Transport, binding, and metabolism of sulfate conjugates in the liver

Cytosolic sulfotransferases in endocrine disruption

The mammalian cytosolic sulfotransferases (SULTs) catalyze the sulfation of endocrine hormones as well as a broad array of drugs, environmental chemicals, and other xenobiotics. Many endocrine-disrupting chemicals (EDCs) interact with these SULTs as substrates and inhibitors, and thereby alter sulfation reactions responsible for metabolism and regulation of endocrine hormones such as estrogens and thyroid hormones. EDCs or their metabolites may also regulate expression of SULTs through direct interaction with nuclear receptors and other transcription factors. Moreover, some sulfate esters derived from EDCs (EDC-sulfates) may serve as ligands for endocrine hormone receptors. While the sulfation of an EDC can lead to its excretion in the urine or bile, it may also result in retention of the EDC-sulfate through its reversible binding to serum proteins and thereby enable transport to other tissues for intracellular hydrolysis and subsequent endocrine disruption. This mini-review outlines the potential roles of SULTs and sulfation in the effects of EDCs and our evolving understanding of these processes.

Complex roles for sulfation in the toxicities of polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) are persistent organic toxicants derived from legacy pollution sources and their formation as inadvertent byproducts of some current manufacturing processes. Metabolism of PCBs is often a critical component in their toxicity, and relevant metabolic pathways usually include their initial oxidation to form hydroxylated polychlorinated biphenyls (OH-PCBs). Subsequent sulfation of OH-PCBs was originally thought to be primarily a means of detoxication; however, there is strong evidence that it may also contribute to toxicities associated with PCBs and OH-PCBs. These contributions include either the direct interaction of PCB sulfates with receptors or their serving as a localized precursor for OH-PCBs. The formation of PCB sulfates is catalyzed by cytosolic sulfotransferases, and, when transported into the serum, these metabolites may be retained, taken up by other tissues, and subjected to hydrolysis catalyzed by intracellular sulfatase(s) to regenerate OH-PCBs. Dynamic cycling between PCB sulfates and OH-PCBs may lead to further metabolic activation of the resulting OH-PCBs. Ultimate toxic endpoints of such processes may include endocrine disruption, neurotoxicities, and many others that are associated with exposures to PCBs and OH-PCBs. This review highlights the current understanding of the complex roles that PCB sulfates can have in the toxicities of PCBs and OH-PCBs and research on the varied mechanisms that control these roles.

Metabolism of natural and synthetic bioactive compounds in Cunninghamella fungi and their applications in drug discovery

Investigation of xenobiotic metabolism is a key step for drug discovery. Since the in vivo investigations may be associated with harmful effects attributed to production of toxic metabolites, it is deemed necessary to predict their structure especially at the preliminary clinical studies. Furthermore, the application of microorganisms that are capable of metabolizing drugs mimic human metabolism and consequently may predict possible metabolites. The genus Cunninghamella has been proven to be a potential candidate, which mimics xenobiotic metabolism occurring inside the human body, including phase I and II metabolic reactions. Moreover, biotransformation with Cunninghamella showed chemical diversity, where a lot of products were detected in relation to the initial substrates after being modified by oxidation, hydroxylation, and conjugation reactions. Some of these products are more bioactive than the parent compounds. The current review presents a comprehensive literature overview regarding the Cunninghamella organisms as biocatalysts, which simulate mammalian metabolism of natural secondary and synthetic compounds.

The Role of Uptake and Efflux Transporters in the Disposition of Glucuronide and Sulfate Conjugates

Glucuronidation and sulfation are the most typical phase II metabolic reactions of drugs. The resulting glucuronide and sulfate conjugates are generally considered inactive and safe. They may, however, be the most prominent drug-related material in the circulation and excreta of humans. The glucuronide and sulfate metabolites of drugs typically have limited cell membrane permeability and subsequently, their distribution and excretion from the human body requires transport proteins. Uptake transporters, such as organic anion transporters (OATs and OATPs), mediate the uptake of conjugates into the liver and kidney, while efflux transporters, such as multidrug resistance proteins (MRPs) and breast cancer resistance protein (BCRP), mediate expulsion of conjugates into bile, urine and the intestinal lumen. Understanding the active transport of conjugated drug metabolites is important for predicting the fate of a drug in the body and its safety and efficacy. The aim of this review is to compile the understanding of transporter-mediated disposition of phase II conjugates. We review the literature on hepatic, intestinal and renal uptake transporters participating in the transport of glucuronide and sulfate metabolites of drugs, other xenobiotics and endobiotics. In addition, we provide an update on the involvement of efflux transporters in the disposition of glucuronide and sulfate metabolites. Finally, we discuss the interplay between uptake and efflux transport in the intestine, liver and kidneys as well as the role of transporters in glucuronide and sulfate conjugate toxicity, drug interactions, pharmacogenetics and species differences.

