Monoclonal antibodies to sucrase/isomaltase: probes for the study of postnatal development and biogenesis of the intestinal microvillus membrane.

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Abstract
Two monoclonal antibodies, designated BB 3/34/12 and BB 5/8/40/90, were produced to rat intestinal sucrase/isomaltase (SI) by the hybridoma [mouse spleen cells with PS-NSI/1-AG4-1 mouse myeloma cells] technique using microvillus membranes as antigen. The BB 3/34/12 antibody was specific for the sucrase subunit. These antibodies provided new information regarding the biosynthesis and postnatal development of SI. In rat intestinal fetal transplants, SI was found exclusively in the form of an enzymatically active high molecular weight precursor, confirming previous observations concerning the role of luminal proteases for the processing of SI in the microvillus membrane. The SI precursor, purified by affinity chromatography using the BB 3/34/12 antibody, had both sucrase and isomaltase activities, suggesting that a single precursor protein generates both sucrase and isomaltase subunits by proteolytic cleavage. The initial appearance of SI during normal postnatal development in the rat intestine was confined to the cells present at the base of the villi. The same localization was observed after precocious induction of SI by cortisone acetate. In both cases, no immunofluorescence was observed in the crypts, suggesting that only the differentiated enterocyte is capable of synthesizing this enzyme. Even at the earliest times of appearance, newly synthesized SI was found almost completely split into its subunits, suggesting that the protease(s) responsible for the processing of the precursor in the microvillus membrane develop(s) in parallel with SI or earlier.