High- and Low-Affinity Transport of D-Glucose from Blood to Brain

Abstract: Measurements of the unidirectional blood-brain glucose flux in rat were incompatible with a single set of kinetic constants for transendothelial transport. At least two transfer mechanisms were present: a high-affinity, low-capacity system, and a low-affinity, high-capacity system. The low-affinity system did not represent passive diffusion because it distinguished between D-and L-glucose. The T_max and K _m, for the high-affinity system were 0.16 mmol 100 g⁻¹ min⁻¹ and 1 mM; for the low-affinity system, ∼ 5 mmol 100 g⁻¹ min⁻¹ and ∼ 1 M. With these values, physiological glucose concentrations were not sufficient to saturate the low-affinity system. In normoglycemia, therefore, three independent pathways of glucose transport from blood to brain appear to exist: a high-affinity facilitated diffusion pathway of apparent permeability 235·10⁻⁷ cm s⁻¹, a specific but nonsaturable diffusion pathway of permeability 85·10⁻⁷ cm s^−l, and a nonspecifc passive diffusion pathway of permeability 2·10⁻⁷ cm s⁻¹.     

NADP+-dependent glutamate dehydrogenase from the facultative methylotroph Hyphomicrobium X

Abstract NADP⁺-dependent glutamate dehydrogenase (GDH; EC 1.4.1.4) was purified using acetone precipitation, heat, DEAE-cellulose and dye-ligand Ramazol Red column chromatography. The M _r of the native enzyme was estimated to be 380 000 (± 10 000) by polyacrylamide gel electrophoresis. The same technique in the presence of sodium dodecyl sulphate (SDS) gave one subunit band with an M _r of 63 400 (±4000). Thus the enzyme has a hexameric structure. The enzyme has a pH optimum of 8.5 and has K _m apparent values of 1.6 mM, 0.015 mM and 10.2 mM for α-ketoglutarate, N NADPH and L -glutamate, respectively. Michaelis-Menten kinetics were not observed when the ammonium concentration was increased. A progressive increase in the ammonium concentration resulted in a progressively increasing K _m value. The enzyme was highly specific for all substrates and markedly insensitive to inhibitors.     

Yeast mitochondria import ATP through the calcium-dependent ATP-Mg/Pi carrier Sal1p, and are ATP consumers during aerobic growth in glucose

Sal1p, a novel Ca²⁺-dependent ATP-Mg/Pi carrier, is essential in yeast lacking all adenine nucleotide translocases. By targeting luciferase to the mitochondrial matrix to monitor mitochondrial ATP levels, we show in isolated mitochondria that both ATP-Mg and free ADP are taken up by Sal1p with a K _m of 0.20 ± 0.03 mM and 0.28 ± 0.06 mM respectively. Nucleotide transport along Sal1p is strictly Ca²⁺ dependent. Ca²⁺ increases the V _max with a S _0.5 of 15 μM, and no changes in the K _m for ATP-Mg. Glucose sensing in yeast generates Ca²⁺ transients involving Ca²⁺ influx from the external medium. We find that carbon-deprived cells respond to glucose with an immediate increase in mitochondrial ATP levels which is not observed in the presence of EGTA or in Sal1p-deficient cells. Moreover, we now report that during normal aerobic growth on glucose, yeast mitochondria import ATP from the cytosol and hydrolyse it through H⁺-ATP synthase. We identify two pathways for ATP uptake in mitochondria, the ADP/ATP carriers and Sal1p. Thus, during exponential growth on glucose, mitochondria are ATP consumers, as those from cells growing in anaerobic conditions or deprived of mitochondrial DNA which depend on cytosolic ATP and mitochondrial ATPase working in reverse to generate a mitochondrial membrane potential. In conclusion, the results show that growth on glucose requires ATP hydrolysis in mitochondria and recruits Sal1p as a Ca²⁺-dependent mechanism to import ATP-Mg from the cytosol. Whether this mechanism is used under similar settings in higher eukaryotes is an open question.     