Human hepatic microsomal sulfatase catalyzes the hydrolysis of polychlorinated biphenyl sulfates: A potential mechanism for retention of hydroxylated PCBs

Polychlorinated biphenyls (PCBs) are persistent environmental contaminants that continue to be of concern due to their varied toxicities. Upon human exposure, many PCBs with lower numbers of chlorine atoms are metabolized to hydroxylated derivatives (OH-PCBs), and cytosolic sulfotransferases can subsequently catalyze the formation of PCB sulfates. Recent studies have indicated that PCB sulfates bind reversibly with a high affinity to human serum proteins, and that they are also taken up by cells and tissues. Since PCB sulfates might be hydrolyzed to the more toxic OH-PCBs, we have investigated the ability of human hepatic microsomal sulfatase to catalyze this reaction. Twelve congeners of PCB sulfates were substrates for the microsomal sulfatase with catalytic rates exceeding that of dehydroepiandrosterone sulfate as a comparison substrate for steroid sulfatase (STS). These results are consistent with an intracellular mechanism for sulfation and de-sulfation that may contribute to retention and increased time of exposure to OH-PCBs.

Impact of Dietary Fibers on Nutrient Management and Detoxification Organs: Gut, Liver, and Kidneys

Increased dietary fiber (DF) intake elicits a wide range of physiologic effects, not just locally in the gut, but systemically. DFs can greatly alter the gut milieu by affecting the gut microbiome, which in turn influences the gut barrier, gastrointestinal immune and endocrine responses, and nitrogen cycling and microbial metabolism. These gut-associated changes can then alter the physiology and biochemistry of the body's other main nutrient management and detoxification organs, the liver and kidneys. The molecular mechanisms by which DF alters the physiology of the gut, liver, and kidneys is likely through gut-localized events (i.e., bacterial nitrogen metabolism, microbe-microbe, and microbe-host cell interactions) coupled with specific factors that emanate from the gut in response to DF, which signal to or affect the physiology of the liver and kidneys. The latter may include microbe-derived xenometabolites, peptides, or bioactive food components made available by gut microbes, inflammation signals, and gut hormones. The intent of this review is to summarize how DF alters the gut milieu to specifically affect intestinal, liver, and kidney functions and to discuss the potential local and systemic signaling networks that are involved.

Breast cancer treatment and sulfotransferase

INTRODUCTION

Sustained exposure to excessive estrogen is an established risk factor for breast cancer. Sulfotransferase (SULT)-mediated sulfonation represents an effective approach for estrogen deprivation as estrogen sulfates do not bind and activate estrogen receptors (ERs). The nuclear receptor (NR) superfamily functions as a sensor for xenobiotics as well as endogenous molecules, which can regulate the expression of SULT.

AREAS COVERED

In this review, we summarize the mechanisms of SULT regulation by NRs and inactivation of estrogen by SULT. Furthermore, we discuss the potential of clinical therapy targeting SULT in breast cancer treatment. Gaps in current knowledge that require further study are also highlighted.

EXPERT OPINION

The prevention of estrogen binding to ER by antiestrogen and inhibition of estrogen synthesis by aromatase or sulfatase inhibitor have been used in clinical therapy for breast cancer. Although the induction of SULT has been proven effective to estrogen inactivation, reports on this method applied to breast cancer treatment are rare. Targeted activation of SULT may open up a new means of treating hormone-dependent breast cancer.

