Uptake process

Carboxylic ionophores selectively transport cations by using intramolecular complexation in the uptake process of cations (basic region). A new ion transport system has been developed which incorporates a structural device which assists in the release process by using intramolecular complexation of an [18]crown-6 ring and a primary ammonium ion 48>. The experimental conditions are shown in Fig. 7. All these com-... 

Kenakin, T. P., and Beek, D. (1981). The measurement of antagonist potency and the importance of selective inhibition of agonist uptake processes. J. Pharmacol. Exp. Ther. 219 112-120. 

Several attempts were made to monitor the uptake of Zn as a function of time. As a first attempt, the cells were exposed to 30 ppm Zn at room temperature. The rate of uptake of the Zn by the cells was much faster than was the case with Sn it was impossible to measure an increase in zinc content of the cells above that of the initial Zn -exposed sample either in florescence signal or GFAA signal by the methods described in this work. Reducing both the concentration of Zn in solution (10 ppm and 1 ppm) and the temperature (0°) did not slow down the uptake process enough to monitor an increase in accumulated Zn. By addition of known concentrations of Zn to the cells exposed to a 1.0 ppm Zn solution, it was found by GFAA that the amount of zinc accumulated... 

After release there must be some way of terminating the action of a NT necessitating the presence of an appropriate enzyme and/or uptake mechanism. Such uptake processes can be quite specific chemically. Thus a high-affinity uptake (activated by low concentrations) can be found for glycine in the cord where it is believed to be a NT, but not in the cortex where is has no such action. This specific uptake can be utilised to map terminals for a particular NT, especially if it can be labelled, and also for loading nerves with labelled NT for release studies. 

Toxins that gain access to a neuron through its uptake process and then destroy it in some way. This approach has been used mainly to destroy monoamine neurons with 5,6 or 5,7-dihydroxytryptamine targeting 5-HT neurons, 6-hydroxydopamine for dopamine (and to a lesser extent noradrenergic) neurons and MPTP for dopamine neurons (see Chapter 7). Only the latter is fully specific and effective systemically. The others need to be administered directly into the appropriate brain areas and while they may only affect the intended NT neurons, the injection may not affect all of them. 

In common with other monoamines, the actions of released noradrenaline are terminated by its rapid reuptake from the synaptic cleft. This uptake process relies on membrane-bound noradrenaline transporters which are glycoproteins closely related... 

Feedback inhibition of amino acid transporters by amino acids synthesized by the cells might be responsible for the well known fact that blocking protein synthesis by cycloheximide in Saccharomyces cerevisiae inhibits the uptake of most amino acids [56]. Indeed, under these conditions, endogenous amino acids continue to accumulate. This situation, which precludes studying amino acid transport in yeast in the presence of inhibitors of protein synthesis, is very different from that observed in bacteria, where amino acid uptake is commonly measured in the presence of chloramphenicol in order to isolate the uptake process from further metabolism of accumulated substances. In yeast, when nitrogen starvation rather than cycloheximide is used to block protein synthesis, this leads to very high uptake activity. This fact supports the feedback inhibition interpretation of the observed cycloheximide effect. 

In the first case the mechanisms are based on an increased reducing capacity of Fe(lll)-chelates, a necessary step in the uptake process, with a concurrent increase in acidification and release of organic acids into the rhizosphere in the latter case molecules having high affinity for Fe (phytosiderophores) are synthesized and released into the rhizosphere when Fe is lacking. 

Figure 1.21, like Figure 1.19, shows two Schild plots, one of which (open circles) departs greatly from the expected behavior. The deviation occurs when noradrenaline is the agonist and again it can be accounted for in terms of the reduction in local concentration caused by the uptake, process... 

FIGURE 1.20 Hypothetical concentration-response curves to illustrate how the uptake) process can influence the study of the antagonism of noradrenaline by phentolamine. The two full lines show the response to noradrenaline, first in the absence and then in the presence of phentolamine. If the experiment is repeated, but with the uptake process blocked, the dotted lines would be obtained. Noradrenaline has become more active, and phentolamine now causes a greater shift (compare the lengths of the two horizontal arrows), as explained in the text. 

Iversen, L.L., Uptake processes for biogenic amines, in Biochemistry of Biogenic Amines, Vol. 3, Plenum Press, New York, 1975, 381. 

The logic of the evolution of insertion can now be considered. Much as the most primitive selectivity of the chemistry of the uptake process, pumping, carrying and final binding to form a useful enzyme is not an invention by organisms but is a necessary consequence of inevitable equilibrium considerations (see Section 4.17), so the binding of particular metal ions to particular chelatase proteins was similarly selected... 

Figure 13.9 represents the TEM image of LDH particles and their cellular internalization. As expected, LDH particles are internalized by endocytosis. Figure 13.9(A) shows the cellular uptake process of LDHs after 3h of treatment, and demonstrates a successive entry of LDH by endocytosis first the LDH particles were located around the cell membrane due to their positive charge ( ), then they migrate to the membrane ruffles which are considered as endocytic bodies ( ), finally the coated intracellular vesicles were formed as early endosomes ( ). Figure 13.9(B)...

