Which are the barriers for lipid transport from lumen to cytosol

The lipid solutes in the bulk phase of the intestinal contents are considered to form a relatively homogeneous dispersion of lipids in the aqueous phase. However, 

The thickness of the unstirred water layer of rabbit intestine in vitro has been found to vary between 115 and 334 /im depending on the rate of stirring of the bulk phase [58]. The thickness of the unstirred water layer of the rat small intestine in vivo has been reported to be even greater [60]. If the effective thickness of the unstirred water layer can reach values of around 500 jam, then it is evident that it is a far from negligible compartment of the total luminal volume at least in the case of the rat. 

It has recently been discussed [61-63] whether the diffusional barrier at the intestinal surface can be accounted for solely by an unstirred water layer. It has been proposed that the mucus layer overlying the enterocytes should be regarded as an important diffusion barrier for uptake of lipid solutes from the luminal contents. The mucus adherent to the rat duodenal wall has been found to be approximately 80 jam thick in the fasted state [64]. The intestinal mucus coat is formed by proteoglycans produced by goblet cells, but so far very little is known about the molecular structure of the mucus layer [65]. The possible interaction between mucus constituents and luminal lipid solutes needs to be investigated in detail, since it might reveal key factors which constitute the diffusional barrier of the small intestine. 

One should also consider the glycocalyx, a carbohydrate-containing polymer network between the microvilli and the mucus gel coat. It probably consists mainly of oligosaccharide chains that are covalently linked to the lipids and proteins of the brush border membrane. The definite structure of the glycocalyx is not yet available, and nothing can be said about its possible importance as an absorption barrier. 

The plasma membrane of the brush border microvilli is characterized by certain distinctive structural features, which may be related to the specialized functional properties that distinguish it from plasma membranes of other cells. The width of the microvillus membrane (measured by electron microscopy) is 10-11 nm, whereas the average eukaryotic plasma membrane is only 7-9 nm. This is probably due to the biochemical composition of the membrane, which is characterized by a high protein to lipid ratio (1.7 1) and a unique lipid composition. The cholesterol-phospholipid ratio and the molar ratio of glycolipid to phospholipid are both about 1 1, which is consistent with the low values for membrane fluidity determined in microvillus membranes. This should be compared with the corresponding ratios 

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Which are the barriers for lipid transport from lumen to cytosol ... 

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