Single nucleotide polymorphisms in SULT1A1 and SULT1A2 in a Korean population

SULT1A1 and SULT1A2 are encoded on the same chromatid, and exhibit a 96% amino acid similarity. To screen for genetic variants in these two closely related genes, SULT1A1 and SULT1A2 were directly sequenced in 50 healthy Koreans. A total of 30 variations were identified in SULT1A1: eight in exons, thirteen in introns, and nine in the 5'-untranslated region. With regard to SULT1A2, 21 variants were identified, comprising seven in exons, five in introns, and nine in the 5'-untranslated region. Among these 51 variations, one in SULT1A1 and eight in SULT1A2 were previously unidentified, which include three coding variants (SULT1A2 R37Q, 110G>A; SULT1A2 G50S, 148G>A; SULT1A2 F286L, 3819C>A) and one null allele (SULT1A2 E217Stop, 3542G>T). Two LD blocks, major haplotype structures, and 7 haplotype-tagging SNPs were determined together for SULT1A1 and SULT1A2 as a single set. Frequencies of common functional variants were compared among ethnic groups. Since these two SULT enzymes are on the same chromatid in a parallel direction with overlapping substrate specificities, a combined analysis using LD and haplotype-tagging single-nucleotide polymorphisms (SNPs) will facilitate understanding of the variations in the sulfation reactions of a wide range of substrates, as compared with analysis of individual genes.

Serum levels of total p-cresylsulphate are associated with angiographic coronary atherosclerosis severity in stable angina patients with early stage of renal failure

OBJECTIVE

p-Cresylsulphate (PCS), a protein-bound uraemic retention solute, is known to cause endothelial dysfunction and possibly plays a role in coronary atherosclerosis. We aimed to investigate the relationship of total PCS with traditional biomarkers associated with chronic coronary atherosclerosis. In addition, the relationship between serum total PCS levels and the severity of coronary artery stenosis was also explored.

METHODS AND RESULTS

Serum total PCS concentrations were measured by using the Ultra Performance LC System in 202 consecutive stable angina patients, and their associations with angiographic indexes of the number of diseased vessels and modified Gensini score were estimated. Patients with significant coronary artery stenosis have higher median serum total PCS levels than patients with normal coronary arteries. Statistically significant associations were observed between the serum total PCS levels and the number of diseased vessels (beta=0.261, p=0.0002), and modified Gensini score (beta=0.171, p=0.016). Using multivariate analysis, serum total PCS level was independently associated with the presence and severity of CAD.

CONCLUSIONS

This study indicates that serum total PCS levels are significantly higher in the presence of CAD and are correlated with the severity of the disease, which suggest that increased serum total PCS may be involved in the pathogenesis of coronary atherosclerosis.

The hepatic microcirculation: mechanistic contributions and therapeutic targets in liver injury and repair

The complex functions of the liver in biosynthesis, metabolism, clearance, and host defense are tightly dependent on an adequate microcirculation. To guarantee hepatic homeostasis, this requires not only a sufficient nutritive perfusion and oxygen supply, but also a balanced vasomotor control and an appropriate cell-cell communication. Deteriorations of the hepatic homeostasis, as observed in ischemia/reperfusion, cold preservation and transplantation, septic organ failure, and hepatic resection-induced hyperperfusion, are associated with a high morbidity and mortality. During the last two decades, experimental studies have demonstrated that microcirculatory disorders are determinants for organ failure in these disease states. Disorders include 1) a dysregulation of the vasomotor control with a deterioration of the endothelin-nitric oxide balance, an arterial and sinusoidal constriction, and a shutdown of the microcirculation as well as 2) an overwhelming inflammatory response with microvascular leukocyte accumulation, platelet adherence, and Kupffer cell activation. Within the sequelae of events, proinflammatory mediators, such as reactive oxygen species and tumor necrosis factor-alpha, are the key players, causing the microvascular dysfunction and perfusion failure. This review covers the morphological and functional characterization of the hepatic microcirculation, the mechanistic contributions in surgical disease states, and the therapeutic targets to attenuate tissue injury and organ dysfunction. It also indicates future directions to translate the knowledge achieved from experimental studies into clinical practice. By this, the use of the recently introduced techniques to monitor the hepatic microcirculation in humans, such as near-infrared spectroscopy or orthogonal polarized spectral imaging, may allow an early initiation of treatment, which should benefit the final outcome of these critically ill patients.

