High molecular weight pro-sucrase-isomaltase in human fetal intestine

Sucrase-isomaltase (SI) was isolated by immunoprecipitation from the intestines of human fetuses of 16-20 wk of gestational age as one large polypeptide (pro-SI) with a molecular weight of 260,000. Treatment with elastase converted the pro-SI into two polypeptides of apparent molecular weights as sucrase and isomaltase from normal adult intestine (M sub(r) 140,000 and 150,000, respectively). The pro-SI had full enzymatic activity. The results indicate that human sucrase-isomaltase is synthesized as a single chain and that the pancreatic proteases play a significant role in the final processing of the molecule. The pro-SI is proposed to be present also in patients suffering from pancreatic insufficiency.