Quercetin transport system

Transport systems. Partitioning of various types of molecules such as allelochemlcals into the lipid bilayer of the mitochondrial inner membrane can perturb the membrane and alter the conformation, properties, and function of components of the membranes. Unfortunately, it is not always possible to demonstrate directly the existence of carrier systems, but indirect evidence can be obtained. Alterations induced to the membrane are sometimes reflected in the osmotic behavior of mitochondria. The inner membrane is relatively impermeable to many cations, including K and H, and many solutes (31). Hence, the organelles are osmotic-ally stable under certain conditions. Indications were obtained that the allelochemlcals inhibited the action of carrier-mediated transport processes associated with the mitochondrial inner membrane (as reflected in the osmotic behavior). Responses obtained with quercetin are shown in Figure 3. Mitochondria are osmotically... 

Noteborn, H.P.J.M., Jansen, E., Benito, S., and Mengelers, M.J.B., Oral absorption and metabolism of quercetin and sugar-conjugated derivatives in specific transport systems. Cancer Lett., 114, 175, 1997. 

Effects of Allelochemlcals on ATPases. Several flavonoid compounds inhibit ATPase activity that is associated with mineral absorption. Phloretin and quercetin (100 pM) inhibited the plasma membrane ATPase Isolated from oat roots (33). The naphthoquinone juglone was inhibitory also. However, neither ferulic acid nor salicylic acid inhibited the ATPase. Additional research has shown that even at 10 mM salicylic acid inhibits ATPase activity only 10-15% (49). This lack of activity by salicylic acid was substantiated with the plasma membrane ATPase Isolated from Neurospora crassa (50) however, the flavonols fisetln, morin, myricetin, quercetin, and rutin were inhibitory to the Neurospora ATPase. Flavonoids inhibited the transport ATPases of several animal systems also (51-53). Thus, it appears that flavonoids but not phenolic acids might affect mineral transport by inhibiting ATPase enzymes. 

The clinical relevance of data obtained from studies with single compounds is questionable, because most studies were performed in in vitro systems, limiting the predictability of the effects of the examined compounds in vivo. Moreover, some polyphenolics, such as quercetin, were shown to interact with the absorption or metabolism of drugs only at very high concentrations (50-100 pmol/L), which are likely to exceed the expected in vivo concentration after the consumption of a moderate amount of a grapefruit/ citrus product. Also, flavonoids have been demonstrated to potentially induce apoptosis in cell lines at concentrations comparable to those used for some in vitro drug interaction studies (64-66). This potentially could have impaired the investigation of enzyme and transporter activities. 

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