Heavy metal detoxication in the mussel Mytilus edulis-composition of Cd-containing kidney granules (tertiary lysosomes)

Intra- and Intercellular Silver Nanoparticle Translocation and Transformation in Oyster Gill Filaments: Coupling Nanoscale Secondary Ion Mass Spectrometry and Dual Stable Isotope Tracing Study

The extensive application of silver nanoparticles (AgNPs) requires a full examination of their biological impacts, especially in aquatic systems where AgNPs are likely to end up. Despite numerous toxicity studies from molecular to individual levels, it is still a daunting challenge to achieve in situ subcellular imaging of Ag and to determine the sites of AgNP interaction with organelles or macromolecules simultaneously. Here, by coupling high-resolution nanoscale secondary ion mass spectrometry elemental mapping with scanning electron microscopy ultrastructural characterization, we successfully visualized the subcellular localization and the potential toxicity effects of AgNPs in the oyster gill filaments. The stable isotope tracing method was also adopted to investigate the respective uptake and transport mechanisms of differently labeled ¹⁰⁹AgNPs and ¹⁰⁷Ag⁺ ions. ¹⁰⁹Ag hotspots were colocalized with endosomes or lysosomes, proving an endocytosis-based entry of AgNPs which passed through the barrier of oyster gill epithelium. These ¹⁰⁹Ag hotspots showed a strong colocalization with ³²S^-. For the first time, we provided visualized evidence of AgNP-induced autophagy in the oyster gill cells. We further identified two categories of hemocytes (blood cells) and illustrated their roles in AgNP transport and sequestration. The integration of morphological and functional aspects of Ag subcellular distribution in different target cells suggested that oysters were equipped with a specialized endolysosomal (epithelial cells) or phagolysosomal system (hemocytes) in regulating the cellular process of AgNPs, during which the lysosome was the most involved organelle and sulfur was the most relevant macronutrient element. This study highlighted not only the intracellular but also the intercellular AgNP translocation and transformation, providing important subcellular imaging of silver and reliable methodology regarding bio-nano interactions in natural environments.

In vivo oral bioavailability of Pb sequestered in metal rich granules in bivalves

The present study was designed to evaluate in vivo the oral bioavailability of lead (Pb) present in the marine bivalve Dosinia exoleta. This infaunal clam, despite inhabiting in clean areas, presents Pb concentrations that are over the 1.5 mg kg^-1 wet weight limit for human consumption set by the European Commission. However, Pb is accumulated in this clam in the form of metal rich granules, and it has been shown to be unavailable for trophic transfer to a marine decapod, so it was hypothesised that it might be unavailable for human consumers as well. Twelve Sprague Dawley rats were fed during 14 days with a diet including control mussels (Mytilus galloprovincialis), D. exoleta, or mussels enriched in Pb to the same levels as those found in D. exoleta. Pb accumulation in different rat tissues (blood, bone, kidneys and liver) was analysed. It was observed that Pb assimilation from D. exoleta was about half of Pb assimilation from M. galloprovincialis, and absolute bioavailabilities were around 2% for M. galloprovincialis and 1% for D. exoleta. These results suggest that it might be possible to increase the limit for human consumption for this bivalve to 3 mg kg^-1 wet weight without representing an increase in the risk for consumers.

Cadmium bioaccumulation and elimination in tissues of the freshwater mussel Anodonta woodiana

Experiments were carried out to investigate the bioaccumulation and elimination of cadmium (Cd) in tissues (kidney, gills, digestive gland, mantle, visceral mass, foot, adductor muscle and hemolymph) from the freshwater mussel, Anodonta woodiana. The mussels were exposed to subchronic Cd at concentrations of 0.168 and 0.675 mg L^-1 for 28 d of bioaccumulation and 28 d of elimination. During the bioaccumulation phase, Cd bioaccumulations increased in all tissues. The highest bioaccumulation of Cd was found in the kidney. The second-highest and third-highest bioaccumulations of Cd were found in the digestive gland and gills, respectively. The Cd bioaccumulations in the tissues of A. woodiana increased with exposure time and concentration, except for hemolymph, which reached the highest value on d 14. The bioaccumulation factors (BCFs) increased with exposure time, but an inverse relationship was observed between BCFs and exposure concentration. During the elimination phase, the visceral mass showed the highest Cd elimination rate. In the kidney, digestive gland and gills, the elimination rates almost reached 40%, but their concentrations were still higher than in other tissues. Thus, we concluded that the kidney, gills and digestive gland of A. woodiana are target tissues for subchronic Cd toxicity.

