Energy dependence processes

The dopamine is then concentrated in storage vesicles via an ATP-dependent process. Here the rate-limiting step appears not to be precursor uptake, under normal conditions, but tyrosine hydroxylase activity. This is regulated by protein phosphorylation and by de novo enzyme synthesis. The enzyme requites oxygen, ferrous iron, and tetrahydrobiopterin (BH. The enzymatic conversion of the precursor to the active agent and its subsequent storage in a vesicle are energy-dependent processes. 

The thyroid is able to concentrate T against a strong electrochemical gradient. This is an energy-dependent process and is linked to the Na -K ATPase-dependent thyroidal T transporter. The ratio of iodide in thyroid to iodide in serum (T S ratio) is a reflection of the activity of this transporter. This activity is primarily controlled by TSH and ranges from 500 1 in animals chronically stimulated with TSH to 5 1 or less in hy-pophysectomized animals (no TSH). The T S ratio in humans on a normal iodine diet is about 25 1. 

The accumulation of a number of amino acids from the external medium seems almost irreversible in Saccharomyces cerevisiae. The first detailed study of this phenomenon concerned histidine [13]. Histidine uptake by the specific histidine permease HlPl is an energy dependent process which accumulates free and intact... 

For the internalisation of metals, many examples exist for which transport may be coupled to an energy-dependent process, of which only a few are described here. For example, the well-studied (e.g. [276]) Na+/K+ channel transports 3Na+ out and 2K+ in for each ATP molecule that is hydrolysed [242]. Mg2+ influx (but likely not efflux) is highly regulated in eukaryotes [277]. ATPases have been implicated in certain cases of Fe [278] or Zn [90] uptake by phytoplankton. Finally, although Cd internalisation by a polychaete appeared to be energy independent, accumulation was increased rather than decreased in the presence of ATPase inhibitors, suggesting that the efflux system might depend upon ATP synthesis [279]. 

Active transport of a drug is mediated by a specific carrier. This is of particular interest in neural tissues and choroid plexus. In this case, the actual molecular size and shape are important, because it involves binding to a specific carrier protein that transports it. Active transport is an energy-dependent process, and may work against a concentration gradient. It is also a saturable mechanism and may be competitively inhibited by other ligands. 

The cellular uptake of AS-ODN is an energy-dependent process and takes place in a saturable and sequence-independent manner [120,121]. The exact mechanism of uptake remains controversial. From in vitro experiments, some authors have proposed that the uptake is endocytic and mediated by membrane receptor proteins. The receptor responsible for the cellular uptake of AS-ODNs was reported to consist of both a 30-kDa protein [122] and an 80-kDa membrane protein [121]. However, other workers have argued that AS-ODN binding to membrane proteins is relatively non-specific and is mostly charge associated, consistent with adsorptive endocytosis or fluid-phase pinocytosis [101]. As a result of these conflicting reports, it is unlikely that in vitro data can be safely extrapolated to what occurs in the intact organism. 

With M. gryphiswaldense, Schuler and Bauerlein (1996) recorded an Fe uptake rate from Fe " citrate of 0.86 nmol min mg dry weight and suggested that the major portion of Fe is taken up in an energy-dependent process possibly by a reductive step (Schuler, 1999). Fukumori et al. (1997) proposed that the dissimilatory nitrite reductase of M. magnetotacticum may function as an Fe" oxidizing enzyme. Later, Fuko-mori (2000) suggested an Fe "quinate complex as the source of Fe which is subsequently reduced in the cell in a microaerobic environment at about neutral pH by the iron reductase NADH (an assimilatory enzyme). 

Oral absorption of ascorbic acid is via an energy-dependent process that is saturable and dose-dependent. Ascorbic acid is stored in the body. Excessive amounts of consumed vitamin C, i.e. if daily intake surpasses 100 mg, are rapidly excreted in the urine. 

Tetracyclines enter microorganisms in part by passive diffusion and in part by an energy-dependent process of active transport. Susceptible cells concentrate the drug intracellularly. Once inside the cell, tetracyclines bind reversibly to the 30S subunit of the bacterial ribosome, blocking the binding of aminoacyl-tRNA to the acceptor site on the mRNA-ribosome complex (Figure 44-1). This prevents addition of amino acids to the growing peptide. 

Thermodynamics An area of science concerned with the role energy plays in chemical reactions and other energy-dependent processes. 

Calcium levels are believed to be controlled in part at least by the uptake and release of Ca2+ from mitochondria.172"174 The capacity of mitochondria for Ca2+ seems to be more than sufficient to allow the buffering of Ca2+ at low cytosol levels. Mitochondria take up Ca2+ by an energy-dependent process either by respiration or ATP hydrolysis. It is now agreed that Ca2+ enters in response to the negative-inside membrane potential developed across the inner membrane of the mitochondrion during respiration. The uptake of Ca2+ is compensated for by extrusion of two H+ from the matrix, and is mediated by a transport protein. Previous suggestions for a Ca2+-phosphate symport are now discounted. The possible alkalization of the mitochondrial matrix is normally prevented by the influx of H+ coupled to the influx of phosphate on the H - PCV symporter (Figure 10). This explains why uptake of Ca2+ is stimulated by phosphate. Other cations can also be taken up by the same mechanism. 

Available evidence indicates that CNTs do not enter the cell directly from the plasma membrane, but are endocytosed inside the lumen of vesicular structures in a temperature- and energy-dependent process (Black and Dolly 1986 Critchley et al. 1985 Dolly et al. 1984 reviewed in Schiavo et al. 2000). The finding of SV proteins as receptors of BoNTs support the proposal (Montecucco and Schiavo 1995) that, after binding, BoNTs are endocytosed within synaptic vesicles (SV) via their retrieval to be refilled with neurotransmitter, an hypothesis originally advanced to account for the increased rate of poisoning with NMJ activity (Figure 4) (Hughes and Whaler 1962). 

Mode of action Griseofulvin [gri see oh FUL vin] enters susceptible fungal cells by an energy-dependent process. It is believed to... 

The uptake of iron from transferrin by reticulocytes is a time-, temperature-, and energy-dependent process in which integrity of both protein and cells is required (64, 65). Synthetic iron chelates, once thought to be effective iron donors (66), appear to depend on membrane-bound transferrin as an intermediate agent cells depleted of the protein by preincubation and washing no longer accept iron from such complexes (67). When such cells are reincubated with transferrin, their capacity to accept iron initially bound to synthetic chelators is largely restored. 

Once each enantiomer has been excited to a different state ( 2) or 3)), the pair can be physically separated using a variety of energy-dependent processes, such as ionization, followed by ions extraction by an electric field. If we execute the excitation in the IR range and ionize the chosen excited enantiomer after only a few ns delay, losses from fluorescence, whose typical lifetimes in that regime are in the ms range, are expected to be minimal. 

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