Efflux rate

Another factor to take into account in biouptake studies is the possibility that the organism develops strategies of eliminating toxic species by means of efflux [38,52,101]. As a first approach, the efflux rate can be set proportional to the amount of species taken up that has been internalised, thus converting the boundary condition of flux balances for two sites, equation (4), into ... 

At this time, the proposal of additional access channels is quite conjectural. It seems likely that there is a channel or access route to the proximal side of the heme in order to provide access for the hydrogen peroxide or water needed for heme oxidation and His-Tyr bond formation. Furthermore, the electron density of compoimd I from PMC (97) reveals the presence of an anionic species that is not present in the native enz5une. However, the rapid influx-efflux rates up to 10 per sec needed for such a species to be a component of compoimd I would pose interesting constraints on a channel, and there does not seem to be a likely candidate in the region. Similarly, the potential channel leading to the cavity at the molecular center is not an ideal candidate for substrate or product movement because of its relationship to the active site residues. However, if the lateral channel is truly blocked by NADPH in small-subunit enzymes, this route may provide an alternative access or exhaust route. Both of these latter two channels require further investigation before a clear role can be ascribed to them. 

Figure 3. Return of the K efflux to the control rate after 0 stress. A. Rate after exposure. The cells are exposed to Os as in Figure 1 for 5 min at different temperatures. The efflux rates vary as shown in Figure 2. But when these Os-induced K efflux rates are normalized to 1, it is clear that the time required to return to the control rate also varies with the temperature. B. Arrhenius plot of the half-time for return. The Tl/2 represents the time required for the efflux rate to return half-way to the control rate (0.56 normalized rate).

The efficiency of tap water washing was monitored using the ion flux compartmental analysis (51) in which the efflux rate of stationary state inorganic ions from plant cells may be analyzed into loss from apparent free-space (surface film, cell walls, intercellular space), from cytoplasm, and from tonoplast. 

Fig. 14.9 (a) The efflux rate, (b) Binomial (dashed lines) and gaussian (solid lines) distributions for the cleavage probability. 

When the efflux rate and the cleavage rate have a similar time scale we observe three peaks in the distribution of fragments (see Fig. 14.11b). Similar to what is observed experimentally [23], the third peak is much smaller than the other two, and the second peak is larger than the first peak. In our model, the first peak corresponding to the small fragments reflects an efficient cleavage mechanism where fragments are repeatedly cleaved before they are released from the CP. These rest ... 

Fig. 14.11 Length distributions of the fragments outside the proteasome. From left to right the efflux rate e increases e = (0.1, 1, 10). Each distribution has been taken at the time 170 (a), 86 (b), 76 (c) when 20 % of substrate degraded. The vertical on each plot is the log frequency (from 0 to 0.25). The influx rate a = 0.01. Note that the distributions are insensitive to the variation in the influx rate a [27],...

This obvious dependence on extracellular calcium is somewhat unexpected because (1) the sustained enhancement of calcium influx rate is adequately balanced by an increase in calcium efflux rate so that (2) the calcium concentration in the bulk cytosol is maintained near the basal value. This apparent paradox may be resolved by a model [54] which postulates that during the sustained phase of cellular response the high rate of calcium cycling across the plasma membrane raises the calcium concentration in a region just below the plasma membrane, often called the submembrane domain (see Rasmussen and Barrett, Chapter 4). Because the elevated calcium level in this domain is not conducted into the bulk cytosol, it cannot activate calcium-dependent response elements in the cytosol. Rather it regulates the activity of calcium-sensitive, plasma membrane-associated enzymes such as the calcium pump and PKC, the previously described phospholipid-dependent, calcium-activated protein kinase. 

An additional experiment is performed usually to determine active efflux. For this experiment flux and efflux assays are performed at 4 °C in comparison to 37 °C. Since efflux is an active process requiring ATP an experiment at 4 °C will show a reduced efflux rate if an active process is involved. 

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