The composition and biosynthesis of the glycoproteins and glycolipids of the rabbit small-intestinal brush border.

1. The glycoprotein and glycolipid composition of isolated rabbit small-intestinal brush borders has been studied. 2. The total glycoprotein fraction contains an average 95 microgram carbohydrate per mg protein, composed of mannose, galactose, fucose, N-acetylglucosamine and N-acetylgalactosamine. Glucose is also present but sialic acid is absent. 3. The isolated glycolipids include ceramide lactoside, ceramide trihexoside and two N-acetylglucosamine-containing glycolipids. Sialic acid containing glycolipid (gangliosides) is present only in trace quantities. 4. The biosynthesis of the brush border-bound glycoproteins and glycolipids has been studied following intraperitoneal injection with D-[1-14C]glucosamine and isolation of the brush borders at intervals between 3 and 24 h. 5. The total glycoprotein fraction labels maximally 7.5 h after injection and subsequently exhibits an exponential loss of radioactivity with a half-life of 11.2 h. The labelling kinetics of one of the glucosamine-containing glycolipids is similar to that of the glycoproteins in that it labels maximally between 7.5 and 12 h, but the second glucosamine-containing glycolipid labels later at approximately 18 h. These results indicate that the glycoproteins and glycolipids are actively synthesized and degraded within the mature small intestinal enterocyte and that individual glycolipids turn over independently.