Catalytically inactive sucrase antigen of rabbit small intestine: the enzyme precursor.

By immunofluorescence microscopy using a specific antibody to the active rabbit sucrase-isomaltase complex, a catalytically inactive sucrase antigen was discovered on the small intestinal mucosa of young rabbits still lacking sucrase activity. In adult rabbits, the same antigen was demonstrated on enterocytes of mucosal crypts devoid of sucrase. Catalytically inactive antigen was isolated by means of immobilized antibody to active sucrase, and it was compared with the active sucrase-isomaltase complex. From structural similarities between the 2 proteins and from the fact that active sucrase succeeds the inactive antigen in both the maturing and the mature rabbit, it is concluded that the inactive antigen is the enzyme precursor. In some patients with hereditary sucrase-isomaltase deficiency, abnormal persistence of sucrase precurosr due to a faulty activation mechanism may be the underlying defect.