Cholesteryl ester absorption by small intestinal brush border membrane is protein-mediated

This paper provides unambiguous evidence that brush border membrane vesicles (BBMV) routinely prepared from rabbit small intestine contain a protein that catalyzes the absorption of long-chain cholesteryl ester and ether, The protein is located on the lumenal side of the brush border membrane. The experiments demonstrate that cholesteryl oleate need not be hydrolyzed prior to its incorporation in the BBMV, Unexpectedly and surprisingly, the absorption kinetics of free and esterified cholesterol are very similar in small intestinal BBMV using mixed bile salt micelles and small unilamellar phospholipid vesicles as the donor, The water-soluble form of the protein responsible for this effect is released into the supernatant, probably by autoproteolysis, and catalyzes the exchange of both free and esterified cholesterol between two populations of small unilamellar phospholipid vesicles (SUV). The water-soluble form of the protein was partially purified by a two-step procedure involving gel filtration on Sephadex C-75 and anion-exchange chromatography on Mono Q, yielding a 50-fold increase in the specific activity of the protein. The resulting protein gave two bands on sodium dodecyl sulfate-10% polyacrylamide gel electrophoresis and was used to raise polyclonal antibodies in sheep. The IgG fraction of the sheep antisera blocked the cholesteryl oleate and cholesterol exchange between two populations of SUV mediated by the antigen. The same IgG fraction produced a partial inhibition of cholesterol absorption in small intestinal BBMV. We conclude from the data presented that, contrary to the general belief prevailing in the field of lipid digestion and absorption, long-chain cholesteryl esters may be taken up by the brush border membrane as such and need not be hydrolyzed prior to absorption. The actual contribution of this mechanism to the total absorption of long-chain cholesteryl esters is probably limited by the low solubility of these compounds in mixed bile salt micelles and lipid vesicles.