Choline, transport through membranes

The identity of the moiety (other than glycerol) esterified to the phosphoric group determines the specific phosphoHpid compound. The three most common phosphoHpids in commercial oils are phosphatidylcholine or lecithin [8002-45-5] (3a), phosphatidylethanolamine or cephalin [4537-76-2] (3b), and phosphatidjlinositol [28154-49-7] (3c). These materials are important constituents of plant and animal membranes. The phosphoHpid content of oils varies widely. Laurie oils, such as coconut and palm kernel, contain a few hundredths of a percent. Most oils contain 0.1 to 0.5%. Com and cottonseed oils contain almost 1% whereas soybean oil can vary from 1 to 3% phosphoHpid. Some phosphoHpids, such as dipaLmitoylphosphatidylcholine (R = R = palmitic R" = choline), form bilayer stmetures known as vesicles or Hposomes. The bdayer stmeture can microencapsulate solutes and transport them through systems where they would normally be degraded. This property allows their use in dmg deHvery systems (qv) (8). 

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. 

Choline is an interesting example of a molecule dependent, for its cellular uptake, on facilitated diffusion. Unable, because of its permanently ionized and poorly lipophilic nature, to penetrate by simple diffusion through a Type 1 membrane, choline is rapidly transported into erythrocytes and several other kinds of cell by a specific carrier. Tetramethylammonium cations can also enter cells on this carrier, a facility denied to higher homologues which can, however, block the physiological uptake of choline (Martin, 1969). 

Choline, an amino alcohol, is the precursor of phospholipids, which are ubiquitous components of cell membranes. Choline can be labeled with "C (50) or F (51,52) and is taken up by tumor cells via an active transport mechanism. It is subsequently trapped through phosphorylation by choline kinase to... 

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