Cellular concentrations

Induction With reference to enzymes an increase in activity due to an increase in their cellular concentrations. This may be a response to a xenobiotic, and often involves an increased rate of synthesis of the enzyme. 

Although the cellular concentrations of lycopene or its oxidation products may be too low to have a general antioxidant or pro-oxidant effect on cells, there is sufficient evidence of its in vivo effect on the classical measures of oxidative stress to indicate that its participation in the redox state... 

Because anthralin exerts its clinical effects at low cellular concentrations, therapy usually starts with low concentrations (0.1% to 0.25%) with gradual increases to higher concentrations (0.5% to 1%). Cream and ointment formulations are usually applied in the evening and allowed to remain overnight. 

The use of triphenylethylene SERMs as Pgp inhibitors for clinical application has been hampered by unacceptable toxicity at doses required to achieve adequate cellular concentration, which is likely due to the involvement of proteins with the ability to bind these compounds. For instance, toremifene is able to reverse MDR and to sensitize human renal cancer cells to vinblastine in vitro. However, in vivo toremifene is tightly bound to serum proteins, in particular a 1-acid glycoprotein (AAG), which may limit its tissue availability (Braybrooke et al. 2000). In agreement with this, Chatterjee and Harris (1990) have shown that tamoxifen and 4-OH-tamoxifen were similarly potent in reversing MDR in Chinese hamster ovary (CHO) cells with acquired resistance to adriamycin. However, the addition of AAG (0.5 to 2 mg/ml, the range found in vivo) to cell cultures decreased the effect of tamoxifen on reversing MDR, and at the highest AAG concentration there was a complete reversal of the effects of... 

For limiting nutrients, cellular concentrations are constant under conditions of steady-state growth. To ensure that the limiting nutrient is not diluted in the microbial population, kmt must be greater than the maximal growth rate, /imax. This limiting condition sets a minimum for the value of the Monod constant, Kmd = / max /[7]- Note that while Monod kinetics are more applicable than first-order kinetics for many ecological uptake processes, solutions of the above equations require considerably more a priori information [48]. 

An inherent property of osmolytes is that they lower the freezing point depressions of cells (Kirst 1996). Therefore, DMSP may also be acting as a cryoprotectant in algae exposed to freezing temperatures. However, Kirst (1996) argues that cellular concentrations of DMSP are too low to produce a significant freezing point depression. 

Most microalgal toxicity tests procedures recommend the use of initial cellular concentrations of 104 cells mL 1. This cellular concentration should be selected because it is the minimum cellular concentration that can be measured in haematocytometers (Neubauer chambers). Furthermore, natural cellular concentrations in non-polluted conditions (in marine environments) are often below the concentration mentioned. The importance of cellular density at the beginning of the test has been demonstrated for certain toxicants [43]. The lower the cellular concentration, the higher the sensitivity of the test, at least for certain types of xenobiotics, such as heavy metals. 

Accordingly, the use of flow cytometry can improve the design of toxicity bioassays, as the detection limit of this apparatus includes cellular concentrations equal to those of microalgal populations found in natural conditions. Comparison of compositions utilised in some known toxicity tests for microalgaes are shown in Table 7.1.1. 

Our knowledge of enzymes also tells us that under usual physiological conditions (i.e. at typical cellular concentrations of substrate) most metabolic reactions are reversible. Energetically irreversible reactions, i.e. those with a large positive free energy change, effectively act as one-way valves allowing substrate flow in the forward direction only. 

The effect of sphingosine on other enzymes may also contribute to its apoptotic effect. These include the inhibition of calcium/calmodulin-requiring enzymes and DNA primase and the stimulation of casein kinase II and several unidentified kinases (Alessenko, 2000). In addition, sphingosine can increase the cellular concentration of cyclic AMP, which is inhibitory for proliferation in many cell types (Pyne and Pyne, 1996). 

Methods to lower the cellular concentrations of HMGA proteins... 

Transport of bile acids from blood into bile is rate limited by activity of the BSEP, and plays a crucial role both in control of intra-cellular concentrations... 

Because muscle cells are especially rich in terms of phosphorus-containing metabolites (e.g., ATP, ADP, phos-phocreatine, and orthophosphate), nuclear magnetic resonance " has proved to be a valuable noninvasive probe of metabolic changes attending muscle activity. The spectral sensitivity of P is especially high relative to other nuclei, and one can detect cellular concentrations as low as 0.5 mM as well as utilize chemical shift data to define intracellular pH and free magnesium ion concentrations. See also Nuclear Magnetic Resonance Chemical Shift... 

At high cellular concentrations dFdCTP inhibits CTP synthetase so that CTP and dCTP production is further down regulated. 

Another example is the tissues that are particularly rich in vitamin C, for example, the cortex of the suprarenal gland or the lens here, vitamin C fulfills both antioxidative functions and metabolic ones as it helps in the formation of collagen structures. Approximately 40% of the body s ascorbate is stored in skeletal muscle because this tissue is relatively abundant and its cellular concentration is tenfold higher than the plasma level. Similarly, the intracellular ascorbate concentration in the brain (3 mM) greatly exceeds the level in the extracellular fluid (200-400 zM). The majority of ascorbate is stored in the astroglial cells that are capable of reducing great quantities of DHAA to ascorbate, which then becomes available for release back into the extracellular fluid. 

As illustrated by the [PS / ] phenomenon the relative abundance of the eRFl/eRF3 complexes is an important determinant for the efficiency of translation termination. Similar effects have been obtained in depletion studies and with eRF3 and eRFl mutants that decrease their cellular concentration (Stansfield et al. 1996 Moskalenko et al. 2003 Chabelskaya et al. 2004 Salas-Marco and Bedwell 2004). Wild type yeast contains approximately 10- to 20-fold fewer termination factors compared to ribosomes (Didichenko et al. 1991 Stansfield et al. 1992). The threshold level of eRFl/eRF3 required to maintain viability is even lower. A 10-fold decrease reduces viability by only about 10% (Valouev et al. 2002) and a decrease in eRF3 of 99% does not affect viability of the cells significantly (Chabelskaya et al. 2004). As eRF3 is associated with polysomes or ribosomes, the mechanism of translation termination depends on efficient recycling (Didichenko et al. 1991 and compare below). 

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