Steroids in chicken egg yolk: metabolism and uptake during early embryonic development

Effects of maternal hormones may adaptively adjust offspring development to prevailing conditions. However, Darwinian fitness of parents is maximized by investing in more than one offspring while each individual offspring benefits from receiving maximal investment. The control of mother and offspring over hormone-mediated maternal effects is thought to play a key role in the outcome of parent-offspring conflict, but these control mechanisms have hardly been studied. We investigated the potential embryonic control by analysing the changes in distribution and metabolism of steroid hormones in the egg during the first 6 days of incubation using injections of radiolabelled testosterone and corticosterone in freshly laid eggs. After 1 day of incubation the highest amount of radioactivity was concentrated in a small area at the top of the yolk. This challenges the use of hormones in oil as mimicking natural exposure. During incubation radioactivity spread within the egg with highest concentrations in yolk and yolk sac and lower concentrations in albumen, embryo, allantois, and amnion. Steroids were metabolised to other unconjugated and conjugated steroids, perhaps facilitating embryonic steroid uptake. Our study shows that the injected radiolabel is metabolised in the egg and taken up by the embryo, giving the embryo potential control over the effects of maternal hormones and thereby limiting maternal control over the outcome of hormone-mediated maternal effects.

Profiling of 19-norandrosterone sulfate and glucuronide in human urine: implications in athlete's drug testing

19-Norandrosterone (19-NA) as its glucuronide derivative is the target metabolite in anti-doping testing to reveal an abuse of nandrolone or nandrolone prohormone. To provide further evidence of a doping with these steroids, the sulfoconjugate form of 19-norandrosterone in human urine might be monitored as well. In the present study, the profiling of sulfate and glucuronide derivatives of 19-norandrosterone together with 19-noretiocholanolone (19-NE) were assessed in the spot urines of 8 male subjects, collected after administration of 19-nor-4-androstenedione (100mg). An LC/MS/MS assay was employed for the direct quantification of sulfoconjugates, whereas a standard GC/MS method was applied for the assessment of glucuroconjugates in urine specimens. Although the 19-NA glucuronide derivative was always the most prominent at the excretion peak, inter-individual variability of the excretion patterns was observed for both conjugate forms of 19-NA and 19-NE. The ratio between the glucuro- and sulfoconjugate derivatives of 19-NA and 19-NE could not discriminate the endogenous versus the exogenous origin of the parent compound. However, after ingestion of 100mg 19-nor-4-androstenedione, it was observed in the urine specimens that the sulfate conjugates of 19-NA was detectable over a longer period of time with respect to the other metabolites. These findings indicate that more interest shall be given to this type of conjugation to deter a potential doping with norsteroids.

Apparent differences in mechanisms of harmol sulfate biliary excretion in mice and rats

Previous experiments demonstrated that the biliary excretion of harmol sulfate (HS) was mediated by breast cancer resistance protein (Bcrp) and not by multidrug resistance-associated protein (Mrp)2 or P-glycoprotein in mice. However, recent reports suggested that species differences in hepatic canalicular transport mechanisms for a given substrate exist between mice and rats. In the present study, biliary excretion of HS was examined in perfused livers from mice and rats in the absence or presence of the P-glycoprotein and Bcrp inhibitor N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918). As expected, in mouse liver perfusions, the biliary excretion of HS was decreased approximately 3.5-fold by GF120918, consistent with previous reports of Bcrp-mediated HS biliary excretion. However, despite sufficient hepatic unbound concentrations of GF120918 to achieve extensive inhibition of Bcrp, the biliary excretion of HS was not decreased significantly in wild-type (50 +/- 12 versus 41 +/- 6%) or TR(-) (18 +/- 2 versus 16 +/- 3%) Wistar rats. In summary, biliary excretion of HS was mediated by a GF120918-sensitive mechanism in mice, previously elucidated as Bcrp. In contrast, the pathway responsible for HS biliary excretion in rats was not impaired by GF120918. Thus, transport mechanism(s) responsible for harmol sulfate biliary excretion appear to differ between mice and rats.

