Role of the intestinal brush border in the absorption of cholesterol in rats

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Abstract
Short-term incubation of the everted intestinal sacs of rats in media containing cholesterol plus oleic acid resulted in rapid hydrolysis, but no synthesis, of the sterol ester. On separation of the brush border from the rest of the mucosal cell, almost all of the hydrolytic activity and appreciable amounts of the synthetic activity of the whole cell were present in thebrush-borderfraction. The isolated brush-border fraction contained considerable amounts of cholesterol, which was always present in the unesterified state; the rest of the cell contained about an equal amount of unesterified cholesterol, but, in addition, small and definite amounts of the esterified sterol were also found in this fraction. On feeding with [4-Cl4jcholesterol, diluted with 3 mg of cholesterol, to rats, the brush border very rapidly took up the fed sterol without changing its net content of cholesterol. No traces of radioactive cholesterol ester could ever be detected in the isolated brush border after feeding with Cl4-labeled esterified or unesterified cholesterol. Labeled sterol appeared quite rapidly in the rest of the cells also, where small proportions were found in the esterified state. Therefore the sequence of events in the cholesterol absorption appears to be: Dietary cholesterol esters are hydrolyzed by the cholesterol ester hydrolase of pancreas or of the mucosal brush border or both, after which the brush border rapidly absorbs the de-esterified sterol and transfers it into the mucosal cell, by a mechanism of displacement, where it is slowly re-esterified for transport through the lymph.