Developmental Changes of Cerebral Cortical [3H]Clonidine Binding in Rats: Influences of Guanine Nucleotide and Cations

Abstract: Cerebral cortical [³H]clonidine binding and influences of GTP and cations were investigated in developing rats. The results from Scatchard plots were compatible with the presence of two populations of binding sites [high-affinity binding ( K_D = 0.59 n M ) and low-affinity binding ( K_D = 7.12 n M )] in 70-day-old rats but only high-affinity binding ( K_D = 0.27 n M ) on day 1. Low-affinity binding was detectable on day 7. K_D values in high- and low-affinity binding were not significantly changed during development after 7 days. B_max of high-affinity binding reached a peak on day 15, and the value of low-affinity binding gradually increased with age. The addition of 10 μ M GTP caused a significant reduction in B_max of high-affinity binding after day 7. Neither K _D nor B_max of low-affinity binding was affected by 10 μ M GTP during development. NaCl (10 and 100 m M ) diminished the binding on days 7 and 70. MnCl₂ (0.1 and 1.0 m M ) markedly increased the binding on days 15 and 70 but not on day 7. It is suggested that: (1) single binding sites of α₂-adrenoceptors with higher affinity seem to be present on day 1; (2) low-affinity binding appears on day 7; (3) the number of high-affinity binding sites reaches a peak on day 15, followed by changes in populations of high-affinity as well as low-affinity sites without changing affinity; (4) the regulatory mechanism in α₂-receptors by guanine nucleotide reaches functional maturity between days 1 and 7; and (5) the involvement of Na⁺ and Mn²⁺ in α₂-receptor binding becomes functional by days 7 and 15, respectively.     

Environmental induction of Na+ transporter affinity in Atlantic salmon embryos

Atlantic salmon ( Salmo salar L.) alevins hatched from eggs transferred from high- to low-Na water at 250° days, before the onset of the phase of increasing whole egg sodium content (at ∼380°days), showed a significantly reduced K _m for Na⁺ transport, whereas transfer at 400° days did not produce any change in K _m . Alevins hatched from eggs given acid shocks of 1, 3, 7 or 14 days duration initiated at 250 or 400° days showed no significant changes in Na⁺ transporter K _m . Extended acid exposure (38 days) from 250°days to hatching resulted in a slight lowering of K _m (P<0.05). A 24-day acid exposure from 400°days to hatching had no effect on Na⁺ transporter K _m . Alevins hatched from eggs incubated throughout in acidified water had a significantly reduced K _m compared to controls (P<0.01).
The timing and duration of periods of Na depletion of eggs is considered with respect to environmental induction of increased Na transporter affinity in teleost embryos as a mechanism of long-term physiological adaptation to the gradual acidification of natural waters.     

The contribution of electrostatic forces to the process of adherence of Candida albicans yeast cells to substrates

Abstract Thermoanaerobacter thermohydrosulfuricus Rt8.B1 catabolized xylose by the pentose phosphate pathway, and xylose isomerase and xylulokinase were inducible. The uptake of xylose was by two low-affinity, inducible systems. Both systems were resistant to the protonophore, tetrachlorosalicylanilide, the F₁F₀-ATPase inhibitor, N , N -dicyclohexylcarboiimide, and the sodium/proton antiporter, monensin. The high capacity system (100 nmol min⁻¹ (mg protein)⁻¹) was only expressed when the bacterium was grown with a high concentration of xylose (50 mM). It took more than 60 mM xylose to saturate the high capacity system. When T. thermohydrosulfuricus was grown with a low concentration of xylose (5 mM), xylose uptake was saturated by as little as 10 mM xylose (18 nmol min⁻¹ (mg protein)⁻¹). Cells grown with 50 mM xylose could not transport glucose, and high capacity xylose transport was not inhibited by glucose or non-metabolizable glucose analogues. Cells grown with 5 mM xylose transported glucose at a rapid rate (30 nmol min⁻¹ (mg protein)⁻¹), and low capacity xylose uptake was competitively inhibited by either glucose or 2-deoxy-glucose. Because the glucose uptake of cells grown on 5 mM xylose was competitively inhibited by xylose, it appeared that the low capacity xylose uptake system was a glucose/xylose carrier.     

