Enzyme Distribution in Subcellular Fractions of Intestinal Mucosal Cells

Mucosal cells of rabbit small intestine were homogenized; in 0.3M mannitol and fractionated by differential centrifugation. Sucrase (EC 3.2.1.26) and alkaline phosphatase (EC 3.1.3.1) were recovered mainly in the “nuclear” fraction, though considerable activities of these enzymes were also detected in the other particulate fractions. The similarity in distribution pattern indicated that the two enzymes are located in the same subcellular organelle. Electron-transfer enzymes and hemoproteins were assayed in the mitochondrial and microsomal fractions, and it was found that the microsomal fraction contained cytochrome b5 and a CO-binding hemoprotein resembling hepatic P-450. On subfractionation of the microsomal fraction by sucrose density gradient centrifugation, it was revealed that the sucrase activity of this fraction was associated with a structure which was different from typical microsomal vesicles and carried many small particles on its surface. The possibility is discussed that this structure was small fragments of microvilli.