Uptake mechanism

Uptake of LCFAs across the lipid-bilayer of most mammalian cells occurs through both a passive diffusion of LCFAs and a protein-mediated LCFA uptake mechanism. At physiological LCFA concentrations (7.5 nM) the protein-mediated, saturable, substrate-specific, and hormonally regulated mechanism of fatty acids accounts for the majority (>90%) of fatty acid uptake by tissues with high LCFA metabolism and storage such as skeletal muscle, adipose tissue, liver,... 

Oehlke and coworkers have described the cellular uptake properties of a simple a-hehcal amphipathic model peptide sequence (Lys-Leu-Ala-Leu-Lys-Leu-Ala-Leu-Lys-Ala-Leu-Lys-Ala-Ala-Leu-Lys-Leu-Ala) in the context of a drug delivery vehicle [72]. On the basis of the data presented, it was proposed that non-endocytosis mechanism(s) were involved in the uptake into mammalian cells. The similarity between our b2 aPNA-sequence to that of this amphipathic model peptide makes it tempting to suggest that a similar uptake mechanism is involved in the cellular uptake of aPNAs. Further experimentation is necessary to test this hypothesis. 

Asphyxiants interfere with the body s oxygen uptake mechanisms. Air normally contains 21% oxygen. Oxygen deficiency in inhaled air, e.g. due to the presence of nitrogen, argon, or carbon dioxide in a confined space, depending on the concentration and duration, may affect the body and ultimately cause death from simple anoxia (Table 4.7). 

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. 

Recently much interest has centred on a very specific toxin for DA neurons. This is 1-methyl-4-phenyl-l,2,3,6-tetrahydropyridine (MPTP). It was discovered when a student, who was addicted to pethidine, tried to manufacture l-methyl-4-phenyl-4-propionoxy-piperidine (MPPP) but took a short-cut in synthesis and produced MPTP. When he administered this to himself he developed Parkinsonism. MPTP destroys DA neurons. Again this process depends on the neuronal uptake mechanism, since MPTP itself is not the active material. It needs to be deaminated to MPP+ which is then taken up by DA nerve terminals. 

In many epithelia Cl is transported transcellularly. Cl is taken up by secondary or tertiary active processes such as Na 2Cl K -cotransport, Na Cl -cotransport, HCOJ-Cl -exchange and other systems across one cell membrane and leaves the epithelial cell across the other membrane via Cl -channels. The driving force for Cl -exit is provided by the Cl -uptake mechanism. The Cl -activity, unlike that in excitable cells, is clearly above the Nernst potential [15,16], and the driving force for Cl -exit amounts to some 2(f-40mV. 

Bouchez M, D Blanchet, J-P Vandecasteele (1997) An interfacial uptake mechanism for the degradation of pyrene by a Rhodococcus strain. Microbiology (UK) 143 1087-1093. 

IV. IRON UPTAKE MECHANISMS FROM SIDEROPHORES AND PHYTOSIDEROPHORES... 

A well-known example of active transport is the sodium-potassium pump that maintains the imbalance of Na and ions across cytoplasmic membranes. Flere, the movement of ions is coupled to the hydrolysis of ATP to ADP and phosphate by the ATPase enzyme, liberating three Na+ out of the cell and pumping in two K [21-23]. Bacteria, mitochondria, and chloroplasts have a similar ion-driven uptake mechanism, but it works in reverse. Instead of ATP hydrolysis driving ion transport, H gradients across the membranes generate the synthesis of ATP from ADP and phosphate [24-27]. 

Acetylcholine synthesis and neurotransmission requires normal functioning of two active transport mechanisms. Choline acetyltransferase (ChAT) is the enzyme responsible for ACh synthesis from the precursor molecules acetyl coenzyme A and choline. ChAT is the neurochemical phenotype used to define cholinergic neurons although ChAT is present in cell bodies, it is concentrated in cholinergic terminals. The ability of ChAT to produce ACh is critically dependent on an adequate level of choline. Cholinergic neurons possess a high-affinity choline uptake mechanism referred to as the choline transporter (ChT in Fig. 5.1). The choline transporter can be blocked by the molecule hemicholinium-3. Blockade of the choline transporter by hemicholinium-3 decreases ACh release,... 

Figure 6.1 Histamine synthesis and metabolism in neurons. L-histidine is transported into neurons by the L-amino acid transporter. Once inside the neuron, L-histidine is converted into histamine by the specific enzyme histidine decarboxylase. Subsequently, histamine is taken up into vesicles by the vesicular monoamine transporter and stored there until released. In the absence of a high-affinity uptake mechanism in the brain, released histamine is rapidly degraded by histamine methyltransferase, which is located postsynaptically and in glia, to telemethylhistamine, a metabolite that does not show any histamine-like activity.

Transferrin iron uptake via receptor-mediated endocytosis has clearly appeared fairly late in evolution, when we consider that the bilobal iron-binding protein is found only as far back as insects . As we have seen in the preceding chapters, iron-uptake mechanisms involving the synthesis of more or less specific siderophores have evolved together with strategies implying the solubilization of insoluble ferric iron by the combined effects of pH and reduction, and even the development of receptor proteins capable of taking up transferrin-, lactoferrin- or haem-bound iron from specific hosts. 

Figure 11.1 Schematic representation of iron uptake mechanisms, (a) The transferrin-mediated pathway in animals involves receptor-mediated endocytosis of diferric transferrin (Tf), release of iron at the lower pH of the endocytic vesicle and recycling of apoTf. (b) The mechanism in H. influenzae involves extraction of iron from Tf at outer membrane receptors and transport to the inner membrane permease system by a periplasmic ferric binding protein (Fbp). From Baker, 1997. Reproduced by permission of Nature Publishing Group.

Compounds absorbed by active uptake mechanisms (e.g., glucose and Gly-Pro) and compounds known to be substrates for efflux transport (e.g., digoxin, verapamil) were also included in the list. The applied concentration (10-500 pM) only had minor effects on the permeability values. Thus, the choice of concentration was not critical for this set of compounds with respect to the relationship between permeability and fraction absorbed in humans. Changing the pH on the apical donor side had significant effects on the Papp values of several compounds, the effects being in agreement with the acid-base properties of the compounds. The... 

Han, H.-K., D.-M. Oh, and G. L. Amidon. Cellular uptake mechanism of amino acid ester prodrugs in Caco-2/hPEPTl cells overexpressing a human peptide transporter. Pharm. 

Lundberg, P. and Langel, U. (2006). Uptake mechanisms of cell-penetrating peptides derived from the Alzheimer s disease associated gamma-secretase complex. Int. J. Pept. Res. Therap. 12, 105-114. 

In this chapter, we describe the synthetic strategy for bio-inorganic conjugates which can be used as drug or gene delivery systems and their efficient cellular uptake mechanism. 

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