Studies on the Ultrastructural Localization of Intestinal Disaccharidases

It is now firmly established that disaccharidases are an intrinsic part of the mammalian intestinal (1) and renal (2) brush border. Isolated brush borders of hamster and rabbit intestine (3-6) are studded with knobs (65 A in diameter) or doughnut-shaped particles (45-110 A in diameter) respectively, when viewed as negatively stained preparations . Isolated brush borders of rabbit kidney do not have such particles (2) . Treatment of the intestinal brush borders with papain removes the particles and solubilizes the disaccharidases, sucrase and maltase (3-6) . It was concluded that these enzymic activities are localized on the knobs, in the glycocalyx, external to the plasma membrane of the microvillus . Crane (7, 8) has incorporated this view in his schematic models for digestive-absorptive function . Recent investigations on sugar transport systems in kidney and intestine (2, 9) prompted us to question the conclusion that the different disaccharidases of the brush border are localized exclusively on particles external to the microvillus membrane. Moreover, it is possible that the results of the earlier studies (3-6) may have been overinterpreted, since the precise conditions required to remove (a) the enzymic activities and (b) the 60 A particles were not described . For example, the number of units of papain used to treat a particular quantity of brush borders was not specified . Thus, it is not clear whether or not the disaccharidases and the knobs were removed at the same time . The present paper is a correlated kinetic study of the papain treatment of hamster intestinal brush borders . The removal of disaccharidase activites and of the 60 A particles (as determined by electron microscopy) have been monitored simultaneously with time . The results show that the disaccharidases cannot be correlated quantitatively with the number of knobs on the brush border membrane .