Subcellular distribution and trophic transfer of Pb from bivalves to the common prawn Palaemon serratus

The edible clam Dosinia exoleta has been reported to accumulate high contents of lead (Pb) in soft tissues disregarding the levels of Pb in the environment. This is due to the retention of Pb in the form of metal rich granules (MRG) in their kidneys throughout the mollusc lifespan. The potential for trophic transfer of Pb in this form to predators is expected to be low, since metals in the form of MRG are generally supposed to be trophically unavailable, but this assumption is based on studies with other metals (Ag, Cd, Cu or Zn) and has not been demonstrated with Pb until now. This study was designed to test if the Pb present in D. exoleta in the form of MRG is available to a decapod consumer, the common prawn Palaemon serratus, in comparison with a mussel diet showing a different subcellular distribution of Pb. As hypothesised, despite the high Pb concentrations (15µgg^-1ww) offered to the prawns as D. exoleta tissues, Pb was almost completely unavailable for trophic transfer, and the prawns fed with this diet during 28 days showed the same Pb accumulation as prawns fed with a control diet with a much lower Pb concentration. On the contrary, individuals fed with mussel tissues containing the same Pb concentrations as the diet based on D. exoleta tissues showed 10 times higher Pb bioaccumulation, corresponding to a trophic transfer factor of 1.1%. Subcellular fractionation experiments revealed that the fraction of Pb in the form of MRG was much lower for the mussel, confirming, as observed for other metals, that MRG-associated Pb is not available for trophic transfer to decapod crustaceans.

Investigation of thermostable metalloproteins in Perna perna mussels from differentially contaminated areas in Southeastern Brazil by bioanalytical techniques

Metallomic studies regarding environmental contamination by metals are of value in elucidating metal uptake, trafficking, accumulation and metabolism in biological systems. Many proven bioindicator species, such as bivalves, have not yet, however, been well-characterized regarding their metalloprotein expression in response to environmental contaminants. In this context, the aim of the present study was to investigate metalloprotein expressions in the thermostable protein fraction of muscle tissue and digestive glands from mussels (Perna perna) from three differentially metal-contaminated sites in Southeastern Brazil in comparison with a reference site. The thermostable protein fractions were analyzed by SDS-PAGE and SEC-HPLC-ICP-MS. Metal content was also determined in both the crude and the purified extracts. Several inter-organ differences were observed, which is to be expected, while inter-site differences regarding thermostable protein content were also verified, indicating accumulation of these elements in muscle tissue and digestive glands and disruption of homeostasis of essential elements, with detoxification attempts by metal-bound proteins, since all metalloproteins present in both matrices eluted bound to at least one non-essential metal. These results are also noteworthy with regard to the adopted reference site, that also seems to be contaminated by toxic metals.

Exposure-dose-response of Tellina deltoidalis to contaminated estuarine sediments 3. Selenium spiked sediments

The metalloid selenium is an essential element which at slightly elevated concentrations is toxic and mutagenic. In Australia the burning of coal for power generation releases selenium into estuarine environments where it accumulates in sediments. The relationship between selenium exposure, dose and response was investigated in the deposit feeding, benthic, marine bivalve Tellina deltoidalis. Bivalves were exposed in microcosms for 28 days to individual selenium spiked sediments, 0, 5 and 20 μg/g dry mass. T. deltoidalis accumulated selenium from spiked sediment but not in proportion to the sediment selenium concentrations. The majority of recovered subcellular selenium was associated with the nuclei and cellular debris fraction, probably as protein bound selenium associated with plasma and selenium bound directly to cell walls. Selenium exposed organisms had increased biologically detoxified selenium burdens which were associated with both granule and metallothionein like protein fractions, indicating selenium detoxification. Half of the biologically active selenium was associated with the mitochondrial fraction with up to 4 fold increases in selenium in exposed organisms. Selenium exposed T. deltoidalis had significantly reduced GSH:GSSG ratios indicating a build-up of oxidised glutathione. Total antioxidant capacity of selenium exposed T. deltoidalis was significantly reduced which corresponded with increased lipid peroxidation, lysosomal destabilisation and micronuclei frequency. Clear exposure-dose-response relationships have been demonstrated for T. deltoidalis exposed to selenium spiked sediments, supporting its suitability for use in selenium toxicity tests using sub-lethal endpoints.

