Low affinity transport system

Aminoglycosides have a low volume of distribution and are excreted by glomerular filtration, which is followed by active reuptake via a high-capacity, low-affinity transport system. The aminoglycosides remain in the kidney longer than in other exposed tissues such as the liver. Thus, with gentamycin, the time for the tissue concentration to be reduced by half is... 

In contrast to the relatively simple mode of inactivation described for acteylcholine and perhaps some neuropeptides, the reuptake systems that have been the subject of intensive study, especially for the aromatic monoamines, are considerably more complex. These uptake systems demonstrate a high affinity for the transmitter that they are meant to accumulate. They exist in addition to transport systems that possess a lower affinity for the substrate in question. These low-affinity transport systems may exist for the accumulation of substrate for general metabolic requirements of the cell. Of interest is the fact that all studies on the high-affinity reuptake systems for monoaminergic transmitters have demonstrated an absolute requirement of the system for Na+. The system does not appear to require adenosine triphosphate, however, and so can be readily distinguished from the Na" "-K+ ATPase exchange system described earlier in this article. The high-... 

CHOLINE TRANSPORT Transport of choline from the plasma into neurons is rate-limiting in ACh synthesis and is accomplished by distinct high- and low-affinity transport systems. The high-affinity system (K = 1-5 /xM) is unique to chohnergic neurons, dependent on extracellular Na+, and inhibited by hemicholinium. Plasma concentrations of choline approximate 10 /xM. Much of the choline formed from AChE-catalyzed hydrolysis of ACh is recycled into the nerve terminal. 

The synthesis of acetylcholine from acetyl CoA and choline is catalyzed by the enzyme choline acetyltransferase (ChAT) (Fig. 48.9). This synthetic step occurs in the presynaptic terminal. The compound is stored in vesicles and later released through calcium-mediated exocytosis. Choline is taken up by the presynaptic terminal from the blood via a low-affinity transport system (high and from the synaptic cleft via a high-affmity transport mechanism (low K. It is also derived from the hydrolysis of phosphatidylcholine (and possibly sphingomyelin) in membrane lipids. Thus, membrane lipids may form a storage site for choline, and their hydrolysis, with the subsequent release of choline, is highly regulated. 

Fig. 1.9. Evolution of glucose transport system activity of S. cerevisiae fermenting a medium model (Salmon et al, 1993). LF = Length of the fermentation as a decimal of total time GP = Glucose penetration speed (mmol/h/g of dry weight) 0 = Low affinity transport system activity = High affinity transport system activity...

Martin DL, Shain W (1979) High affinity transport of taurine and beta-alanine and low affinity transport of gamma-aminobutyric acid by a single transport system in cultured glioma cells. J Biol Chem 254(15) 7076-7084... 

Biphasic kinetics of Fe2+ transport in a wild-type strain of El. pylori suggested the presence of high- and low-affinity uptake systems. The high-affinity system (apparent Ks = 0.54 jimoldm ) is absent in a mutant lacking the feoB gene. Transport via FeoB is highly specific for Fe2+, and was inhibited by FCCP,... 

Likewise, for zinc, bacteria have developed active uptake systems (Hantke, 2001). In many bacteria the high-affinity Zn2+ uptake system uses an ABC transporter of the cluster 9 family, which mostly transports zinc and manganese and is found in nearly all bacterial species. First identified in cyanobacteria and pathogenic streptococci, but also found in E. coli, the system is encoded by three genes ZnuABC and consists of an outer membrane permease ZnuB, a periplasmic-binding protein ZnuA and a cytoplasmic ATPase ZnuC. Low-affinity transporters of the ZIP family, described later in this chapter, such as ZupT, have also been shown to be involved in bacterial zinc uptake. 

A particular ion or uncharged molecule can be transported by different transporters depending on its concentration. For example NH4+ may be absorbed by a passive low-affinity uptake system when its external concentration is large and by an active high-affinity system when its external concentration is small. Figure 6.10 summarizes the main transport processes on the plasma membrane and tonoplast of plant cells. 

The fact that Mn2+ may also be used105 as a probe for Ca2+ serves to emphasize those problems, as Ca2+ and Mg2+ tend to be mutually inhibitory. It appears that Mn2+ resembles Ca2+ in terms of movement in and out of certain cells. However, Mn2+ is taken up by some microbes through low affinity magnesium transport systems. These organisms usually have specific high affinity transport systems for Mn2+ as well.106... 

Figure 1. Schematic of the two iron transport systems of microorganisms. The high affinity system is comprised of specific carriers of ferric ion (siderophores) and their cognate membrane hound receptors. Both components of the system are regulated by iron repression through a mechanism which is still poorly understood. The high affinity system is invoked only when the available iron supply is limiting otherwise iron enters the cell via a nonspecific, low affinity uptake system. Ferri-chrome apparently delivers its iron by simple reduction. In contrasty the tricatechol siderophore enterobactin may require both reduction and ligand hydrolysis for release...

Phosphate transporters have been characterized in many model organisms, though relatively little mechanistic work has been done in marine phytoplankton. Phosphate transport is elfected by high and low affinity transporters and dependent on ATP, Na, and Mg " " in several diatoms (Cembella et al., 1984). These observations are found to be consistent with the well known active transport system of yeast (Raghothama, 1999). The dependence of phosphate transport on Mg " " in diatoms and yeast suggests that eukaryotes may transport an uncharged cation phosphate complex (MeHP04, where Me may be Ca +, Mg +, Co +, Mn ) as has been observed in heterotrophic bacteria (van Veen, 1997). 

It is clear that freshly isolated hepatocytes in suspension and in primary monolayer culture exhibit different kinetic parameters for Na -de-pendent AIB uptake (17,24,25). Morin et al. (25) reported two systems for Na" -dependent AIB transport in freshly isolated cells in suspension with values of 0.13 0.05 and 43.6 10.0 mM. After 20 hours of culture, they found that the cells contained only the high-aflfinity portion of AIB uptake. The loss of the low-affinity transport during culture may explain why a single K value of about 1 mM has been reported by those laboratories that have used cultured hepatocytes exclusively for their studies (24, 26). Gurr and Potter have used both transport and enzymatic activities to stress the inherent differences between freshly isolated cell suspensions and hepatocytes in primary culture (27,28). Our laboratory has shown that during the initial 24 hours of hepatocyte culture dramatic decreases occur in the transport activity of System ASC, yet System A activity remains unaltered (29-31). Therefore, although it is recognized that System ASC mediates part of the Na" -dependent AIB uptake in hepatocyte cell suspensions (10,17,22), as the time in culture is increased the contribution of System ASC is minimized. Inhibition analysis in cultured hepatocytes supports the view that most, if not all, of the Na" -dependent AIB uptake in these cells occurs via System A (Table II and 26). 

Fe2+ is also transported by the CorA protein (Hantke, 1997), a divalent cation transporter for Mg2+, Co2+, Mn2+, and Ni2+ in E. coli and S. typhimurium. CorA may represent the often-mentioned low-affinity iron-uptake system of E. coli that is suppressed in Fe3+ uptake studies by the addition of 0.2 mM dipyridyl and 0.1 mM nitrilotriacetate. 

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