Does calcium ameliorate the negative effect of NaCl on melon root water transport by regulating aquaporin activity?

The hydraulic conductance ( L ₀) of detached, exuding root systems from melon ( Cucumis melo cv. Amarillo oro) was measured. All plants received a half-strength Hoagland nutrient solution, and plants stressed either solely with NaCl (50 mM) or with NaCl (50 mM) following treatment (2 d) with CaCl₂ (10 mM) were compared with controls and CaCl₂-treated (10 mM) plants. The L ₀ of NaCl-treated plants was markedly decreased when compared to control and CaCl₂-treated plants, but the decrease was smaller when NaCl was added to plants previously treated with CaCl₂. A similar effect was observed when the flux of Ca²⁺ into the xylem and the Ca²⁺ concentration in the plasma membrane of the root cells were determined. In control, CaCl₂- and NaCl + CaCl₂-treated plants, HgCl₂ treatment (50 μM) caused a sharp decline in L ₀ to values similar to those of NaCl-stressed roots, but L ₀ was restored by treatment with 5 mM DTT. However, in NaCl roots only a slight effect of Hg²⁺ and DTT were observed. The effect of all treatments on L ₀ was similar to that on osmotic water permeability ( P _f) of individual protoplasts isolated from roots. The results suggest that NaCl decreased the passage of water through the membrane and roots by reducing the activity of Hg-sensitive water channels. The ameliorative effect of Ca²⁺ on NaCl stress could be related to water-channel function.     

Kinetics of Regional Blood–Brain Barrier Transport and Brain Phosphorylation of Glucose and 2-Deoxyglucose in the Barbiturate–Anesthetized Rat

Abstract: Recent studies indicate the lumped constant (LC), which defines the relative rates of brain utilization of glucose and 2-deoxyglucose (2-DG), doubles to values > 1.0 under conditions of hypoglycemia. Since changes in the LC should be predictable given the kinetic parameters of blood-brain barrier (BBB) transport and brain phosphorylation of glucose and 2-DG, the present studies were designed to measure the necessary kinetic parameters. The carotid injection technique was used to determine cerebral blood flow and the K_m , V_max, and K _D of glucose and 2-DG transport through the BBB in seven brain regions in rats anesthetized with 50 mg/kg i.p. pentobarbital. Regional glucose transport through the BBB was characterized by an average K_m = 6.3 m m , average V_max = 0.53 μmol min⁻¹g⁻¹, and average K _D= 0.022 ml min⁻¹g⁻¹. The nonsaturable route of transport of glucose represented on the average 40% of the total glucose influx into brain regions at an arterial glucose concentration of 10 m m . In addition, the rate constants of phosphorylation of glucose and 2-DG were measured for each region. Substitutions of the measured kinetic parameters for sugar transport and phosphorylation into equations defining the LC confirm the observation that the LC would be expected to vary under extreme conditions such as hypoglycemia and to exceed values of 1.0 under these conditions.     