Exposure-dose-response of Tellina deltoidalis to metal contaminated estuarine sediments 2. Lead spiked sediments

Lead accumulation in estuarine sediments, as a result of activities such as mining and ore smelting, and through urban runoff is a continuing problem in the increasingly developed world. Marine organisms accumulate lead, which is known to be highly toxic to biological processes and to degrade organism and ecosystem health. Here the relationship between lead exposure, dose and response was investigated in the sediment dwelling, deposit feeding, marine bivalve Tellina deltoidalis. Bivalves were exposed in the laboratory to individual lead spiked sediments at < 0.01, 100 and 300 μg/g dry mass, for 28 days and accumulated total tissue lead concentrations of 4, 96 and 430 μg/g, respectively. Subcellular fractionation indicated that around 70% of the total accumulated tissue lead was detoxified, three quarters of the detoxified lead fraction was converted into metal rich granules, with the remainder in the metallothionein like protein fraction. The majority of biologically active lead was associated with the mitochondrial fraction with up to a 128 fold increase in lead burden in exposed organisms compared to controls. This indicates lead detoxification was occurring but the organism was unable to prevent lead interacting with sensitive organelles. With increased lead exposure T. deltoidalis showed a suppression in glutathione peroxidase activity, total glutathione concentration and reduced GSH:GSSG ratios, however, these differences were not significant. Lead exposed T. deltoidalis had a significantly reduced total antioxidant capacity which corresponded with increased lipid peroxidation, lysosomal destabilisation and micronuclei frequency. The exposure-dose-response relationships demonstrated for lead exposed T. deltoidalis supports its potential for the development of sublethal endpoints in lead toxicity assessment.

Exposure-dose-response of Tellina deltoidalis to metal-contaminated estuarine sediments: 1. Cadmium spiked sediments

Cadmium is a ubiquitous environmental metal contaminant with an affinity for biological membranes; it can enter cells by facilitated transport and it binds therein to various biomolecules and affects membrane system function. The relationship between cadmium exposure, dose and response was investigated in the benthic, deposit feeding, marine bivalve Tellina deltoidalis, using 28 day microcosm spiked cadmium exposures. Tissue cadmium reached steady state with the exposure concentration. Half the accumulated cadmium was detoxified and with increased exposure more was converted into metal rich granules. Most biologically active cadmium was in the mitochondrial fraction, with up to 7320-fold cadmium increases in exposed organisms. Cadmium exposed T. deltoidalis generally had reduced glutathione peroxidase enzyme activity. An increase in total glutathione concentrations, due to a build up of oxidised glutathione, was indicated by the reduced to oxidised glutathione ratio. All cadmium exposed T. deltoidalis had reduced total antioxidant capacity that corresponded with increased lipid peroxidation, lysosomal destabilisation and micronuclei frequency. Clear exposure-dose-response relationships have been demonstrated for T. deltoidalis exposed to cadmium-spiked sediments, supporting this organism's suitability for laboratory or in situ evaluation of sediment cadmium toxicity.

Subcellular differences in handling Cu excess in three freshwater fish species contributes greatly to their differences in sensitivity to Cu

