggest that alterations in blood components in vivo may influence the metabolic disposition of this carcinogen.

Hydrocarbon transport in chylomicrons and high-density lipoproteins in rat

A lipoprotein system is described that transports gut hydrocarbons of low polarity in chylomicrons of intestinal lymph and plasma to plasma high density lipoproteins (HDL) in rat. Four highly lipophilic aryl and alkyl hydrocarbons [benzo(alpha)pyrene; 1,1,1-trichloro-2,2-bis(p-chlorophenol)ethane (DDT), hexadecane and octadecane] were selected to give a graded range of polarity. Chylomicrons were labeled doubly with radioisotopes in triacylglycerol and a single hydrocarbon by feeding [3H]-glycerol and [14C]hydrocarbon. All hydrocarbons were transported in the triacylglycerol oil phase of chylomicrons. Injected chylomicron triacylglycerol and 3 of 4 hydrocarbons were cleared simultaneously from plasma consistent with lipoprotein-lipase dependent hydrocarbon clearance but DDT was cleared more rapidly. HDL was the major plasma acceptor of all labelled hydrocarbons. Plasma chemical fluxes were measured for octadecane and DDT and both showed net fluxes from chylomicrons to HDL. HDL selectively concentrated chylomicron hydrocarbons from chylomicron triacylglycerol. Lipoprotein lipase stimulation by intravenous heparin significantly increased transfer of alkanes from chylomicrons to HDL. These results indicate that (a) chylomicrons transport gut-derived hydrocarbons with a wide range of structure and polarity as triacylglycerol solutes; (b) HDL are a major plasma acceptor of all these hydrocarbons, demonstrating both selective solute uptake from triacylglycerol and net chemical uptake for the 2 hydrocarbons studied and (c) efflux of these chylomicron hydrocarbons from plasma and into HDL is regulated partly by hydrolysis of chylomicron triacylglycerol.

Association of aflatoxin B1 with plasma components in vitro

The in vitro association of [3H]aflatoxin B1 with plasma from man, pig and rat was compared. The uptake of aflatoxin B1 into plasma was rapid and did not differ between species. Its association with lipoprotein fractions was minimal when incubated with whole plasma; column chromatography of plasma from all 3 species resulted in the elution of the aflatoxin B1 with the low Mr plasma proteins. It is concluded that although aflatoxin B1 is a hydrophobic compound, it does not partition into the plasma lipoproteins and that its in vitro uptake into plasma does not differ among the 3 species studied. Plasma transport is probably not one of the factors determining species specificity for carcinogenesis by aflatoxin B1.