Chitinolytic properties of Bacillus pabuli K1

The chitinolytic properties of Bacillus pabuli K1 isolated from mouldy grain was studied. Chitinase activity was measured as the release of p -nitrophenol from p -nitrophenyl-N, N'-diacetylchitobiose. Influences of substrate concentration and different environmental variables on growth and chitinase activity were determined. The optimum environmental conditions for chitinase production were: 30°C, initial pH 8, initial oxygen 10% and a_w > 0.99. Chitinase production was induced when B. pabuli K1 was grown on colloidal chitin. The smallest chito-oligosaccharide able to induce chitinase production was N, N'-diacetylchitobiose, (GlcNAc)₂. Production was also induced by (GlcNAc)₃ and (GlcNAc)₄. When the bacterium was grown on glucose or N -acetylglucosamine, no chitinases were formed. The highest chitinase production observed was obtained with colloidal chitin as substrate. The production of chitinases by B. pabuli K1 growing on chitin was repressed by high levels (0.6%) of glucose. The production was also repressed by 0.6% starch, laminarin and β-glucan from barley and by glycerol. The addition of pectin and carboxymethyl cellulose increased chitinase production.     

D-GALACTOSE TRANSPORT BY SYNAPTOSOMES ISOLATED FROM RAT BRAIN

Abstract— Synaptosomes prepared by differential and Ficoll density gradient centrifugation took up d -galactose by two saturable transport systems: one. a high affinity system with a K _m of 0-25 mn and V_max of 075 nmol/mg protein 3 min, the other, a low affinity system with a K_m of 47 mM and a V_max of 135 nmol/mg protein/3 min. The high affinity system was inhibited by 1-5 mM phlorizin but was unaffected by the absence of sodium ion or the presence of 1 mM ouabain. The low affinity system was unaffected by phlorizin or ouabain. Both systems were inhibited by high concentrations of glucose. 2-deoxyga-lactose. and inositol, and by 2.4-dinitrophcnol. Galactose was rapidly converted in synaptosomes to phos-phorylatcd intermediates and was more slowly oxidized to ¹⁴CO₂     

The Uptake of Carnitine by Slices of Rat Cerebral Cortex

Abstract: The properties of carnitine transport were studied in rat brain slices. A rapid uptake system for carnitine was observed, with tissue-medium gradients of 38 ± 3 for L-[¹⁴CH₃]carnitine and 27 ± 3 for D-[¹⁴CH₃]carnitine after 180 min incubation at 37°C in 0.64 mM substrate. Uptake of L- and D-carnitine showed saturability. The estimated values of K _m for L- and D-carnitine were 2.85 mM and 10.0 mM, respectively; but values of V _max (1 μmol/min/ml in-tracellular fluid) were the same for the two isomers. The transport system showed stereospecificity for L-carnitine. Carnitine uptake was inhibited by structurally related compounds with a four-carbon backbone containing a terminal carboxyl group. L-Carnitine uptake was competitively inhibited by γ-butyrobetaine ( K _i= 3.22 mM), acetylcarnitine ( K _i= 6.36 mM), and γ-aminobutyric acid ( K _i= 0.63 mM). The data suggest that carnitine and γ-aminobutyric acid interact at a common carrier site. Transport was not significantly reduced by choline or lysine. Carnitine uptake was inhibited by an N₂ atmosphere, 2,4-dinitrophenol, carbonylcyanide- N -chlorophenylhydrazone, potassium cyanide, n-ethylmaleimide, and ouabain. Transport was abolished by low temperature (4°C) and absence of glucose from the medium. Carnitine uptake was Na⁺-dependent, but did not require K⁺ or Ca²⁺.     

Reduced Development of Grey Mould (Botrytis cinerea) in Bean and Tomato Plants by Calcium Nutrition