Since changes in metal distribution among tissues and subcellular fractions can provide insights in metal toxicity and tolerance, we investigated this partitioning of Cu in gill and liver tissue of rainbow trout (Oncorhynchus mykiss), common carp (Cyprinus carpio) and gibel carp (Carassius auratus gibelio). These fish species are known to differ in their sensitivity to Cu exposure with gibel carp being the most tolerant and rainbow trout the most sensitive. After an exposure to 50 μg/l (0.79 μM) Cu for 24h, 3 days, 1 week and 1 month, gills and liver of control and exposed fish were submitted to a differential centrifugation procedure. Interestingly, there was a difference in accumulated Cu in the three fish species, even in control fishes. Where the liver of rainbow trout showed extremely high Cu concentrations under control conditions, the amount of Cu accumulated in their gills was much less than in common and gibel carp. At the subcellular level, the gills of rainbow trout appeared to distribute the additional Cu exclusively in the biologically active metal pool (BAM; contains heat-denaturable fraction and organelle fraction). A similar response could be seen in gill tissue of common carp, although the percentage of Cu in the BAM of common carp was lower compared to rainbow trout. Gill tissue of gibel carp accumulated more Cu in the biologically inactive metal pool (BIM compared to BAM; contains heat-stable fraction and metal-rich granule fraction). The liver of rainbow trout seemed much more adequate in handling the excess Cu (compared to its gills), since the storage of Cu in the BIM increased. Furthermore, the high % of Cu in the metal-rich granule fraction and heat-stable fraction in the liver of common carp and especially gibel carp together with the better Cu handling in gill tissue, pointed out the ability of the carp species to minimize the disadvantages related to Cu stress. The differences in Cu distribution at the subcellular level of gills and liver of these fish species strongly reflects their capacity to handle Cu excess and is one of the greatest contributors to their difference in sensitivity to Cu.

Copper and zinc detoxification in Gammarus pulex (L.)

To negate the toxicity of labile intracellular metals, some aquatic organisms partition metals into specific subcellular locations for detoxification, namely the soluble heat-stable cytosol and insoluble metal-rich granules. The aim of the present study was to characterise these subcellular storage sites in the freshwater crustacean Gammarus pulex (Linnaeus) following in situ exposures upstream (Drym, low metal) and downstream (Relubbus, elevated metal) of copper- and zinc-rich inflows into the River Hayle (Cornwall, UK). In the cytosol of gammarids exposed at Relubbus, copper and zinc associated to a 7.5-kDa metallothionein-like protein (MTLP) that was largely absent from gammarids prior to exposure. Exposure at Relubbus caused MTLP concentrations to increase 4- to 5-fold between days 2 and 4, indicating an induction response to increased labile intracellular metal. On day 16, spherical calcium-rich granules (0.5–2.5 μmol l–1) were visualised and analysed in the posterior caeca of gammarids exposed at both sites. Following exposure at Relubbus, granules contained trace amounts of copper, but zinc was absent. Granules in gammarids exposed at Drym contained no detectable copper or zinc. Granule formation appeared to be independent of exposure. Within the posterior caeca, granules have been associated with calcium storage during the crustacean molt, rather than in detoxification of trace metals. However, the granular copper burden appeared to follow environmental Cu availabilities. Thus, we describe Cu sequestration within molt-cycle calcium storage granules. As both MTLP concentrations and granule formation in crustaceans are affected upon by molting, we hypothesise that detoxification might impact upon this existing process.

The metal-binding features of the recombinant mussel Mytilus edulis MT-10-IV metallothionein

In contrast with the paradigmatic mammalian metallothioneins (MTs), mollusc MT systems consist at least of a high-cadmium induced form, possibly involved in detoxification, and another isoform either constitutive or regulated by essential metals and probably associated with housekeeping metabolism. With the aim of providing a deeper characterization of the coordination features of a molluscan MT peptide of the latter kind, we have analyzed here the metal-binding abilities of the recombinant MeMT-10-IV isoform of Mytilus edulis (MeMT). Also, comparison with other MTs of this type has been undertaken. A synthetic complementary DNA was constructed, cloned and expressed into two Escherichia coli systems. Upon zinc coordination, MeMT folds in vivo into highly chiral and stable Zn(7) complexes, with an exceptional reluctance to fully substitute cadmium(II) and/or copper(I) for zinc(II). In vivo cadmium binding leads to homometallic Cd(7) complexes that structurally differ from any of the in vitro prepared Cd(7) complexes. Homometallic Cu-MeMT can only be obtained in vitro from Zn(7)-MeMT after a great molar excess of copper(I) has been added. In vivo, two different heterometallic Zn,Cu-MeMT complexes are recovered, which nicely correspond to two distinct stages of the in vitro zinc/copper replacement. These MeMT metal-binding features are consistent with a physiological role related to basal/housekeeping metal, mainly zinc, metabolism, and confirm the correspondence between the MeMT gene response pattern and the functional properties of the encoded protein.

