Vitamin membrane vesicles

Hydrophobic species bearing hydrocarbon chains present vitamin B12 or vitamin B6 type activity [5.37]. Such systems lend themselves to inclusion in membrane or micellar media. They thus provide a link with catalysis in more or less organized media such as membranes, vesicles, micelles, polymers [5.39-5.41] (see Section 7.4). Water soluble cyclophanes showing, for example, transaminase [5.42], acetyl transfer [5.43], pyruvate oxidase [5.44] or nucleophilic substitution [5.45] activity have been described. 

Malo C and Wilson JX (2000) Glucose modulates vitamin C transport in adult human small intestinal brush border membrane vesicles. Journal of Nutrition 130,63-9. 

After the procollagen polypeptides are assembled into a triple helix, they are secreted by the classical route. They pass through the smooth endoplasmic reticulum and the Golgi complex, where they are packaged into membranous vesicles and secreted into the extracellular space by exo-cytosis. This process requires ATP and may involve microtubules and microfilaments. The conformation of procollagen markedly affects the rate of secretion. Prevention of the formation of a triple helix (e.g., lack of 4-hydroxylation due to vitamin C deficiency) leads to the accumulation of nonhelical propolypeptides within the cistemae of the rough endoplasmic reticulum and a delayed rate in its secretion. 

Although only CaBP mRNA is known to increase in response to vitamin D, other vitamin D-dependent changes occur in the intestinal epithelium, including increases in activity of alkaline phosphatase, calcium ATPase, adenylate cyclase, and RNA polymerase. In response to vitamin D, several brush-border membrane proteins increase in concentration as does a calcium-binding complex. Non-cAMP-dependent phosphorylation of a brush-border membrane protein, increased synthesis and turnover of microvillar membrane phospholipids, and effects on mitochondria, Golgi membranes, and intracellular membrane vesicles are observed. Increased transport of Ca " " across the basolateral membrane may be produced by... 

Some recent studies on vitamin transport using membrane vesicles include those of vitamin B6 by rat kidney brush border membranes (Bowman et al, 1990), ascorbic acid by teleost intestinal brush border membranes (Mafha et ai, 1993), biotin by human kidney brush border membranes (Baur and Baumgartner, 1992), pantothenate by human placental brush border membranes (Grassl, 1992), folate and riboflavin by rabbit intestinal brush border membranes (Said and Mohammadkhani, 1993a,b Said et al, 1993), and thiamine by rat small intestine basolateral membranes (Laforenza et al, 1993). Bile acid transport in human placental, rat ileal, and rabbit small intestinal brush border membrane vesicles (Dumaswala et al, 1993 Gong et al, 1991 Kramer et al, 1993) and the effect of vitamin D status... 

In particular, the use of intestinal brush border membranes has offered much potential for the detailed investigation of absorptive phenomena. It is likely that the absorption of lipid nutrients including fat-soluble vitamins for example, which may intercalate transiently into the bilayer and affect the physical properties of the membrane, will concurrently influence the entry of other substances. Alternatively, interactions between nutrients in the gut might be expected to mutually influence absorption of these substances. Also, there may be competition between nutrients for the same transport system. Membrane vesicles are very much suited for studying this type of problem and their use has helped resolve, for example, the mechanism whereby lecithins inhibit cholesterol uptake (Merrill et al, 1987). Again, the use of isolated brush order membranes has been conducive to a better understanding of the mechanisms of interaction between lipid micelles or lipid vesicles and the intestinal luminal membrane. 