Fertilization of bean plants grown in perlite with 1 and 3 mM CaCl₂ or Ca(NO₃)₂ reduced severity of grey mould as compared with control plants or plants fertilized with 5 mM of the compounds. Fertilization with Ca(NO₃)₂ reduced severity leaf grey mould and fruit ghost spots of tomato plants grown in perlite by 70 and 45%, respectively. The rate of decrease varied with the position of the fruits on the plants. Leaves from plants treated with calcium or otherwise [KNO₃, (NH₄)₂SO₄] produced less ethylene than leaves of nontreated plants. Rate of growth of B. cinerea was lower on growth medium prepared from washings from leaves of calcium fertilized plants than from leaves from other treatments. The fertilizer combination Ca(H₂PO₄)₂+ CaSO₄ (1 and 3 g/kg soil) applied once to tomato plants grown in soil reduced severity of leaf grey mould by 80 % (significant at P = 0.05) but 1–3 g CaSO₄/kg soil only tended to reduce disease severity (30–40 %, not significant) as compared with the control. The compounds CaCl₂ and Ca(NO₃)₂ increased significantly ( P = 0.05) the growth of B. cinerea on synthetic medium when applied at rates of 1 0–10.0 mM whereas reduction of growth was observed with 0.1 mM of the compounds and of CaSO₄.     

Nitrogen use efficiency. 3. Nitrogen fixation: genes and costs

The nitrogen use efficiencies (NUE) of N₂ fixation, primary NH ₄⁺ assimilation and NO ₃⁻ assimilation are compared. The photon and water costs of the various biochemical and transport processes involved in plant growth, N-assimilation, pH regulation and osmolarity generation, per unit N assimilated are respectively likely to be around 5 and 7% greater for N₂ fixation than for a combination of NH ₄⁺ and root and shoot NO ₃⁻ assimilation as occurs with most crops. Studies on plant and rhizobial genes involved in nodulation and N₂ fixation may lead to more rapid nodulation, production of more stress-tolerant N₂ fixing systems and transfer of the hydrogenase system to rhizobium/legume symbioses which currently do not have this ability. The activity of an uptake hydrogenase is predicted to decrease the photon cost of diazotrophic plant growth by about 1%.     

THE CHEMICAL FORM OF DISSOLVED SI TAKEN UP BY MARINE DIATOMS

Results of past studies of the pH-dependent Si uptake kinetics of Phaeodactylum tricornutum Bohlin suggested that the anion SiO(OH)     is the chemical form of dissolved Si taken up by marine diatoms. We determined the chemical form of Si taken up by three other marine diatom species and P. tricornutum by examining the kinetics of Si use under two dramatically different SiO(OH)     :Si(OH)₄ ratios in seawater by varying pH from ≈8 to ≈9.6. Uptake rates were determined using a precise and sensitive ³²Si tracer methodology. The pH-dependent uptake kinetics obtained for all species except P. tricornutum suggest that marine diatoms transport Si(OH)₄. The half-saturation constant (K _m ) varies strongly as a function of pH for all species when the substrate of transport is assumed to be SiO(OH)     . Kinetic curves for Thalassiosira pseudonana (Hustedt) Hasle et Heimdal, Thalassiosira weissflogii (Grunow) G. Fryxell et Hasle, and Cylindrotheca fusiformis Reimann et Lewin have statistically identical values of K _m at each pH when the substrate for transport is assumed to be Si(OH)₄ ( T. pseudonana and T. weissflogii ) or total dissolved silicon ( C. fusiformis ). In contrast, P. tricornutum exhibits unusual biphasic uptake kinetics: uptake conforms to Michaelis–Menten kinetics up to 15 to 25 μM, above which uptake increases linearly. This enigmatic response may have biased conclusions drawn from past experiments using this species. However, based on the consistency of the results for the three other species, a new model of Si transport in marine diatoms is proposed on the basis of the direct formation of a complex between the Si-transport protein and Si(OH)₄.     

