Intracellular transformation rate

The enzymatic activity in soil is mainly of microbial origin, being derived from intracellular, cell-associated or free enzymes. Only enzymatic activity of ecto-enzymes and free enzymes is used for determination of the diversity of enzyme patterns in soil extracts. Enzymes are the direct mediators for biological catabolism of soil organic and mineral components. Thus, these catalysts provide a meaningful assessment of reaction rates for important soil processes. Enzyme activities can be measured as in situ substrate transformation rates or as potential rates if the focus is more qualitative. Enzyme activities are usually determined by a dye reaction followed by a spectrophotometric measurement. 

Bronk, D. A. (1999). Rates of NH4+ uptake, intracellular transformation, and dissolved organic nitrogen release in two clones of marine Synechococcus spp.J. Plankton Res. 21, 1337-1353. 

VLDLs are synthesized in the liver and transport triacylglycerols, cholesterol and phospholipids to other tissues, where lipoprotein lipase hydrolyzes the triacylglycerols and releases the fatty acids for uptake. The VLDL remnants are transformed first to IDLs and then to LDLs as all of their apoproteins other than apoB-100 are removed and their cholesterol esterified. The LDLs bind to the LDL receptor protein on the surface of target cells and are internalized by receptor-mediated endocytosis. The cholesterol, which is released from the lipoproteins by the action of lysosomal lipases, is either incorporated into the cell membrane or re-esterified for storage. High levels of intracellular cholesterol decrease the synthesis of the LDL receptor, reducing the rate of uptake of cholesterol, and inhibit HMG CoA reductase, preventing the cellular synthesis of cholesterol. 

This two-variable system (Goldbeter et al, 1978) presents the additional advantage of being formally identical with the system of eqns (2.7) studied in chapter 2 for glycolytic oscillations. This similarity stems from the basic structure common to the two models a substrate, injected at a constant rate, is transformed in a reaction catalysed by an allosteric enzyme activated by the reaction product. In the cAMP-synthesizing system in D. discoideum, activation is indirect as extracellular cAMP enhances the synthesis of intracellular cAMP, which is then transported into the extracellular medium. However, the hypothesis of a quasi-steady state for intracellular cAMP is tantamount to considering that the variation of )8 is so fast that the enzyme is, de facto, activated directly by its apparent product, extracellular cAMP. 

The fifth and final step is the transformation of the enzyme from its occluded 2.K form to a non-occluded ,.K form, which readily exchanges its K for intracellular Na" and releases it to the intracellular compartment. In the absence of ATP, but with saturating Na" ", this transition is extremely slow with a rate constant of 0.26/s for enzyme from pig and rabbit kidney outer medulla [47] or 6/s for enzyme from electrical eel electroplax [51], It is substantially enhanced by binding of ATP to 2-K at a site of low affinity = 0.45 mM). At saturating ATP concentrations the rate constant of the transition is increased to 54/s for the kidney enzyme [47]. In the absence of ATP the equilibrium is towards 2.K, as the rate constant for the... 

Section 11.2.2). In normal cells, they are expressed at low levels, whereas in transformed cells the rate of expression is increased and so also is the rate of cell growth and division. The oncoproteins can be classified according to their intracellular location (i) nuclear proteins, (ii) proteins present in the cytosol, (iii) transmembrane proteins spanning the plas-malemma, and (iv) proteins loosely associated with the plasmalemma on the cytoplasmic surface (Fig. 11.5). They can also be classified according to probable mode of action (Reddy et ah, 1988) into the following ... 

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