Low density lipoprotein uptake: holoparticle and cholesteryl ester selective uptake.

Low density lipoproteins (LDL) contain apolipoprotein B-100 and are cholesteryl ester-rich, triglyceride-poor macromolecules, arising from the lipolysis of very low density lipoproteins. This review will describe the receptors responsible for uptake of whole LDL particles (holoparticle uptake), and the selective uptake of LDL cholesteryl ester. The LDL-receptor mediates the internalization of whole LDL through an endosomal-lysosomal pathway, leading to complete degradation of LDL. Increasing LDL-receptor expression by pharmacological intervention efficiently reduces blood LDL concentrations. The lipolysis stimulated receptor and LDL-receptor related protein may also lead to complete degradation of LDL in presence of free fatty acids and apolipoprotein E- or lipase-LDL complexes, respectively. Selective uptake of LDL cholesteryl ester has been demonstrated in the liver, especially in rodents and humans. This activity brings five times more LDL cholesteryl ester than the LDL-receptor to human hepatoma cells, suggesting that it is a physiologically significant pathway. The lipoprotein binding site of HepG2 cells mediates this process and recognizes all lipoprotein classes. Scavenger receptor class B type I and CD36, which mediate the selective uptake of high density lipoprotein cholesteryl ester, are potentially involved in LDL cholesteryl ester selective uptake, since they both bind LDL with high affinity. It is not known whether they are identical to the uncloned lipoprotein binding site and if the selective uptake of LDL cholesteryl ester produces a less atherogenic particle. If this is verified, pharmacological up-regulation of LDL cholesteryl ester selective uptake may become another therapeutic approach for reducing blood LDL-cholesterol levels and the risk of atherosclerosis.     

Anniversary uptake

DNA at 50: Media editors have been thinking hard about the best ways to cover the DNA landmark and subsequent developments. Richard Harris looks at how the story came out.     

Nitrogen uptake

Summary Nitrogen uptake from applied nutrient solutions was evaluated in two old fields, in a pine plantation, and in a hardwood stand, to test the idea that plant communities become more efficient trappers and retainers of plant nutrients during succession. Uptake was estimated as the difference between nutrient concentrations in water collected from beneath soil profiles with and without roots by lysimeters within each successional stage. Results suggest that nitrate uptake decreased while ammonia uptake increased with succession. This apparent shift from a nitrate to an ammonia nitrogen economy during succession has been reported by other workers and is evolutionarily significant as an energy, nitrogen, and cation saving mechanism.     

Synaptosomal uptake

Using purified synaptosomal preparations from rat brain, the uptake ofl-tryptophan and norepinephrine was studied. We were unable to replicate some of the results of the experiments obtained with crude mitochondrial, fractions (P₂). Thus we examined the validity of the results of uptake studies obtained with the crude synaptosomes and established conditions which would simulate the biochemical milieu in which the nerve terminals functionin vivo, such as active substrate-dependent respiration, respiratory coupling on addition of ADP, low impurity of noncharacteristic markers, exogenous added proteins (e.g. bovine serum albumin), and verification by electron microscopy. All uptake studies withl-TRP and NE were completed in a system designed for simultaneous recording of respiration and the effect of added ADP. This system was also employed in comparative studies with mitochondria purified by multiple density gradients derived either from the perikaryon or from synaptosomes. Synaptosomal or mitochondrial preparations which did not conform to the above criteria invariably showed significantly lowered ability of uptake ofl-TRP or NE. This was found to be related to impairment in their respiratory and coupling ability. When the experimental conditions of others were employed, the time course of uptake of TRP for crude synaptosomes (P₂) was 100 nmol/g/min and was linear for 2.5 min, while for the purified synaptosomes it was 20 nmol/g/min with a l-min linearity. The mitochondria purified from P₂ displayed 30 nmol/g/min uptake withl-TRP with a linearity of 2.5 min. Reconstituted system of purified synaptosomes and mitochondria gave 60 nmol/g/min ofl-TRP transport with 2.5 min linearity. Also examined was the effect of eight different media. It was found that Krebs-Ringer solution containing glucose (40 mM), pyruvate and malate (10 mM), and ADP (250 nmol) gave optimal uptake of TRP both for synaptosomes and for mitochondria, increasing it to 60 and 86 nmol/g/min. The above conditions also enhanced the uptake of NE by synaptosomes and mitochondria. Uptake of NE was not proportional to protein concentration when the protein content exceeded 0.4 mg. Purified synaptosomal mitochondria accumulated NE more actively than the purified nonsynaptic free mitochondria, albeit at the same rate. Synaptic and free mitochondria had an impaired uptake of NE in presence of DNP, antimycin A, and rotenone, and unlike withl-TRP, pyruvate and malate also reduced uptake of NE. Significant differences were noted for the cytochrome oxidase activity between the synaptosomal and free michondria when compared to that of the homogenate.     