Biotransformation and pharmacokinetics of 4-(3,4-dihydroxybenzoyloxymethyl)phenyl-O-beta-D-glucopyranoside, an antioxidant isolated from Origanum vulgare

4-(3,4-Dihydroxybenzoyloxymethyl)phenyl- O-beta-D-glucopyranoside (OV-16) is a polyphenolic glycoside isolated from oregano (Origanum vulgare L.), which is a popular Chinese herb and a common spice in Western diet. To understand the biotransformation and pharmacokinetics of OV-16, rats were orally administered OV-16 and oregano decoction. Blood samples were withdrawn at specific time points. The presence of OV-16 and its metabolites protocatechuic acid (PCA) and p-hydroxybenzyl alcohol (HBA) in serum were determined by HPLC method, whereas their conjugated metabolites were assayed indirectly through hydrolysis with beta-glucuronidase and sulfatase. Our results showed that when OV-16 was orally administered, free forms of OV-16, PCA, and HBA were not present in blood and the major metabolites were the glucuronides/sulfates of PCA and HBA sulfate. The serum metabolites of OV-16 exhibited free radical scavenging activity. When oregano decoction was given, the glucuronides and sulfates of PCA were the major metabolites in blood.

Obstructive cholestasis induces TNF-alpha- and IL-1 -mediated periportal downregulation of Bsep and zonal regulation of Ntcp, Oatp1a4, and Oatp1b2

Inverse acinar regulation of Mrp2 and 3 represents an adaptive response to hepatocellular cholestatic injury. We studied whether obstructive cholestasis (bile duct ligation) and LPS treatment affect the zonal expression of Bsep (Abcb11), Mrp4 (Abcc4), Ntcp (Slc10a1), and Oatp isoforms (Slco1a1, Slco1a4, and slco1b2) in rat liver, as analyzed by semiquantitative immunofluorescence. Contribution of TNF-alpha and IL-1beta to transporter zonation in obstructive cholestasis was studied by cytokine inactivation. In normal liver Bsep, Mrp4, Ntcp, and Oatp1a1 were homogeneously distributed in the acinus, whereas Oatp1a4 and Oatp1b2 expression increased from zone 1 to 3. Glutamine synthetase-positive pericentral hepatocytes exhibited markedly lower Oatp1a4 expression than the remaining zone 3 hepatocytes. In cholestatic liver Bsep and Ntcp immunofluorescence in periportal hepatocytes significantly decreased to 66 +/- 4% (P < 0.01) and 67 +/- 7% (P < 0.05), whereas it was not altered in pericentral hepatocytes. Oatp1a4 was significantly induced in hepatocytes with a primarily low expression, i.e., in periportal hepatocytes and in glutamine synthetase-positive pericentral hepatocytes. Likewise, Oatp1b2 was upregulated in periportal hepatocytes. Mrp4 zonal induction was homogeneous. Inactivation of TNF-alpha and IL-1beta prevented periportal downregulation of Bsep. Recruitment of neutrophils and polymorphonuclear cells mainly occurred in the periportal zone. Likewise, IL-1beta induction was largely found periportally. No significant transporter zonation was seen following LPS treatment. In conclusion, zonal downregulation of Bsep in obstructive cholestasis is associated with portal inflammation and is mediated by TNF-alpha and IL-1beta. Periportal downregulation of Ntcp and induction of Oatp1a4 and Oatp1b2 may represent adaptive mechanisms to reduce cholestatic injury in hepatocytes with profound downregulation of Bsep and Mrp2.

The characterization and molecular structure of hepatoproliferin: a liver regeneration factor from rat hepatocytes

Hepatoproliferin (HPF) was purified from regenerating rat livers as an oligomeric entity (big-HPF) from which the monomeric form (small-HPF) could be obtained using disaggregating conditions. By using a solid-phase ion-exchange method, small-HPF was forced to dissociate into two charged ionic species, namely norepinephrine (NE) and a sulfonated disaccharide with a molecular structure consisting of D-glucuronic acid bound to glucosamine 2,6-disulfate by a beta-glycosidic linkage having a beta, 1 --> 4 configuration. Monomeric HPF stemmed from the formation of three electrostatic bonds between the protonated amine groups of three norepinephrines, of which two bind to the deprotonated sulfonic groups of glucosamine 2,6-disulfate and one to the deprotonated carboxylic group of glucuronic acid, to constitute a tightly associated complex with a molecular mass of 1046 Da. This represents one of the two purified isoforms of small-HPF. The other isoform, which has a lower molecular mass of 877 Da, lack one NE, leaving the weaker carboxylic group of glucuronic acid unoccupied, to constitute a more acidic form of HPF.

