Characterization of proteins in rat and human intestinal surfactant-like particles.

Phospholipid-rich particles isolated from the apical surface of rat enterocytes have surfactant-like properties and are enriched for intestinal alkaline phosphatase. Purified intact rat particles were used to produce antibodies in rabbits. Antiserum against the rat particle identified major proteins of 48, 68, 98, and 118 kDa on Western blots of isolated rat surfactant-like particle and did not detect any protein in rat intestinal basolateral membranes, rat brush border membranes, or human particles, but did detect a single 180-kDa protein in a preparation of rat milk fat globules, and two proteins (66 and 103 kDa) in rat pulmonary surfactant. The proteins detected on Western blot corresponded with the major proteins identified by Coomassie blue staining. Similar particles were isolated from the apical surface of human intestine and had an enzyme composition, buoyant density, and lipid content similar to those of the rat particles. Antiserum raised against the human jejunal particle detected proteins in the human particle by Western blot that were similar in size with the rat particle proteins (33, 52, 75, 82, and 118 kDa), but did not cross-react with human brush border or rat particle proteins. These studies demonstrate that the surfactant-like particles are present in human as well as rat intestinal tissue, demonstrate similar enzyme and protein content, and confirm their unique identity, distinct from apical brush border or basolateral membranes.