This project will continue to examine in detail the regulation of intestinal sucrase-isomaltase synthesis and its intracellular processing. This glycoprotein enzyme will also serve as a marker for the insertion of macromolecules into the microvillous membrane. Since surface remodeling is an important feature of intestinal development, ontogenic control of sucrase-isomaltase synthesis and processing will also be studied. Ihtense focus on this enzyme is now possible because of two major advances in this laboratory in the last two years of this grant; namely, the preparation of highly specific antibody against the enzyme which permits immunochemical analysis, and, secondly, the identification of an intracellular (microsomal) phase in the synthesis of sucrase-isomaltase. The studies described in this application are designed to characterize translational modification of the enzyme if such occur. Synthesis and subcellular distribution of sucrase-isomaltase will be investigated in the rat and rabbit under basal conditions, after the adminstration of agents which inhibit different steps in the processing of the enzyme, namely actinomycin, colchicine and tunicamycin, and after excess carbohydrate feeding, fasting, and pancreatic and/or biliary diversion. Evidence for an enzymatically inactive sucrase precursor protein ("pro-sucrase") in crypt cells will be sort using immunochemical techniques and an attempt may be made to isolate nascent sucrase using cell-free synthesis techniques. Mechanisms controlling differentiation of sucrase-isomaltase will be studied in developing animals and in a fetal intestinal transplant system developed in this laboratory. Experiments in animals will serve as the basis for investigation of synthesis and intracellular processing of sucrase-isomaltase in human diseases known to be accompained by disaccharidase deficiency (e.g., sucrase-isomaltase deficiency, celiac sprue, chronic enteritis of infancy). Information may thus be obtained concerning reconstitution of the human microvillus membrane after intestinal injury.