Identification of sulfation sites of metabolites and prediction of the compounds' biological effects

Characterizing the biological effects of metabolic transformations (or biotransformation) is one of the key steps in developing safe and effective pharmaceuticals. Sulfate conjugation, one of the major phase II biotransformations, is the focus of this study. While this biotransformation typically facilitates excretion of metabolites by making the compounds more water soluble, sulfation may also lead to bioactivation, producing carcinogenic products. The end result, excretion or bioactivation, depends on the structural features of the sulfation sites, so obtaining the structure of the sulfated metabolites is critically important. We describe herein a very simple, high-throughput procedure for using mass spectrometry to identify the structure—and thus the biological fate—of sulfated metabolites. We have chemically synthesized and analyzed libraries of compounds representing all the biologically relevant types of sulfation products, and using the mass spectral data, the structural features present in these analytes can be reliably determined, with a 97% success rate. This work represents the first example of a high-throughput analysis that can identify the structure of sulfated metabolites and predict their biological effects.

Integration of hepatic drug transporters and phase II metabolizing enzymes: Mechanisms of hepatic excretion of sulfate, glucuronide, and glutathione metabolites

The liver is the primary site of drug metabolism in the body. Typically, metabolic conversion of a drug results in inactivation, detoxification, and enhanced likelihood for excretion in urine or feces. Sulfation, glucuronidation, and glutathione conjugation represent the three most prevalent classes of phase II metabolism, which may occur directly on the parent compounds that contain appropriate structural motifs, or, as is usually the case, on functional groups added or exposed by phase I oxidation. These three conjugation reactions increase the molecular weight and water solubility of the compound, in addition to adding a negative charge to the molecule. As a result of these changes in the physicochemical properties, phase II conjugates tend to have very poor membrane permeability, and necessitate carrier-mediated transport for biliary or hepatic basolateral excretion into sinusoidal blood for eventual excretion into urine. This review summarizes sulfation, glucuronidation, and glutathione conjugation reactions, as well as recent progress in elucidating the hepatic transport mechanisms responsible for the excretion of these conjugates from the liver. The discussion focuses on alterations of metabolism and transport by chemical modulators, and disease states, as well as pharmacodynamic and toxicological implications of hepatic metabolism and/or transport modulation for certain active phase II conjugates. A brief discussion of issues that must be considered in the design and interpretation of phase II metabolite transport studies follows.

Removal of P-cresol sulfate by hemodialysis

Protein-bound solutes are poorly cleared by dialysis. Among the most extensively studied of these solutes is p-cresol, which has been shown to be toxic in vitro. This study examined the form in which p-cresol circulates and quantified its removal by hemodialysis. HPLC analysis of plasma from hemodialysis patients contained a peak whose mobility corresponded to synthetic p-cresol sulfate (PCS) but no detectable unconjugated p-cresol. Treatment with sulfatase resulted in recovery of this peak as p-cresol, confirming its identity. Subsequent studies compared the removal of PCS and another protein-bound solute, indican, to the removal of urea during clinical hemodialysis treatments. PCS and indican were 94 +/- 1% and 93 +/- 2% bound to plasma protein, respectively. Protein-binding caused a predictable decrease in measured dialytic clearance, which averaged 20 +/- 4 ml/min for PCS and 25 +/- 5 ml/min for indican as compared with 260 +/- 20 ml/min for urea. Volumes of distribution for the protein-bound solutes were greater than the plasma volume, averaging 15 +/- 7 L for PCS and 14 +/- 3 L for indican as compared with 37 +/- 7 for urea. Solute reduction ratios were 20 +/- 9% for PCS, 30 +/- 7% for indican, and 69 +/- 5% for urea. We conclude that p-cresol circulates in the form of its sulfate conjugate, PCS. PCS is poorly removed by hemodialysis because its clearance is limited by protein binding and the ratio of its volume of distribution to its clearance is high.

