Transporter tools membrane vesicles

Solute uptake can also be evaluated in isolated cell suspensions, cell mono-layers, and enterocyte membrane vesicles. In these preparations, uptake is normalized by enzyme activity and/or protein concentration. While the isolation of cells in suspension preparations is an experimentally easy procedure, disruption of cell monolayers causes dedifferentiation and mucosal-to-serosal polarity is lost. While cell monolayers from culture have become a popular drug absorption screening tool, differences in drug metabolism and carrier-mediated absorption [70], export, and paracellular transport may be cell-type- and condition-depen-dent. 

These models also suffer from distinct limitations. The ability to translate transport parameters in this model to in vivo rate or extent of absorption is limited. Contamination due to heterogeneity of tissue source or other membranes, as well as proper orientation after vesicle resealing, are additional issues. One of the major drawbacks of this technique is the need for a sensitive analytical methodology. Use of radiolabeled solutes is a viable alternative when available. In other cases, the availability of sensitive and specific analytical tools such as HPLC/MS/MS would be well suited. Depending on the method used to quench the uptake reaction, the matrix for assay may be a dilute buffer solution or a miscible mixture of buffer and organic solvent (e.g, acetonitrile). 

Because Ras proteins are farnesylated in the cytosol, then targeted to the endoplasmic reticulum for subsequent processing, and, if required, directed to the Golgi for palmitoylation before their ultimate signaling destination, the question arises how the specific distribution of these proteins is maintained. Traffic by vesicles or diffusion processes were proposed for N- and H-Ras proteins, and the involvement of transport proteins has been suggested for K-Ras. However, the factors regulating the transport or distribution of Ras proteins over the different cellular membranes are still poorly understood, mainly due to the lack of appropriate tools enabling this study. 

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