Cholesterol exchange

Another pathway of some importance occurs in the brain this is the cholesterol 24-hydroxylase pathway. About 25% of the body s cholesterol exists in the plasma membranes of myelin sheaths. Here, the blood-brain barrier prevents cholesterol exchanges with the circulating lipoproteins, which makes it difficult for cholesterol to leave the brain. The cytochrome P-450 enzymes (CYP 46), expressed almost exclusively in the endoplasmic reticula of the brain, allows formation of 24-hydroxycholesterol. 

B10. Barter, P. J., Hopkins, G. J., Gorjatschko, L., and Jones, M. E., A unified model of esterified cholesterol exchanges between human plasma lipoproteins. Atherosclerosis 44, 27-40 (1982). 

Urquhart R, Chan RY, Li OT, TiUey L, Grieser F, Sawyer WH. Omega-6 and omega-3 fatty acids monolayer packing and effects on bilayer permeability and cholesterol exchange. Biochem Int 1992 26 831-841. 

Maguire, P. A., and Sherman, I. W. (1990). Phospholipid composition, cholesterol content and cholesterol exchange in Plasmodium falciparum-infected red cells. Mol. Biochem. Parasitol. 38,105-112. 

In blood, unesterified or free cholesterol experiences countless exchanges among the following blood components [17] (i) between lipoproteins of the same or different types, (ii) between plasma membranes and all types of lipoproteins, and (iii) between red cells and various lipoproteins. These cholesterol exchanges among various lipoprotein species are extremely rapid because they involve transfer of cholesterol molecules mostly situated at the surface of lipoprotein complexes. On the other hand, the exchange rates of cholesterol esters among lipoproteins are much slower. This is because transfer of the cholesterol ester molecule between two lipoprotein complexes must involve first the exit of the ester molecule from the nonpolar core of one... 

It has been shown that free cholesterol molecules can transfer between membranes by diffusion through the intervening aqueous layer [17], Desorption of free cholesterol molecules from the donor lipid-water interface is rate-limiting for the overall transfer process and the rate of this step is influenced by interactions of free cholesterol molecules with neighboring phospholipid molecules. The influence of phospholipid unsaturation and sphingomyelin content on the rate of free cholesterol exchange are known in pure phospholipid bilayers and similar effects probably occur in cell membranes. The rate of free cholesterol clearance from cells is determined by the structure of the plasma membrane [17] It follows that the physical state of free cholesterol in the plasma membrane is important for the kinetics of cholesterol clearance and cell cholesterol homeostasis, as well as the structure of the plasma membrane. 

Upon calculation of these results in terms of cholesterol exchange (in nmoles/ml/h) there was little difference between all three groups. 

Stanley, M. M., and S. H. Cheng Cholesterol exchange in the gastrointestinal tract in normal and abnormal subjects. Gastroenterology 30, 62 (1956). 

Bile Acid Sequestrants. The bile acid binding resins, colestipol [26658424] and cholestyramine, ate also effective in controlling semm cholesterol levels (150). Cholestyramine, a polymer having mol wt > ICf, is an anion-exchange resin. It is not absorbed in the gastrointestinal tract, is not affected by digestive enzymes, and is taken orally after being suspended in water (151). 

The earliest attempts to prepare deuterated steroids were carried out by exchange reactions of aliphatic hydrogens with deuterium in the presence of a surface catalyst. Cholesterol, for example, has been treated with platinum in a mixture of deuterium oxide and acetic acid-OD, and was found to yield... 

Anion exchange resins are basic polymers with a high affinity for anions. Because different anions compete for binding to them, they can be used to sequester anions. Clinically used anion exchange resins such as cholestyramine are used to sequester bile acids in the intestine, thereby preventing their reabsorption. As a consequence, the absorption of exogenous cholesterol is decreased. The accompanying increase in low density lipoprotein (LDL)-receptors leads to the removal of LDL from the blood and, thereby, to a reduction of LDL cholesterol. This effect underlies the use of cholestyramine in the treatment of hyperlipidaemia. 

Extraction of Aesculus hippocastanum L. (horse-chestnut) and purification on cation-exchanger (H -form), resp. precipitation with cholesterol. 

This protein is found in plasma of humans and many other species, associated with HDL. It facilitates transfer of cholesteryl ester from HDL to VLDL, IDL, and LDL in exchange for triacylglycerol, relieving product inhibition of LCAT activity in HDL. Thus, in humans, much of the cholesteryl ester formed by LCAT finds its way to the hver via VLDL remnants (IDL) or LDL (Figure 26-6). The triacylglycerol-enriched HDL2 delivers its cholesterol to the hver in the HDL cycle (Figure 25-5). 

Synthesis of glycogen, fatty acids, protein, and nucleic acids does not occur in the RBC however, some lipids (eg, cholesterol) in the red cell membrane can exchange with corresponding plasma lipids. 

ATPase also catalyzed a passive Rb -Rb exchange, the rate of which was comparable to the rate of active Rb efflux. This suggested that the K-transporting step of H,K-ATPase is not severely limited by a K -occluded enzyme form, as was observed for Na,K-ATPase. Skrabanja et al. [164] also described the reconstitution of choleate solubilized H,K-ATPase into phosphatidylcholine-cholesterol liposomes. With the use of a pH electrode to measure the rate of H transport they observed not only an active transport, which is dependent on intravesicular K, but also a passive H exchange. This passive transport process, which exhibited a maximal rate of 5% of the active transport process, could be inhibited by vanadate and the specific inhibitor omeprazole, giving evidence that it is a function of gastric H,K-ATPase. The same authors demonstrated, by separation of non-incorporated H,K-ATPase from reconstituted H,K-ATPase on a sucrose gradient, that H,K-ATPase transports two protons and two ions per hydrolyzed ATP [112]. 

Lipids are transported between membranes. As indicated above, lipids are often biosynthesized in one intracellular membrane and must be transported to other intracellular compartments for membrane biogenesis. Because lipids are insoluble in water, special mechanisms must exist for the inter- and intracellular transport of membrane lipids. Vesicular trafficking, cytoplasmic transfer-exchange proteins and direct transfer across membrane contacts can transport lipids from one membrane to another. The best understood of such mechanisms is vesicular transport, wherein the lipid molecules are sorted into membrane vesicles that bud out from the donor membrane and travel to and then fuse with the recipient membrane. The well characterized transport of plasma cholesterol into cells via receptor-mediated endocytosis is a useful model of this type of lipid transport. [9, 20]. A brain specific transporter for cholesterol has been identified (see Chapter 5). It is believed that transport of cholesterol from the endoplasmic reticulum to other membranes and of glycolipids from the Golgi bodies to the plasma membrane is mediated by similar mechanisms. The transport of phosphoglycerides is less clearly understood. Recent evidence suggests that net phospholipid movement between subcellular membranes may occur via specialized zones of apposition, as characterized for transfer of PtdSer between mitochondria and the endoplasmic reticulum [21]. 

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