The Hepatic Sinusoid in Aging and Cirrhosis

The fenestrated sinusoidal endothelium ('liver sieve') and space of Disse in the healthy liver do not impede the transfer of most substrates, including drugs and oxygen, from the sinusoidal lumen to the hepatocyte. Plasma components transfer freely in both directions through the endothelial fenestrations and into the space of Disse. The endothelium is attenuated, there is no basement membrane and there is minimum collagen in the space of Disse, thus minimising any barriers to substrate diffusion. Both cirrhosis and aging are associated with marked structural changes in the sinusoidal endothelium and space of Disse that are likely to influence bulk plasma transfer into the space of Disse, and diffusion through the endothelium and space of Disse. These changes, termed capillarisation and pseudocapillarisation in cirrhosis and aging, respectively, impede the transfer of various substrates. Capillarisation is associated with exclusion of albumin, protein-bound drugs and macromolecules from the space of Disse, and the progressive transformation of flow-limited to barrier-limited distribution of some substrates. There is evidence that the sinusoidal changes in cirrhosis and aging contribute to hepatocyte hypoxia, thus providing a mechanism for the apparent differential reduction of oxygen-dependent phase I metabolic pathways in these conditions. Structural change and subsequent dysfunction of the liver sieve warrant consideration as a significant factor in the impairment of overall substrate handling and hepatic drug metabolism in cirrhosis and aging.

Sulfotransferases in the bioactivation of xenobiotics

Conjugation of xenobiotics is often associated with detoxification. However, this traditional view is one-sided. In particular, numerous compounds are known that are metabolized to chemically reactive metabolites via sulfation (O-sulfonation). This can be rationalized by the fact that the sulfate group is electron-withdrawing and may be cleaved off heterolytically in appropriate molecules, thus leading to the formation of a strongly electrophilic cation. The heterologous expression of sulfotransferases in indicator cells of standard mutagenicity tests has substantially improved the accessibility of this activation pathway. The use of this technology is important, since many reactive sulfate conjugates only show strong toxicological effects if they are generated directly within the indicator cell, due to their insufficient penetration of cell membranes. Xenobiotic-metabolizing sulfotransferases are cytosolic enzymes, which form a superfamily (SULT). Eleven distinct human SULT forms are known, which strongly differ in their tissue distribution and their substrate specificity. Common functionally relevant genetic polymorphisms of the transcribed region are known for two of the forms, SULT1A1 and 1A2. Studies using recombinant test systems demonstrate that many promutagens are activated with high selectivity by an individual SULT form. Pronounced differences in promutagen activation were detected between the different human forms, including their allelic variants, and also between orthologous SULTs from different species. Therefore, SULTs may be involved in the individual genetic disposition, species differences, and organotropisms for toxicological effects of chemicals. Activation by SULTs differs from other activation pathway in its cyclic nature: reaction of a sulfuric acid ester with water usually regenerates the hydroxylated compound, which becomes available for a new cycle of activation. SULT-mediated reactivation may even occur if another initial reactive species, e.g. an epoxide, has reacted with water.

Availability and mean transit times of phenol and its metabolites in the isolated perfused rat liver: normal and retrograde studies using tracer concentrations of phenol

Phenolic compounds are frequently detoxified by the formation of sulphate and glucuronic acid conjugates in the liver. These conjugates are formed in the hepatocytes and then either transported into the bile or back into the blood. In this study, we examined the transport kinetics of phenol and its metabolites in the isolated perfused rat liver by monitoring the outflow profiles of these compounds after a bolus input in a single pass preparation. Phenol was almost exclusively metabolized to phenyl sulphate (97%) at the trace concentrations used, with the amount of phenol and metabolites excreted into the bile being minimal (3.5%). The metabolite formed was rapidly transported back into the perfusate, with mean transit times of 17.4 and 12.3 s anterograde and 24.9 and 24.2 s retrograde at flow rates of 15 and 30 mL min-1 respectively, which were intermediate between those of Evans blue and water. The outflow concentration-time profile for phenyl sulphate formation was unaffected by the addition of another organic anion (bromosulphophthalein). The effect of enzyme zonation on outflow concentration-time profiles was also investigated using retrograde perfusions. The transit time ratios for generated metabolite to water for anterograde perfusions (0.6) was found to be more than twice that for retrograde perfusions (0.23) at 15 mL min-1 and approximately 1.6 times greater at 30 mL min-1, being 0.58 and 0.37 respectively. The relative ratios obtained are consistent with previous findings that normalized variance of solutes in the retrograde perfusions is greater than that for anterograde perfusions.