Intracellular storage pools

I When Insulin levels decreese, glucose transporters move from cell membrane to intracellular storage pool, where they can be recycled. 

Many of these reactions are compositely triggered by a flux of extracellular Ca into the cells, combined with liberation of Ca from intracellular storage pools. The present discussion is oversimplified and will not take the latter event into account. 

PTH can be secreted, sequestered in an intracellular storage pool, or degraded within the parathyroid gland. Secretion is thought to occur by exocytosis, although the number of secretory granules is inadequate to maintain the observed rate of sustained release of PTH. It appears that... 

If different cells use calcium from different sources to mediate secretion, then it is likely that methods for removal of cytoplasmic calcium also vary. A plasma membrane calcium pump is important in adrenal medulla to extrude mediator calcium (22). In exocrine pancreas, calcium may be returned to intracellular storage pools, as well as extruded from the cell to terminate secretion. 

Carbohydrates in algae and plants are often classified based on methodological discrimination. The structural carbohydrates are not water-soluble, whereas the other types of carbohydrates are water-soluble and typically extracted by hot water. In Phaeocystis five different pools of carbohydrates can be distinguished. Like all algal and plant cells, both solitary and colonial cells produce (1) structural carbohydrates, polysaccharides that are mainly part of the cell wall, (2) mono- and oligosaccharides, which are present as intermediates in the synthesis and catabolism of cell components, and (3) intracellular storage glucan. Colonial cells of Phaeocystis excrete (4) mucopolysaccharides, heteropolysaccharides that... 

Different species of macroalgae would be expected to have different N release rates. For example, some macroalgae are known to have large intracellular inorganic N storage pools (e.g., McGlathery et ah, 1996). Release of NH4", as measured with isotope dilution, was observed in Ulva fenestrata (0.08-11.27 pmol N g h ) and Gracilaria pacifica (0.12-0.77 pmol N g h ) when NH4+ was added to the medium. However, no net NH4+ release, over uptake, was observed. Small pulses of free amino acids were released and then rapidly reincorporated (Naldi and Wheeler, 2002), but no protein release was detectable. 

Most models of algal growth combine the steps of nutrient uptake into the cell and growth on intracellular nutrients into a single step of uptake and growth. In the models that separate those two steps for phosphorus (e.g. Droop, 1973, 2003), an intracellular phosphorus pool must be included, in addition to the phosphorus that is part of the organic molecules. We denote the mass fraction of intracellular phosphorus contained in nutrient storage pools as... 

The transition metals, especially copper and iron ions, catalyse the formation of harmful hydroxyl radicals ( OH) from hydrogen peroxide (Haber-Weiss reaction). Because iron mediates oxidative damage, the substantial intracellular pool of free iron must be regulated by iron chelators, e.g. intracellular storage proteins such as ferritin. 

The measurements of the labeled metabolites may be performed with GC- or LC-MS, or by NMR. Because it is the most commonly used method, we will only consider GC-MS based approaches here. Obviously and unfortunately, it is not possible to directly measure the isotopomer enrichments by GC-MS, because the apparatus only yields total masses of molecules or fractions thereof, but not directly the position of a label. Each MS peak is produced by all isotopomers with the same molecular weight that is, the same number of labeled carbon positions. Sometimes this concept is also called mass isotopomers [264]. In a so-called retrobiosynthetic approach, it has been shown that the labeling state of many intracellular pools can be determined indirectly by measuring the labels in macromolecular biomass components at steady state for example, the labeling state of alanine from hydrolyzed protein reflects the label of pyruvate [265]. Using this approach, it is possible to quantify fluxes into storage components. 

The predominant class of intracellular iron-storage compounds is represented by ferritin in eukaryotes and bacterioferritin in prokaryotes (see Iron Proteins for Storage Transport their Synthetic Analogs). In various in vivo Mdssbauer spectroscopic studies on siderophore uptake in fungi, it was realized that siderophores can also function as intracellular iron-storage compounds. In the ascomycete Neurospora crassa, the transport siderophore coprogen represents an intracellular transient iron pool. A major part of coprogen-bound iron is transferred to a... 

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