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High-affinity transport systems compounds

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. [Pg.893]

Cefadroxyl and cefaclor are beta-lactam antibiotics which show high affinity for the PepTl carrier system, whereas the other two beta-lactams, cephalotin and cef-metazole, are not recognized by PepTl protein and are not actively transported in the intestine. However, as the VolSurf Caco-2 model predicts that all the beta-lactams are nonpenetrating compounds, it is very probable that, as they rely only the diffusion mechanism, cefadroxyl and cefaclor will not cross the cell monolayer. [Pg.413]

Negative rejections were consistently measured for several compounds by using the cellulose acetate membrane system. Compounds of this nature must possess a strong affinity for the membrane material and have relatively high transport rates through the membrane. [Pg.438]

Noradrenaline is transported by uptake systems that have been extensively studied. On release of noradrenaline from sympathetic nerve varicosities in the peripheral nervous system, it is subject to two uptake systems. Uptake 1 (UJ is a reuptake process where the noradrenaline is recovered by the nerve via a process that has a high affinity but relatively low maximum rate, whereas a second process, uptake 2 (Uj), clears noradrenaline from the tissues into extraneuronal sites by a low affinity, but fast, process (which is inhibited by GLUCOCORTICOIDS, phenoxybenzamine and normetanephrine). The first - the neuronal system - has been studied in detail, and is essentially the same process as used for dopamine and 5-hydroxytryptamine in the CNS. The U transport protein has now been cloned, and is one of a famiiy of transporter proteins which act as co-transporters for Na, Cl and the amine, driven by the ATP-generated electrochemical gradient for Na . This Ui noradrenaline reuptake process is inhibited by cocaine and amphetamine (thus accounting for some of their actions, particularly within the CNS), phenoxybenzamine and the extensive class of tricyclic and related compounds that are used as ANTIDEPRESSANTS (e.g. desipramine). [Pg.284]

This pathway was first found in microorganisms which produce SA or the related compound 2,3-DHBA. The function of these compounds is different from that in plants. Under aerobic growth conditions, iron occurs in the environment as the highly insoluble Fe(OH)j. To overcome the problem of Fe " deficiency almost all bacteria and fungi have evolved high-affinity Fe transport systems based on the synthesis of low-molecular-mass... [Pg.301]

The transport of MTA in mammalian cells has been investigated only in isolated and perfused rat liver l. Fig. 10 shows that the thioethers methionine and MTA are incorporated at a much higher rate than Ado-Met, probably because of its charged sulphonium pole. The existence of a high affinity permease specific for thioethers and a low affinity system specific for sulphonium compounds has been postulated. [Pg.141]


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Compounding systems

High-affinity

High-affinity transport systems

Systemic Transport

Transport systems

Transport systems/transporters

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