Other systems like electroporation have no lipids that might help in membrane sealing or fusion for direct transfer of the nucleic acid across membranes they have to generate transient pores, a process where efficiency is usually directly correlated with membrane destruction and cytotoxicity. Alternatively, like for the majority of polymer-based polyplexes, cellular uptake proceeds by clathrin- or caveolin-dependent and related endocytic pathways [152-156]. The polyplexes end up inside endosomes, and the membrane disruption happens in intracellular vesicles. It is noteworthy that several observed uptake processes may not be functional in delivery of bioactive material. Subsequent intracellular obstacles may render a specific pathway into a dead end [151, 154, 156]. With time, endosomal vesicles become slightly acidic (pH 5-6) and finally fuse with and mature into lysosomes. Therefore, polyplexes have to escape into the cytosol to avoid the nucleic acid-degrading lysosomal environment, and to deliver the therapeutic nucleic acid to the active site. Either the carrier polymer or a conjugated endosomolytic domain has to mediate this process [157], which involves local lipid membrane perturbation. Such a lipid membrane interaction could be a toxic event if occurring at the cell surface or mitochondrial membrane. Thus, polymers that show an endosome-specific membrane activity are favorable. 

The polyvalent nature of dendrimers has been investigated as vehicles for carrying multiple chelator groups to enhance signals in various imaging applications (Barthand Soloway, 1994 Yoo et al., 1999 Kobayashi et al., 2000, 2001 Sato et al., 2001). In addition, in certain chelate-dendrimer constructs, excess amines on the dendrimer surface can aid in the cellular uptake process through charge-mediated endocytosis. 

As mentioned previously, siderophores must selectively bind iron tightly in order to solubilize the metal ion and prevent hydrolysis, as well as effectively compete with other chelators in the system. The following discussion will address in more detail the effect of siderophore structure on the thermodynamics of iron binding, as well as different methods for measuring and comparing iron-siderophore complex stability. The redox potentials of the ferri-siderophore complexes will also be addressed, as ferri-siderophore reduction may be important in the iron uptake process in biological systems. 

Drugs that block the catecholamine uptake process (e.g., cocaine, tricyclic antidepressants, and phenothiazines) are apt to block the anti hypertensive action of which of the following drugs ... 

The series of molecular events responsible for the uptake process constitutes the endocytic pathway, which enables cells to internalize macromolecules from the cell exterior, forming an endosome. The endosome is an intermediate organelle that serves as an essential component for many receptor-mediated signaling pathways and as a transport vector for eventual delivery to a specialized organelle known as the lysosome. Once in the lysosomal lumen, digestive enzymes provide essential metabolites from these macromolecules (i.e. free amino acids and lipids) directly to the cytosol for their use. 

The action of catecholamines released at the synapse is modulated by diffusion and reuptake into presynaptic nerve terminals. Catecholamines diffuse from the site of release, interact with receptors and are transported back into the nerve terminal. Some of the catecholamine molecules may be catabolized by MAO and COMT. The cate-cholamine-reuptake process was originally described by Axelrod [18]. He observed that, when radioactive norepinephrine was injected intravenously, it accumulated in tissues in direct proportion to the density of the sympathetic innervation in the tissue. The amine taken up into the tissues was protected from catabolic degradation, and studies of the subcellular distribution of catecholamines showed that they were localized to synaptic vesicles. Ablation of the sympathetic input to organs abolished the ability of vesicles to accumulate and store radioactive norepinephrine. Subsequent studies demonstrated that this Na+- and Cl -dependent uptake process is a characteristic feature of catecholamine-containing neurons in both the periphery and the brain (Table 12-2). 

The uptake process is energy-dependent since it can be inhibited by incubation at a low temperature or by metabolic inhibitors. The energy requirements reflect a coupling of the uptake process with the Na+ gradient across the... 

Biological systems are open to the exchange of matter with their environment. If species from the medium (either nutrients or pollutants) travel towards the membrane and a net increase in their concentration arises in the cell, then an uptake process is occurring. Apart from the obvious importance for the organism itself, there is an impact on the medium (e.g. regulating the fate of pollutants in the environment). 

It is apparent that the kinetics of the homogeneous reaction can have a dramatic impact on the overall uptake process by controlling the ratio of com-plexed to free M, which affects the velocity of transport towards the organism surface. Therefore, kinetics do matter and all the dynamic effects must be properly taken into account. 

The free energy of the phosphorylated histidine (P His) or cysteine (P Cys) is comparable with the free energy of PEP (AG° = — 61.5 kJ mol ). The reactions (1) to (4) are therefore fully reversible under physiological conditions, whereas reaction (5) is irreversible. The substrate when bound to the domain IIC (or IID) obtains the phosphoryl group from the unit IIB, via unit IIA, which is rephosphorylated by P HPr. Efficient translocation of carbohydrates depends on the phosphorylated IIB domain. The release of the phosphorylated substrate terminates the uptake process. 

The main objective of this section is to examine the implications of the different transport mechanisms on the kinetics of the uptake process. Section... 

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