Active transport mechanisms

Biochemical studies of plasma membrane Na /H exchangers have been directed at two major goals (1) identification of amino acids that are involved in the transport mechanism and (2) identification and characterization of the transport pro-tein(s). To date, most studies have been performed on the amiloride-resistant form of Na /H" exchanger that is present in apical or brush border membrane vesicles from mammalian kidney, probably because of the relative abundance of transport activity in this starting material. However, some studies have also been performed on the amiloride-sensitive isoform present in non-epithelial cells. 

If cellular redox state, determined by the glutathione status of the heart, plays a role in the modulation of ion transporter activity in cardiac tissue, it is important to identify possible mechanisms by which these effects are mediated. Protein S-,thiolation is a process that was originally used to describe the formation of adducts of proteins with low molecular thiols such as glutathione (Miller etal., 1990). In view of the significant alterations of cardiac glutathione status (GSH and GSSG) and ion-transporter activity during oxidant stress, the process of S-thiolation may be responsible for modifications of protein structure and function. 

In the lattice-gas model, as treated in Section IV.D above, ion transfer is viewed as an activated process. In an alternative view it is considered as a transport governed by the Nernst-Planck or the Langevin equation. These two models are not necessarily contra-dictive for high ionic concentrations the space-charge regions and the interface have similar widths, and then the barrier for ion transfer may vanish. So the activated mechanism may operate at low and the transport mechanism at high ionic concentrations. 

Changes in transporter proteins by high-frequency genetic mutation mechanisms, while maintaining transport activity, helps to evade the host s immune response, and in cases where the substrate specificity is changed, helps the microorganism to adapt to new iron sources. 

A carrier mechanism was excluded for these molecules when ion-transport activity was undiminished in membranes in the gel state. The transport rates... 

Fig. 4.10. Principle of signal transduction by steroid hormone receptors. The steroid hormone receptors in the cytosol are found in the form of an inactive complex with the heat shock proteins Hsp90 and Hsp56 and with protein p23. The binding of the hormone activates the receptor so that it can be transported into the nucleus where it binds to its cognate HRE. It remains unclear in which form the receptor is transported into the nucleus, and to which extent the associated proteins are involved in the transport. One mechanism of activation of transcription initiation involves activation of a histone acetylase and remodehng of chromatin. Furthermore, the receptors interact directly or indirectly with components of the RNA polymerase II holoenzyme (e.g. SUGl).

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