Editing ABC transporter (section)

=== Membrane assays ===
The ''vesicular transport assay'' detects the translocation of molecules by ABC transporters.<ref>{{cite journal | vauthors = Horio M, Gottesman MM, Pastan I | title = ATP-dependent transport of vinblastine in vesicles from human multidrug-resistant cells | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 85 | issue = 10 | pages = 3580–4 | date = May 1988 | pmid = 3368466 | pmc = 280257 | doi = 10.1073/pnas.85.10.3580 | bibcode = 1988PNAS...85.3580H | doi-access = free }}</ref> Membranes prepared under suitable conditions contain inside-out oriented vesicles with the ATP binding site and substrate binding site of the transporter facing the buffer outside. Substrates of the transporter are taken up into the vesicles in an ATP dependent manner. Rapid filtration using glass fiber filters or nitrocellulose membranes are used to separate the vesicles from the incubation solution and the test compound trapped inside the vesicles is retained on the filter. The quantity of the transported unlabelled molecules is determined by HPLC, LC/MS, LC/MS/MS. Alternatively, the compounds are radiolabeled, fluorescent or have a fluorescent tag so that the radioactivity or fluorescence retained on the filter can be quantified.

Various types of membranes from different sources (e.g. insect cells, transfected  or selected mammalian cell lines) are used in vesicular transport studies. Membranes are commercially available or can be prepared from various cells or even tissues e.g. liver canalicular membranes. This assay type has the advantage of measuring the actual disposition of the substrate across the cell membrane. Its disadvantage is that compounds with medium-to-high passive permeability are not retained inside the vesicles making direct transport measurements with this class of compounds difficult to perform.

The vesicular transport assay can be performed in an "indirect" setting, where interacting test drugs modulate the transport rate of a reporter compound. This assay type is particularly suitable for the detection of possible drug-drug interactions and drug-endogenous substrate interactions. It is not sensitive to the passive permeability of the compounds and therefore detects all interacting compounds. Yet, it does not provide information on whether the compound tested is an inhibitor of the transporter, or a substrate of the transporter inhibiting its function in a competitive fashion.  A typical example of an indirect vesicular transport assay is the detection of the inhibition of taurocholate transport by ABCB11 ([[ABCB11|BSEP]]).