ISOLATION AND CHARACTERIZATION OF SURFACTANT-LIKE PARTICLES IN RAT AND HUMAN COLON

The source of a phospholipid-rich layer recovered from the surface of the mammalian colon has been obscure. This report describes the isolat ion of a low-density membrane from the surface of rat and human colons (d = 1.07-1.08 g/ml), with a low cholesterol-to-phospholipid ratio an d phosphatidylcholine as its major phospholipid. Electron microscopy s hows unilamellar and partially coiled membranes. Compared with microvi llous membranes isolated from underlying mucosa, this extracellular me mbrane is enriched for tissue-unspecific alkaline phosphatase and surf actant protein A. It does not contain small intestinal marker proteins (intestinal alkaline phosphatase and sucrase-isomaltase). The human m embrane contains only traces of the colonic microvillous membrane mark er, carcinoembryonic antigen. Antiserum against the rat colonic membra ne does not recognize colonic microvillous membrane or small intestina l surfactant-like particle proteins. Antiserum against human colonic m embrane identifies one protein in the surfactant-like particle from th e adjacent small intestine and two proteins in the colonic microvillou s membrane. These data show that the colonocyte microvillous membrane is covered by another membrane with a different protein composition. E nrichment for surfactant protein A suggests that this colonic membrane is another example of a surfactant-like particle sharing proteins wit h pulmonary surfactant.