Intracellular concentration

Km for an enzymatic reaction are of significant interest in the study of cellular chemistry. From equation 13.19 we see that Vmax provides a means for determining the rate constant 2- For enzymes that follow the mechanism shown in reaction 13.15, 2 is equivalent to the enzyme s turnover number, kcat- The turnover number is the maximum number of substrate molecules converted to product by a single active site on the enzyme, per unit time. Thus, the turnover number provides a direct indication of the catalytic efficiency of an enzyme s active site. The Michaelis constant, Km, is significant because it provides an estimate of the substrate s intracellular concentration. 

As stated above, calcium is an extremely important cellular ion for several cellular functions. The concentration of calcium in human extracellular fluid is about 2.5 mM, while the intracellular concentration is only 100-200 nM depending on the cell type. Thus, there is 10 000-20 000 fold concentration difference between the cell interior and exterior that has to be maintained by cellular pumping mechanisms. This requires a large amount of energy. " ... 

Fructose is present outside a cell at 1 /iM concentration. An active transport system in the plasma membrane transports fructose into this cell, using the free energy of ATP hydrolysis to drive fructose uptake. Assume that one fructose is transported per ATP hydrolyzed, that ATP is hydrolyzed on the intracellular surface of the membrane, and that the concentrations of ATP, ADP, and Pi are 3 mM, 1 mM, and 0.5 mM, respectively. T = 298 K. What is the highest intracellular concentration of fructose that this transport system can generate Hint Kefer to Chapter 3 to recall the effects of concentration on free energy of ATP hydrolysis.)... 

Another new modification is the 2 -deoxy-2 flouro-Darabinonucleic acid (2 F-ANA), which increases the strength of the oligonucleotide-mRNA hybrids, elicits efficient RNaseH-mediated degradation of the target, is more nuclease resistant and reaches high intracellular concentrations for prolonged time. Similar results could be obtained with oxetane modified ASONs. 

The higher molecular weight organotin(IV)s, such as TBT and TFT, are known to be immunotoxic and to cause renal and hepatic damage. TBT at environmentally relevant concentrations increases intracellular concentration of Ca(II) ([Ca(II)],) in murine thymocytes by increasing membrane Ca(II) permeability and... 

Continuous and detailed knowledge of process conditions is necessary for the control and optimization of bioprocessing operations. Because of containment and contamination problems, this knowledge must often be obtained without sampling the process stream. At present, conditions such as temperatme, pressure, and acidity (pH) can be measured rapidly and accurately. It is more difficult to monitor the concentrations of the chemical species in the reaction medium, to say nothing of monitoring the cell density and intracellular concentrations of hundreds of compounds. 

Osmotic adjustment by plant cells in response to an increasing saline environment can be mediated by an alteration in intracellular concentrations of both inorganic and organic ions (Wyn Jones, 1980,1984 Aspinall, 1986 Flowers Yeo, 1986 Grumet Hanson, 1986 Moftah Michel, 1987). 

Enzymes that operate at their maximal rate cannot respond to an increase in substrate concentration, and can respond only to a precipitous decrease in substrate concentration. For most enzymes, therefore, the average intracellular concentration of their substrate tends to be close to the value, so that changes in substrate... 

As expected, ionomycin, a calcium ionophore, causes a sustained rise in the free intracellular concentration to approximately 450 nM (Figure 4). In this system, pardaxin induced an increase in intracellular [Ca ] only in the presence of extracellular Ca (Figure 4). These results indicate that pardaxin mediated a Ca influx but did not release Ca from intracellular stores. This influx is most probably mediated directly by pardaxin channels and possibly also indirectly by activation of the Ca channels of the chromaffin cells by the depolarization produced by the pardaxin channels (data not shown). These observations further substantiate our hypothesis 10) that transmembrane fluxes of Na and Ca are involved in the pathological action of pardaxin. 

Figure 5.12 Diagramatic illustration of the possible correlation between compound potency in cellular and enzymatic activity assays when the cellular phenotype is a direct result of inhibition of the target enzyme. Compounds that fall into the lower left and upper right quadrants demonstrate a correlation of rank-order potency between the cellular and cell-free assays. Compounds in the upper left quadrant may represent potent enzyme inhibitors that for some reason do not achieve adequate intracellular concentrations, as described in the text. Note the absence of any compound points in the lower right quadrant. Population of this quadrant would usually be inconsistent with enzyme inhibition being the direct cause of the observed cellular phenotype.

All of these factors (ANS stimulation, blood-borne and locally produced substances) alter smooth muscle contractile activity by altering the intracellular concentration of calcium. An increase in cytosolic calcium leads to an increase in crossbridge cycling and therefore an increase in tension... 

Acetylcholine is formed from acetyl CoA (produced as a byproduct of the citric acid and glycolytic pathways) and choline (component of membrane lipids) by the enzyme choline acetyltransferase (ChAT). Following release it is degraded in the extracellular space by the enzyme acetylcholinesterase (AChE) to acetate and choline. The formation of acetylcholine is limited by the intracellular concentration of choline, which is determined by the (re)uptake of choline into the nerve ending (Taylor Brown, 1994). 

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