High Molecular Weight Pro-Sucrase-Isomaltase in Human Fetal Intestine

Abstract

SummarySucrase-isomaltase (SI) was isolated by immunoprecipitation from the intestines of human fetuses of [16][17][18][19][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. 140,000 and 150,000, respectively). The pro-SI had full enzymatic activity. SpeculationThe 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.Sucrase-isomaltase (SI) is a digestive enzyme responsible for all the sucrase activity, nearly all the isomaltase activity, and for most of the maltase activity (-70%) of the small intestine. SI is localized with its major part exposed to the luminal content of the intestine and anchored to the small intestinal brush border membrane by a small hydrophobic segment [for review see (12)]. In the majority of mammals including man, SI occurs as an enzyme complex built up of two polypeptides one of which has sucrase the other isomaltase activity.By observation on pigs whose pancreas has been totally disconnected it has been shown that SI is synthesized as a single-chain, assigned pro-SI (13). Treatment of this pro-SI with pancreatic proteases splits it into two polypeptides of apparent molecular sizes similar to those of the "final" SI (13), suggesting that the final step in the processing of SI occurs at the luminal surface of the intestine by action of the pancreatic enzymes.In the developing human fetus zymogen granules have been detected in the pancreas as early as the 5th month of gestation [for review see (9)]; however, the intestinal enterokinase, which is the activator for trypsinogen, does not appear until the 26-30 wk of gestation (I). This implies that the fetal pancreas does not have any proteolytic activity until this time. Fetal intestines before 26 wk of gestational age therefore provide a unique model to investigate if the biosynthesis of the human SI is likely to proceed via a large pro-SI and the possible role of pancreatic proteases in the processing of the SI. MATERIALS AND METHODSAnti-human-SI-IgG was obtained against normal, adult SI as previously described (14).Two human fetuses of 16-20 wk of gestational age were obtained after abortus provocatus. Their full length intestines were immediately cut out and frozen at minus 80°C until use. As control material small jejunum pieces of normal adult intestine were used (15).Solubilization of the brush border proteins. The proximal part of each fetal intestine (approximately 0.3 g) was homogenized in crushed ice in 4 volumes of a 50 mM potassium phosphate buffer, pH 6.0, containing 1% Triton X-100 and aprotinin 28 p,g X ml-The homogenate was incubated at 4°C for I h and subsequently centrifuged 50,000 X g for I h at 4°C. The sup...

Maturation of Sucrase-Isomaltase Complex in Human Fetal Small and Large Intestine during Gestation

“…They are in good accordance with previous reported values for fetal intestine (5,8,15). For the 1 5 , 18-, and 24-wk samples, the results were obtained from homogenate of the whole intestinal mucosa, and we observed a progressive rise in activity between 15 and 24 wk; at 24 wk the activity was near adults values.…”

“…allows the activation of pancreatic proteases into their mature form, and pancreatic proteases have been shown to be responsible for sucrase-isomaltase cleavage (3,7,8).…”

“…Recently it has been shown in mammals to be synthetized as a single chain secondarily split to two subunits by pancreatic proteases (3,7,13). In early human fetal small intestine the enzyme is present as high molecular weight single chain (8), which is split in vitro by pancreatic elastase. This enzyme also is expressed at a lower level in human fetal colonic mucosa between 10 and 30 wk.…”

Fetal Forms of Oligoaminopeptidase, Dipeptidylaminopeptidase IV, and Sucrase in Human Intestine and Meconium

“…This may be due to altered activity of the enzymes involved in the intracellular processing of the carbohydrate moiety of these glycoproteins. Age-dependent variations of posttranslational modifications of brush border enzymes may also occur in the brush border itself, brought about by age-dependent variations of luminal agents capable of modifying intestinal glycoproteins such as bacterial glycosidases (31) or pancreatic enzymes (32).…”

Developmental pattern of calmodulin-binding proteins in rat jejunal epithelial cells

“…In other words, the 110K subunit might be synthesized in undifferentiated cells as a high-molecular-mass form which cleaves into a 110 000-dalton polypeptide during differentiation. Despite the immaturity of the pancreas in 18-to 19-day fetuses [21], a role for pancreatic proteases would not be excluded as already described in the case of brush border hydrolases [20,45,461. As a third hypothesis, it might be proposed that the 110K subunit is an artefactual Note that, in 15-day fetuses, for caldesmon and fodrin a positive reaction was also seen in the smooth muscle anlage of the intestinal wall (a, d).…”