Bianchi, J. Rose, R. C. 1985. Transport of L-ascorbic acid and dehydro-L-ascorbic acid across renal cortical basolateral membrane vesicles. Biochim. Biophys. Acta 820(2) 265-273. Bode, A. M. Yavarow, C. R. 1993. Enzymatic basis for altered ascorbic acid and dehydro-ascorbic acid levels in diabetes. Biochem. Biophys. Res. Commun. 191 1347-1353. Biondi, C. Pavan, B. Dalpiaz, A. Medici, S. Lunghi, L. Vesce, F. 2007. Expression and characterization of vitamin C transporter in the human trophoblast cell line HTR-8/ SVneo Effect of steroids, flavonoids, and NSAIDs. Mol. Hum. Reprod 13(1) 77-83. Boyer, J. C. Campbell, C. E. Sigurdson, W. J. Kuo, S. M. 2005. Polarized localization of vitamin C transporters, SVCTl and SVCT2, in epithelial cells. Biochem. Biophys. Res. Commun. 334 150-156. 

A few substances are so large or impermeant that they can enter cells only by endocytosis, the process by which the substance is bound at a cell-surface receptor, engulfed by the cell membrane, and carried into the cell by pinching off of the newly formed vesicle inside the membrane. The substance can then be released inside the cytosol by breakdown of the vesicle membrane. Figure 1-5D. This process is responsible for the transport of vitamin B12, complexed with a binding protein (intrinsic factor) across the wall of the gut into the blood. Similarly, iron is transported into hemoglobin-synthesizing red blood cell precursors in association with the protein transferrin. Specific receptors for the transport proteins must be present for this process to work. 

With the isolated perfused duodenum, there is a rapid increase in calcium transport in response to the addition of calcitriol to the perfusion medium. Isolated enterocytes and osteoblasts also show a rapid increase in calcium uptake in response to calcitriol. It is not associated with changes in mRNA or protein synthesis, but seems to be because of recruitment of membrane calcium transport proteins from intracellular vesicles to the cell surface. It is inhibited by the antimicrotubule compound colchicine. It can only be demonstrated in tissues from animals that are adequately supplied with vitamin D in vitamin D-deficient animals, the increase in intestinal calcium absorption occurs only more slowly, together with the induction of calbindin. 

Osteoblasts take up Ca2+ ions from the periosteal extracellular fluid using Na+/Ca2+-exchangers NCX1 and NCX3. Once in the cytosol, the Ca2+ ions must be transported to the osteoid matrix side (basal side) by calbindins, which require the active form of vitamin D (calcitriol) for synthesis and expression. The Ca2+ ions are passed out to the osteoid matrix through an ATP-dependent plasma membrane Ca2+-ATPase lb (PMCAlb). The orientations of the cells, the transporters, and the calbindins are described in detail in Sect. 10.4.1. Once in the osteoid matrix, the matrix vesicles take up the Ca2+ ions via an annexin transporter. 

Why is vitamin C used so extensively as an electron donor There are two main reasons. First, vitamin C is very soluble in water, so it can be concentrated in confined spaces surrounded by membranes (which are made of lipids impermeable to vitamin C). The synthesis of noradrenaline from dopamine, for example, takes place in small membrane-bounded spaces, or vesicles, within cells of the cortex of the adrenal glands. The vitamin C concentration inside these vesicles reaches about 100 times that of blood plasma. As vitamin C is consumed by the enzyme dopamine mono-oxygenase, electrons are passed across the vesicle membrane (via an iron-containing protein, cytochrome b65l), to regenerate vitamin C within the vesicles. Thus, for periods of days or weeks, the intracellular vitamin C needed for physiological tasks can be insulated from changes in plasma levels caused by variations in diet, and maintained at the ideal levels for a particular reaction. 

Plausible as the above mechanism may seem, it may, however, not be the whole truth. An alternative mechanism is vesicular transport. In chicken intestine it has been shown that the only epithelial organelles that increased in Ca content as a result of calcitriol treatment were the lysosomes." The result lends support to a transport mechanism involving Ca + uptake across the brush-border membrane by endocytic vesicles, fusion of these vesicles with lysosomes, and possibly also delivery of Ca to the basal lateral membrane of the epithelial cell by exocytosis. This process would also explain the vitamin-D-induced alterations in brush-border-membrane lipid compositions as a consequences of preferential incorporation of certain types of lipids into the vesicles. Interestingly, the lysosomes in the chicken studies also contained high levels of calbin-... 

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