Effects of sulfate and nitrate on K+ uptake and growth of wheat and cucumber

The uptake of K⁺ ion was studied in the roots of wheat ( Triuicum aestivum L. cv. GK Szeged) and cucumber ( Cucumis sativus L. cv. Budai csemege) seedlings grown in nutrient solution under nitrogen and sulfate stress conditions. Seedlings pretreated with 1 or 10 m M NaNO₃, absorbed more K⁺ than those treated with 0.1 m M NaNO₃. However, the posteffect of NaNO₃ was considerably influenced by the Na₂SO₄, treatment. The results suggest that, at least partly, a feed-back regulation of K⁺ uptake may occur. However, due to the high Na⁺ contents of the roots, a Na⁺ effect in this process cannot be excluded. The growth and dry matter yields of the roots and shoots were strongly influenced by the SO²⁻/₄ and NO⁻/₃ supply of the plants. Appreciable differences were experienced between wheat and cucumber seedlings. The optimum SO²⁻/₄ concentration of the growth solution for maximal growth varied considerably between the species, and was also different for the roots and the shoots in a given species.     

Competitive Inhibition of Rat Brain Hexokinase by 2-Deoxyglucose, Glucosamine, and Metrizamide

Abstract: The effects of metrizamide on the kinetics of rat brain hexokinase were compared in vitro with those of 2-deoxyglucose and glucosamine. Although metrizamide, 2-deoxyglucose, and glucosamine are known to be competitive inhibitors of approximately equal potency for glucose of yeast hexokinase ( K ₁ approximately 0.7 m m for all three), metrizamide is a much weaker competitive inhibitor ( K _i about 20 m m ) of rat brain hexokinase than either 2-deoxyglucose or glucosamine ( K _i about 0.3 m m for both). This indicates a greater active site specificity of rat brain hexokinase than of yeast hexokinase. Rat brain hexokinase activity is enhanced approximately threefold in the presence of 0.05, 0.2, and 0.8 mg/ml bovine serum albumin, while yeast hexokinase is only enhanced by 50% under these conditions. Despite the high K _i value for metrizamide, interference with glucose metabolism may occur whenever metrizamide is present in much higher concentrations than glucose. Myelography in humans is one such situation.     

Behavioural and physiological adaptations to a burrowing lifestyle in the snake blenny, Lumpenus lampretaeformis, and the red band-fish, Cepola rubescens

Observations have been made on the mode of burrow construction in the snake blenny, Lumpenus lampretaeformis , under laboratory conditions. It appears that head probing and lateral oscillations of the body are principally responsible for the excavation of the burrow which is completed within 24 h. The burrow structure has been analysed in detail, showing a mean depth of 7.2 cm with a maximum observed length of 73 cm, with most systems between 20 and 35 cm in length. Initially linear burrows with two openings are usually provided with a small side tunnel, giving the system a characteristic Y-shape.
Burrow irrigation was investigated for the first time in L. lampretaeformis. The mean duration of burrow irrigation, by flexions of the tail of the fish, was 21 s with over 13 min h⁻¹ spent in irrigating the burrow. The mean water displacement per irrigation period was 3.1 ml. The PO ₂ and PCO ₂ were measured in both surface water and within the burrow system of L. lampretaeformis. Surface water values for PO ₂ were high (> 150 Torr) and PCO ₂ low (<0.4 Torr). Hypoxic and hypercapnic conditions were measured in the burrow system itself, with PO ₂ values ranging between 57 and 129 Torr and PCO ₂ rising to > 1.3 Torr in some burrows.
A comparative study of Cepola rubescens burrows indicated similar surface water PO ₂ and PCO ₂ values as in L. lampretaeformis. Burrow water PO ₂ values ranged between 60 and 94 Torr, with PCO ₂ values as high as 1.5 Torr being recorded. These results are discussed in relation to the adaptation of both species to a burrowing lifestyle.     