Seizure proneness and neurotransmitter uptake

The ability of midbrain homogenates from two strains of mice to accumulate several putative neurotransmitters, or their precursor in the case of acetylcholine, has been examined. The high-affinity transport mechanisms toward glutamate, GABA, dopamine, and glycine were similar in both strains. The seizure-prone DBA21BG strain had a significantly higher capacity to transport choline than did the relatively seizure-resistant C57BL/6 IBC mice. Howaver, no difference in the density of muscarinic binding sites in the two mouse strains was found.     

Blood rheology and oxygen uptake

Continuously measured oxygen uptake during constant work exercise (15' 50W) reveals increasing oxygen consumption in individuals with elevated blood viscosity parameters, indicating persistent contribution of anaerobic glycolysis during steady state exercise far below expected "anaerobic threshold". Improvement of viscosity parameters by prostaglandin E1--infusion (Prostavasin) 40 micrograms i.v., naftidrofurylhydrogenoxalat (Dusodril pi) 400 mg i.v. or hemodilution with 500 ml 6% hydroxyethylamylum MW 40000 (Onkohaes) in 5 patients results in significant reduction of this oxygen gradient in subsequent exercise test. Integrated VO2 during exercise above the mean value at rest or the quotient of VO2 during 15 min by VO2 during 30 min (including recovery time) are not differing significantly due to high variations inter- and intraindividually. Oxygen gradient during submaximal constant exercise permits direct clinical determination of microcirculatory performance in involved muscle tissue as a function of blood viscosity.     

Bacterial zinc uptake and regulators

Many bacteria use an ABC transporter for high-affinity uptake of zinc with a cluster 9 solute-binding protein. Other members of this protein family transport manganese. At present, it is not always possible to distinguish zinc-specific and manganese-specific transporters on the basis of sequence analysis. Low-affinity ZIP-type zinc transporters in bacteria have also been identified. Most high-affinity zinc uptake systems are regulated by Zur proteins, which form at least three unrelated subgroups of the Fur protein family (regulators of iron transport). High-affinity transport of zinc out of the periplasmic space poses a problem to the cell because zinc is a cofactor of several periplasmic enzymes. Certain zinc-binding proteins in the periplasm might function as chaperones to supply these enzymes with zinc.     

Silver uptake, distribution, and effect on calcium, phosphorus, and sulfur uptake

Bean, corn, and tomato plants were grown in a nutrient solution labeled with ³²P, ⁴⁵Ca, or ³⁵S and varying concentrations of AgNO₃. Following a 6-hour treatment period, plants were harvested and analyzed. A low Ag⁺ concentration (50 nanomolar) inhibited the shoot uptake of the ions investigated. In the roots, Ca uptake increased whereas P and S uptake decreased.     

