Inactivation steady-state

As was recalled by Keynes (1994a), it was first observed by Chandler Meves (1970) that in squid axons perfused with NaF a small flow of Na ions persisted in the inactivated steady state. Their tentative conclusion based on the Hodgkin-Huxley model was to suppose that the inactivation parameter h was the sum of two components h, and Zij, where A, predominated at negative potentials, and Zij predominated at positive ones. This proposition predicted the existence of one type of Na+ conductance that increased transiently with depolarization as in the Hodgkin—Huxley system, and a second type that persisted with depolarization to give a steady low level of conductance. Combined with m kinetics the idea fitted well with the experimental data. 

Measurements have been reported by Keynes Meves (1993) of the probability functions both for the initial opening of the Na channels in squid axons, and for the reopenings that generate the late current in the inactivated steady state. Plots... 

Figure 4. Effects of dihydro-brevetoxin B (H2BVTX-B) on Na currents in crayfish axon under voltage-clamp. (A) A family of Na currents in control solution each trace shows the current kinetics responding to a step depolarization (ranging from -90 to -I-100 mV in 10 mV increments). Incomplete inactivation at large depolarizations is normal in this preparation. (B) Na currents after internal perfusion with H2BVTX-B (1.2 a M). inactivation is slower and less complete than in the control, and the current amplitudes are reduced. (C) A plot of current amplitudes at their peak value (Ip o, o) and at steady-state (I A, A for long depolarizations) shows that toxin-mOdified channels (filled symbols) activate at more negative membrane potentials and correspond to a reduced peak Na conductance of the axon (Reproduced with permission from Ref. 31. Copyright 1984 American Society for Pharmacology and Experimental Therapeutics).

The distribution of open channel times is mainly determined by the rate constants S and K (2 is assumed to be very small). Mutations which change the C to O transition (e.g., the burst size of channel opening) have not been characterized yet. However, structural alterations which affect k and thereby the level of steady state inactivation have been described for Sh channels [29,60]. Different splice variants of Sh channels... 

Choi, Y.J., Chae, H.J., and Kim, E.Y., Steady-state oxidation model by horseradish peroxidase for the estimation of the non-inactivation zone in the enzymatic removal of pentachlorophenol, J. Biosci. Bioeng., 88, 368-373, 1999. 

The inactivation must be placed in context with the steady-state enzyme synthesis rate, which can be simply described ... 

Dekker et al. [170] studied the extraction process of a-amylase in a TOMAC/isooctane reverse micellar system in terms of the distribution coefficients, mass transfer coefficient, inactivation rate constants, phase ratio, and residence time during the forward and backward extractions. They derived different equations for the concentration of active enzyme in all phases as a function of time. It was also shown that the inactivation took place predominantly in the first aqueous phase due to complex formation between enzyme and surfactant. In order to minimize the extent of enzyme inactivation, the steady state enzyme concentration should be kept as low as possible in the first aqueous phase. This can be achieved by a high mass transfer rate and a high distribution coefficient of the enzyme between reverse micellar and aqueous phases. The effect of mass transfer coefficient during forward extraction on the recovery of a-amylase was simulated for two values of the distribution coefficient. These model predictions were verified experimentally by changing the distribution coefficient (by adding... 

Dekker et al. [170] have also shown that the steady state experimental data of the extraction and the observed dynamic behavior of the extraction are in good agreement with the model predictions. This model offers the opportunity to predict the effect of changes, both in the process conditions (effect of residence time and mass transfer coefficient) and in the composition of the aqueous and reverse micellar phase (effect of inactivation rate constant and distribution coefficient) on the extraction efficiency. A shorter residence time in the extractors, in combination with an increase in mass transfer rate, will give improvement in the yield of active enzyme in the second aqueous phase and will further reduce the surfactant loss. They have suggested that the use of centrifugal separators or extractors might be valuable in this respect. 

Tyrrell, L., Renganathan, M., Dib-Hajj, S. D., Waxman, S. G. Glycolization alters steady-state inactivation of sodium channel Nav1.9 / NaN in dorsal root ganglion neurons and is developmentally regulated, J. Neurosci. 2001, 21, 9629-9637. 

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