Triiodothyronine Is a High-Affinity Inhibitor of Amino Acid Transport System L1 in Cultured Astrocytes

Abstract: The relationship between the transport of thyroid hormones and that of amino acids was examined by measuring the uptake of amino acids that are characteristic substrates of systems L, A, and N, and the effect of 3,3',5-triiodo-L-thyronine (T₃) on this uptake, in cultured astrocytes. Tryptophan and leucine uptakes were rapid, Na⁺-independent, and efficiently inhibited by T₃ (half-inhibition at ∼ 2 μ M ). Two Na⁺-independent L-like systems (L₁ and L₂), common to leucine and aromatic amino acids, were characterized kinetically. System L₂ had a low affinity for leucine and tryptophan ( K _m= 0.3–0.9 m M ). The high-affinity system L₁ ( K _m∼ 10 μ M for both amino acids) was competitively inhibited by T₃ with a K _i of 2–3 μ M (close to the T₃ transport K _m). Several T₃ analogues inhibited system L₁ and the T₃ transport system similarly. Glutamine uptake and α-(methylamino)isobutyric acid uptake were, respectively, two and 200 times lower than tryptophan and leucine uptakes. T₃ had little effect on the uptakes of glutamine and α-(methylamino)isobutyric acid. The results indicate that the T₃ transport system and system L₁ are related.     

Expression of Bradyrhizobium japonicum cbb3 terminal oxidase under denitrifying conditions is subjected to redox control

Bradyrhizobium japonicum utilizes cytochrome cbb ₃ oxidase encoded by the fixNOQP operon to support microaerobic respiration under free-living and symbiotic conditions. It has been previously shown that, under denitrifying conditions, inactivation of the cycA gene encoding cytochrome c ₅₅₀, the electron donor to the Cu-containing nitrite reductase, reduces cbb ₃ expression. In order to establish the role of c ₅₅₀ in electron transport to the cbb ₃ oxidase, in this work, we have analyzed cbb ₃ expression and activity in the cycA mutant grown under microaerobic or denitrifying conditions. Under denitrifying conditions, mutation of cycA had a negative effect on cytochrome c oxidase activity, heme c (FixP and FixO) and heme b cytochromes as well as expression of a fixP '–' lacZ fusion. Similarly, cbb ₃ oxidase was expressed very weakly in a napC mutant lacking the c -type cytochrome, which transfers electrons to the NapAB structural subunit of the periplasmic nitrate reductase. These results suggest that a change in the electron flow through the denitrification pathway may affect the cellular redox state, leading to alterations in cbb ₃ expression. In fact, levels of fixP '–' lacZ expression were largely dependent on the oxidized or reduced nature of the carbon source in the medium. Maximal expression observed in cells grown under denitrifying conditions with an oxidized carbon source required the regulatory protein RegR.     

Isoform-Specific Effects of Apolipoproteins E2, E3, and E4 on Cerebral Capillary Sequestration and Blood-Brain Barrier Transport of Circulating Alzheimer's Amyloid β

Abstract: Cerebral capillary sequestration and blood-brain barrier (BBB) permeability to apolipoproteins E2 (apoE2), E3 (apoE3), and E4 (apoE4) and to their complexes with sAβ_1–40, a peptide homologous to the major form of soluble Alzheimer's amyloid β, were studied in perfused guinea pig brain. Cerebrovascular uptake of three apoE isoforms was low, their blood-to-brain transport undetectable, but uptake by the choroid plexus significant. Binding of all three isoforms to sAβ_1–40 in vitro was similar with a K _D between 11.8 and 12.9 n M . Transport into brain parenchyma and sequestration by BBB and choroid plexus were negligible for sAβ_1–40-apoE2 and sAβ_1–40-apoE3, but significant for sAβ_1–40-apoE4. After 10 min, 85% of sAβ_1–40-apoE4 taken up at the BBB remained as intact complex, whereas free sAβ_1–40 was 51% degraded. Circulating apoE isoforms have contrasting effects on cerebral capillary uptake of and BBB permeability of sAβ. ApoE2 and apoE3 completely prevent cerebral capillary sequestration and blood-to-brain transport of sAβ_1–40. Conversely, apoE4, by entering brain microvessels and parenchyma as a stable complex with sAβ, reduces peptide degradation and may predispose to cerebrovascular and possibly enhance parenchymal amyloid formation under pathological conditions.