Metal influence on metallothionein synthesis in the hydrothermal vent mussel Bathymodiolus thermophilus

The present study reports on the metallothionein expression in the hydrothermal vent mussel Bathymodiolus thermophilus. Metallothioneins (MT) are proteins involved in intracellular metal regulation and conserved throughout the animal kingdom. The hydrothermal vent environment presents peculiarities (high levels of sulfides and metals, low pH, anoxia) that may have driven associated species to develop original evolutionary ways to face these extreme living conditions. Mussels were exposed to different metal solutions at the atmospheric pressure. The MT mRNA levels and MT contents were measured in gills and mantles of each exposed mussel. The intracellular metal distribution was estimated in fractions obtained after the centrifugation of tissue homogenates. A few of the tested metals (Ag, Cu, Cd, Hg and Zn) were able to significantly induce MT mRNA levels. Silver was the only one that produced a significant increase of the MT protein level in both mantle and gills. The gills always presented higher MT protein levels than the mantle did, while their MT mRNA levels were similar. Our data show that MT mRNA and MT protein levels do not follow a clear relationship in the gills and mantle of B. thermophilus and we assume that a posttranscriptional control occurs in these mussels.

65Zn2+ Transport by lobster hepatopancreatic lysosomal membrane vesicles

In crustaceans, the hepatopancreas is the major organ system responsible for heavy metal detoxification, and within this structure the lysosomes and the endoplasmic reticulum are two organelles that regulate cytoplasmic metal concentrations by selective sequestration processes. This study characterized the transport processes responsible for zinc uptake into hepatopancreatic lysosomal membrane vesicles (LMV) and the interactions between the transport of this metal and those of calcium, copper, and cadmium in the same preparation. Standard centrifugation methods were used to prepare purified hepatopancreatic LMV and a rapid filtration procedure, to quantify 65Zn2+ transfer across this organellar membrane. LMV were osmotically reactive and exhibited a time course of uptake that was linear for 15-30 sec and approached equilibrium by 300 sec. 65Zn2+ influx was a hyperbolic function of external zinc concentration and followed Michaelis-Menten kinetics for carrier transport (Km = 32.3 +/- 10.8 microM; Jmax = 20.7 +/- 2.6 pmol/mg protein x sec). This carrier transport was stimulated by the addition of 1 mM ATP (Km = 35.89 +/- 10.58 microM; Jmax = 31.94+/-3.72 pmol/mg protein/sec) and replaced by an apparent slow diffusional process by the simultaneous presence of 1 mM ATP+250 microM vanadate. Thapsigargin (10 microM) was also a significant inhibitor of zinc influx (Km = 72.87 +/- 42.75 microM; Jmax =22.86 +/- 4.03 pmol/mg protein/sec), but not as effective in this regard as was vanadate. Using Dixon analysis, cadmium and copper were shown to be competitive inhibitors of lysosomal membrane vesicle 65Zn2+ influx by the ATP-dependent transport process (cadmium Ki = 68.1 +/- 3.2 microM; copper Ki = 32.7 +/- 1.9 microM). In the absence of ATP, an outwardly directed H+ gradient stimulated 65Zn2+ uptake, while a proton gradient in the opposite direction inhibited metal influx. The present investigation showed that 65Zn2+ was transported by hepatopancreatic lysosomal vesicles by ATP-dependent, vanadate-, thapsigargin-, and divalent cation-inhibited, carrier processes that illustrated Michaelis-Menten influx kinetics and was stimulated by an outwardly directed proton gradient. These transport properties as a whole suggest that this transporter may be a lysosomal isoform of the ER Sarco-Endoplasmic Reticulum Calcium ATPase.

Sub-cellular partitioning of Cd, Cu and Zn in tissues of indigenous unionid bivalves living along a metal exposure gradient and links to metal-induced effects

We studied organ and sub-cellular distributions of several trace metals in a freshwater bivalve that has been proposed for use as a metal biomonitor. Specimens of Pyganodon grandis were collected from nine lakes located along a Cd, Cu and Zn concentration gradient (Rouyn-Noranda area, Quebec). Gills and digestive gland were isolated, homogenized and six sub-cellular fractions were separated by differential centrifugation and analysed for their Cd, Cu and Zn content. Metallothionein was quantified independently. Gill tissues contained abundant calcium concretions that accounted for over 60% of the total gill burden of each metal. Cadmium and Zn concentrations in this granule fraction reflected ambient metal concentrations. Metal concentrations in the digestive gland also responded to the metal contamination gradient, but to a lesser extent than the gills, reflecting the lower abundance of granules in the digestive gland. Metals (Cd, Cu) in this organ were present largely in the "heat-stable proteins" fraction, and metal concentrations in this fraction were strongly correlated with those of both metallothionein and, to a lesser extent, the "lysosomes+microsomes" and "mitochondria" fractions. In both organs, Cd concentrations in the "heat-denaturable protein" fraction remained low and constant, suggesting reasonably effective metal detoxification. Some evidence for oxidative stress was noted in the gills but not in the digestive gland. Overall, we conclude that in nature metals in P. grandis are bound differently in the gills and in the digestive gland and that metal detoxification in the former organ may be less effective than in the latter.