Relations between K+ uptake and photosynthetic uptake of inorganic carbon by aquatic plants

The uptake of K⁺ by the leafy shoots of four submersed higher aquatic plants (Elodea canadensis, Ranunculus aquatilis, R. trichophyllus, and Callitriche hamulata) with different HCO₃ ^- affinity was measured in successive 2-h periods under the conditions of high or low photosynthetic rates (i.e. at pH 7.5 or 10). At pH 7.5 the uptake of K⁺ by species with the higher HCO₃ ^- affinity (E. canadensis, R. trichophyllus) was significantly faster than that by species with a lower HCO₃ ^- affinity (R. aquatilis, C. hamulata). In the former group of species, the K⁺ uptake rate at pH 7.5 was 1.7 - 3.5 times higher than at pH 10. At pH 10, the soft-water species, R. aquatilis, had the lowest net photosynthetic rate (PN) of the three HCO₃ ^- users but, in contrast to the relative hard-water species, R. trichophyllus, showed a small K⁺ efflux (47 nmol kg^-1 s^-1). Thus, K⁺ uptake by shoots was not strictly correlated with PN. A significant K⁺ efflux (73 - 86 nmol kg^-1 s^-1) occurred from all HCO₃ ^- users in darkness. The relatively low K⁺ uptake by the strict CO₂ user, C. hamulata, was quite independent of PN and light or darkness. It may be suggested that uptake of K⁺ by shoots of submersed plants depends on their HCO₃ ^- affinity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Prion uptake in the gut: identification of the first uptake and replication sites

After oral exposure, prions are thought to enter Peyer's patches via M cells and accumulate first upon follicular dendritic cells (FDCs) before spreading to the nervous system. How prions are actually initially acquired from the gut lumen is not known. Using high-resolution immunofluorescence and cryo-immunogold electron microscopy, we report the trafficking of the prion protein (PrP) toward Peyer's patches of wild-type and PrP-deficient mice. PrP was transiently detectable at 1 day post feeding (dpf) within large multivesicular LAMP1-positive endosomes of enterocytes in the follicle-associated epithelium (FAE) and at much lower levels within M cells. Subsequently, PrP was detected on vesicles in the late endosomal compartments of macrophages in the subepithelial dome. At 7-21 dpf, increased PrP labelling was observed on the plasma membranes of FDCs in germinal centres of Peyer's patches from wild-type mice only, identifying FDCs as the first sites of PrP conversion and replication. Detection of PrP on extracellular vesicles displaying FAE enterocyte-derived A33 protein implied transport towards FDCs in association with FAE-derived vesicles. By 21 dpf, PrP was observed on the plasma membranes of neurons within neighbouring myenteric plexi. Together, these data identify a novel potential M cell-independent mechanism for prion transport, mediated by FAE enterocytes, which acts to initiate conversion and replication upon FDCs and subsequent infection of enteric nerves.     

Identification and characterization of plasmids in hydrogen uptake positive and hydrogen uptake negative strains ofRhizobium japonicum

Modifications were made of published procedures to allow routine isolation of plasmids fromRhizobium japonicum. The plasmid profiles of a series of H₂ uptake positive and H₂ uptake negative strains were compared. None of the strains ofR. japonicum with high H₂ uptake activities exhibited discernible plasmids, while most of the strains, with little or no H₂ uptake activity, showed plasmids with molecular weights ranging from approximately 49–290 x10⁶. An examination of H₂ uptake negative mutants derived from an H₂ uptake positive parent revealed two discernible plasmid bands in nonrevertible mutants but no detectable plasmids in revertible mutants or in the parent strain from which mutants were derived.     

Topographic distribution of dopamine uptake,choline uptake,choline acetyltransferase,and GABA uptake in the striata of weaver mutant mice

The topographic distribution of dopamine (DA) uptake, choline uptake, choline acetyltransferase (ChAT) activity and GABA uptake within the striata of weaver mutant mice and control mice was determined. Uptake of [³H]dopamine, [³H]choline and [¹⁴C]GABA, as well as ChAT activity were determined in samples prepared from the dorsolateral, dorsomedial, ventrolateral and ventromedial portions of the striatum. In 45–60 day old control mice, dopamine uptake was homogeneously distributed throughout the striatum. On the other hand, striata from weaver mice exhibited an uneven distribution with the ventral aspects having greater uptake activity than the dorsal regions. Thus, although the ventral portion of the striatum is less severely affected than the dorsal portion, all areas of the striatum exhibited significantly reduced uptake rates. In 9 and 12 month old mice, choline uptake was higher in lateral than medial zones of the striatum of both genotypes and no differences were observed between genotypes. GABA uptake was higher in the ventral striatum than in the dorsal striatum but again no differences were found between weaver and control mice. The results of this study indicate that the entire weaver striatum is severely deficient in its ability to recapture dopamine and thus is functionally compromised. The results also indicate that the striatal cholinergic and GABAergic interneurons are not directly or indirectly affected by the weaver gene.Special ïssue dedicated to Dr. Morris H. Aprison     

