Internal pool

Experiments using fluorescein-labelled fMet-Leu-Phe indicate that the majority of re-expressed receptors that appear on the plasma membrane within 4-10 min after stimulation arise from the mobilisation of internal pools. Subcellular fractionation studies indicate that the pools of these receptors are the membranes of specific granules, although it is possible that these are on other membranes (e.g. on gelatinase-containing granules or se-... 

Lanphear BP et al Low-level environmental lead exposure and children s intellectual development An international pooled analysis. Environ Health Perspect 2005 113 894. [PMID 16002379]... 

Sodium is toxic to some plants at high concentrations. It has been shown by Cramer et al. (1985) that excess NaCl leads to the loss of K+ due to membrane depolarisation and that there is a loss of Ca2+ ions from the cell plasmalemmae and internal pool due to displacement by Na+ ions (Cramer et al., 1987, 1991 Lauchli, 1990). 

Lanphear BP, Hornung R, Khoury J, Yolton K, Baghurst P, Bellinger DC, Canfield RL, Dietrich KN, Bornschein R, Greene T, Rothenberg SJ, Needleman HL, Schnaas L, Wasserman G, Graziano J, Roberts R (2005) Low-level environmental lead exposure and children s intellectual function An international pooled analysis. Environ Health Perspect, 113(7) 894-899. 

B. A. Lawrence, C. Suarez, A. DePina, E. Click, N. H. Kolodny and M. M. Allen (1998). Two internal pools of soluble polyphosphate in the cyanobacterium Synestocystis sp. strain PCC 6308 an in vivo 31P NMR spectroscopic study. Arch. Microbiol., 169, 195-200. 

Although slightly attenuated, the rise in cytosolic calcium proceeds in the absence of extracellular calcium (as mentioned above), indicating that upon All stimulation calcium is released into the cytosol from an internal pool. The identity of this mobilized internal pool was initially inferred from cellular studies in which treatment with dantrolene inhibited the redistribution of intracellular calcium [39,40], Be-... 

Figure 30.3 Difference between nitrogen uptake and growth kinetics from experiments, after Zehr et al. (1988). Short-term uptake kinetics (A) are driven by membrane transport processes, and usually exceed assimilation and incorporation into protein. Short-term uptake declines as feedback from internal pools and regulatory mechanisms are affected by influx of N. (B) Longer term N uptake is driven by protein synthesis rate, which is equivalent to the growth rate. (C) Model of competition between two species showing that competition can be affected by both transport and growth.

Finally, transport can also be driven by the conversion of intracellular substrate to another chemical form. For example, in the case of nucleoside drugs, conversion to the corresponding nucleotides by appropriate kinases may be the limiting factor in cellular uptake and activation. The same principle applies to sulfation, glu euro nidation, prodrug activations, or other metabolic processes that provide a removal of the transported species from the transportable (free) internal pool. In some cases, transport is directly coupled to substrate modification, as in the uptake of sugars into bacterial cells by phosphoenolpyruvate (PEP)-coupled phosphorylation systems. 

The enzyme is also subject to allosteric inhibition. When glucose binds at the active site, it stabiles the T-state conformation of the enzyme. The T state is also stabilised by bi- or tricyclic aromatic compounds such as caffeine or flavins, which bind at the entrance to the active site tunnel,and by acylated p-glucopyranosylamine derivatives, which bind similarly to glucose, but more tightly. A third allosteric site, formed at the interface of two subunits and normally an internal pool of water molecules , has recently been discovered in rabbit muscle phosphorylase b " and human liver phosphorylase a. Occupancy of this site freezes the enzyme in the T state and inhibitors with 10 M dissociation constants from the site are being investigated in the treatment of diabetes. 

The movement of the endocytosed fluorescent SM from the internalized pool back to the plasma membrane has also been examined in fibroblasts (M. Koval, 1989). This transport process occurs via vesicles. The properties of the recycling pool of NBD-SM are distinct from those observed for export of the newly synthesized SM out of the Golgi (Fig. 12). As stated above, monensin and brefeldin A arrest newly synthesized NBD-SM transport from the Golgi to the cell surface but the recycling of endocytosed fluorescent SM is insensitive to monensin. The overall process of internalization of SM from the plasma membrane to the intracellular pool and transport back to the cell surface occur with a of 40 min. These time constants are similar to those for membrane protein recycling processes from the plasma membrane. 

This internal pool contains at least 30% of the receptors normally present on the surface of promastigotes. De novo protein synthesis is not required for recruitment and cell surface expression of the internal receptor pool. A comparison of heparin binding affinities between the regenerated and the native receptors indicated similar binding constants. Finally, the heparin receptors are localized almost exclusively on the flagella of promastigotes. 

Butcher, B. A., Shome, K., Estes, L. W. et al. Leishmania donovani cell-surface heparin receptors of promastigotes are recruited from an internal pool after trypsinization. Exp. Parasitol. 71 49-59. 

No method is totally accurate unless the cells are totally and irreversibly dead. They can be a reflection of the situation during the throes of death. If a herbicide suppresses the biosynthesis of a given amino acid, and the assay for phytotoxicity happens to be incorporation of that amino acid into protein, there will be a strong stimulation of incorporation while the cells are dying due to diminished internal pools. 

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