Small metallothionein MT-10 genes in coastal and hydrothermal mussels

Metallothioneins (MTs) are important proteins in the intracellular regulation of metals. In the Mytilidae family, which includes many economically important species, 2 major forms of MTs have been reported: MT-10 (10 kDa) and MT-20 (20 kDa). Many different MT-10 proteins have been isolated from the common species Mytilus edulis, which suggests that distinct MT-10 genes may occur in a single specimen. Some MT genes, involving 3 exons and 2 large introns, have been isolated in Mytilidae. Our aim was to determine whether intron-free forms of the MT-10 genes can exist, which could allow rapid transcription in response to exposure to metals. Our study focused on 2 species living under very different environmental conditions: Mytilus edulis (a coastal mussel) and Bathymodiolus thermophilus (a hydrothermal mussel). We report here the first description of small, intron-free MT-10 genes, possessing a correct open reading frame in these 2 species.

Copper transport by lobster (Homarus americanus) hepatopancreatic lysosomes

Lysosomes are known centers for sequestration of calcium and a variety of heavy metals in many invertebrate tissues, and as a result of this compartmentalization these organelles perform important detoxification roles in the animals involved. The present investigation uses a centrifugation method to isolate and purify hepatopancreatic lysosomes from the American lobster, Homarus americanus. Purified lysosomal preparations were used to characterize membrane transport mechanisms in these organelles for transferring and sequestering cytoplasmic copper following its absorption across the plasma membrane from dietary constituents. The copper-specific fluorescent dye, Phen Green, was employed to quantify transmembrane fluxes of this metal as has been recently used to investigate copper movements across hepatopancreatic mitochondrial and plasma membranes. Results indicated the presence of a vanadate-sensitive, calcium-stimulated, copper ATPase in the membranes of these organelles that displayed high affinity carrier-mediated transport kinetics and may significantly contribute to organismic copper homeostasis. Together with a putative bafilomycin-sensitive V-ATPase in the membrane of the same organelles, importing hydrogen ions into the organellar interior, this copper ATPase may function as part of a physiological mechanism for precipitate formation between metallic cations and anions. These ionic precipitate complexes may then act as a sink for excess metals and thereby reduce the circulating concentrations of these elements.

Copper transport by lobster (Homarus americanus) hepatopancreatic mitochondria

Mechanisms of copper transport into purified mitochondrial suspensions prepared from the hepatopancreas of the Atlantic lobster Homarus americanus were investigated. Mitochondria were purified by combining methods of differential and Percoll-gradient centrifugation, and copper transport was studied using the copper-sensitive fluorescent dye Phen Green. Copper transport by this mitochondrial preparation was kinetically the sum of saturable and non-saturable transfer components. Addition of 500 μmol l–1 Ca2+ or 500 nmol l–1 Ruthenium Red abolished the non-saturable copper transport component, significantly (P&lt;0.01) reduced the apparent binding affinity of the saturable transport component, but was without effect (P&gt;0.05) on the apparent maximal transport velocity of the saturable transfer process. The antiport inhibitor diltiazem (500 μmol l–1) acted as a mixed inhibitor of the saturable transport mechanism, but had no effect on the non-saturable component of transfer. These results suggest that the non-saturable copper influx process was probably by way of the well-known Ruthenium-Red-sensitive Ca2+ uniporter and that the saturable transport component was probably due to a combination of both the Na+-dependent, diltiazem-sensitive 1Ca2+/2Na+ antiporter and the Na+-independent, diltiazem-insensitive 1Ca2+/2H+ antiporter. A model is discussed relating these mitochondrial copper uptake processes to the transfer of metal ions across the epithelial brush-border membrane.