A single channel for nitrate uptake, nitrite export and nitrite uptake by Escherichia coli NarU and a role for NirC in nitrite export and uptake

Two related polytopic membrane proteins of the major facilitator family, NarK and NarU, catalyse nitrate uptake, nitrite export and nitrite uptake across the Escherichia coli cytoplasmic membrane by an unknown mechanism. A 12-helix model of NarU was constructed based upon six alkaline phosphatase and beta-galactosidase fusions to NarK and the predicted hydropathy for the NarK family. Fifteen residues conserved in the NarK-NarU protein family were substituted by site-directed mutagenesis, including four residues that are essential for nitrate uptake by Aspergillus nidulans: arginines Arg(87) and Arg(303) in helices 2 and 8, and two glycines in a nitrate signature motif. Despite the wide range of substitutions studied, in no case did mutation result in loss of one biochemical function without simultaneous loss of all other functions. A NarU+ NirC+ strain grew more rapidly and accumulated nitrite more rapidly than the isogenic NarU+ NirC(-) strain. Only the NirC+ strain consumed nitrite rapidly during the later stages of growth. Under conditions in which the rate of nitrite reduction was limited by the rate of nitrite uptake, NirC+ strains reduced nitrite up to 10 times more rapidly than isogenic NarU+ strains, indicating that both nitrite efflux and nitrite uptake are largely dependent on NirC. Isotope tracer experiments with [15N]nitrate and [14N]nitrite revealed that [15N]nitrite accumulated in the extracellular medium even when there was a net rate of nitrite uptake and reduction. We propose that NarU functions as a single channel for nitrate uptake and nitrite expulsion, either as a nitrate-nitrite antiporter, or more likely as a nitrate/H+ or nitrite/H+ channel.     

Copper toxicity and uptake in microorganisms

Summary Copper is a required trace element for growth of microorganisms since it is a cofactor for numerous enzymes. Also, proteins containing copper are important electron transfer carriers. However, at elevated concentrations, copper can be highly toxic to microorganisms. This review examines copper toxicity and uptake in microorganisms, with an emphasis on copper-resistance mechanisms.     

Nickel uptake and utilization by microorganisms

Nickel is an essential nutrient for selected microorganisms where it participates in a variety of cellular processes. Many microbes are capable of sensing cellular nickel ion concentrations and taking up this nutrient via nickel-specific permeases or ATP-binding cassette-type transport systems. The metal ion is specifically incorporated into nickel-dependent enzymes, often via complex assembly processes requiring accessory proteins and additional non-protein components, in some cases accompanied by nucleotide triphosphate hydrolysis. To date, nine nickel-containing enzymes are known: urease, NiFe-hydrogenase, carbon monoxide dehydrogenase, acetyl-CoA decarbonylase/synthase, methyl coenzyme M reductase, certain superoxide dismutases, some glyoxylases, aci-reductone dioxygenase, and methylenediurease. Seven of these enzymes have been structurally characterized, revealing distinct metallocenter environments in each case.     

Bacterial solute uptake and efflux systems

The recent discovery of binding protein dependent secondary transporters and the ever-growing family of membrane potential generating secondary transporters emphasize the diversity of transport systems in both the mechanistical and physiological sense. The vast amount of data on the lactose permease is now beginning to crystallize in a model that relates functional events to structural changes of the protein. Evidence has been presented that multidrug transporters pick up their substrates from the membrane, and the binding of a number of substrates to the binding-protein components of ATP-driven transporters is now understood in detail.