Molecular Pathophysiology of Small-intestinal Sucrase—Isomaltase

Biosynthesis and transport of plasma membrane glycoproteins in the rat intestinal epithelial cell: studies with sucrase-isomaltase

Sucrase-isomaltase (SI), an integral heterodimeric glycoprotein of the intestinal microvillus membrane, is synthesized as a single enzymically active precursor protein (pro-SI) of high relative molecular mass. After glycosylation in the Golgi complex pro-SI is transferred to the microvillus membrane where it is cleaved into the two subunits by pancreatic elastase. Pro-SI was purified by monoclonal antibody-affinity chromatography from microvillus membranes of fetal intestinal transplants in which SI is found exclusively in the non-cleaved precursor form. The N-terminal amino acid sequence of pro-SI was identical to that of the isomaltase subunit of SI which anchors the mature enzyme complex to the lipid bilayer, but it differed from the N-terminal sequence of the sucrase subunit of SI. This structural comparison indirectly gave insight into the mechanisms of membrane insertion and assembly of pro-SI during its biosynthesis. Subcellular fractionation studies indicate transient structural association of newly synthesized pro-SI with the basolateral membrane on its transfer from the Golgi complex to the microvillus membrane, suggesting that part of the basolateral membrane or its associated structures might be involved in the sorting-out processes of microvillar membrane proteins. This concept may have general relevance for the mechanisms of membrane insertion, intracellular transport and sorting of other microvillar membrane glycoproteins in the intestinal epithelial cell.

Molecular cloning of sucrase-isomaltase cDNA in the house musk shrew Suncus murinus and identification of a mutation responsible for isolated sucrase deficiency

Isolated sucrase deficiency has been demonstrated in a line of house musk shrew, Suncus murinus (laboratory name: suncus). This animal belongs to the order Insectivore and is phylogenetically different from ordinarily used laboratory animals. They are believed to have evolved with mainly animal food without sucrose. To study the molecular basis of the sucrase deficiency in suncus, we cloned 6. 0-kilobase (kb) sucrase-isomaltase (SI, EC 3.2.1.48-10) cDNA from suncus intestinal cDNA library. The cDNA clone contained a 5442-base pair (bp)-long open reading frame preceded by an in frame termination codon. The deduced 1813-amino acid sequence showed 68.6, 71.2, and 74.7% similarity with those of rat, rabbit, and human, respectively. A cleavage site between isomaltase and sucrase as well as the region surrounding the catalytic sites for sucrase and isomaltase were conserved among the species. Out of 18 potential N-linked glycosylation sites, 5 were common among all 4 species. In the connecting segment which was enriched with O-linked glycosylation sites in the other species, only two sites were present in suncus. Northern blot analysis revealed that the 6.0-kb SI mRNA was expressed in the KAT line with intact sucrase-isomaltase activity. In contrast, 3.0-kb SI mRNA was expressed in suncus of the MI line with isolated sucrase deficiency. The 3.0-kb mRNA cosegregated with sucrase deficiency phenotype as an autosomal recessive trait. Sequence analysis revealed a 2-nucleotide deletion at position 2767-2768, which results in a frameshift and an immature termination codon. The cDNA of the MI line diverged from that of the KAT line at position 2865, having an 18-bp unique sequence followed by a poly(A) tail. The mutant cDNA encodes 922 amino acid residues which preserves the region for isomaltase but lacks that for whole sucrase. While the cells transfected with the plasmids expressing SI in the KAT line showed both sucrase and isomaltase activity, the plasmids expressing MI line cDNA showed only isomaltase activity. Thus it was concluded that the mutation in the SI gene was responsible for isolated sucrase deficiency in the MI line.

Synthesis and intracellular processing of sucrase-isomaltase in rat jejunum

To examine directly the synthesis and intracellular processing of rat intestinal sucrase-isomaltase, [3H]leucine-labeled enzyme was solubilized from purified microsomal and microvillus fractions, immunoprecipitated, and analyzed. Immunologic identity between the microsomal and microvillus forms of the enzyme was demonstrated. Time-course studies using both intravenous and intraluminal labeling demonstrated an initial peak of microsomal sucrase-isomaltase labeling at 30 min followed by a sharp decline, and a subsequent rise in microvillus sucrase-isomaltase labeling, suggesting a precursor-product relationship. Fluorography and quantitative analysis of electrophoresed immunoprecipitates confirmed these results, and revealed the presence of two high molecular weight proteins in the microsomes. Susceptibility of the smaller high molecular weight precursor to cleavage with endo-beta-N-acetyl-glucosaminidase H indicated that this was the initially synthesized form of the glycoprotein. These data are consistent with the membrane flow hypothesis and the single polypeptide model suggested for sucrase-isomaltase synthesis.

Influence of a dorsomedial hypothalamus lesion on the circadian changes in the enzyme development and activity of intestinal brushborder membranes in the rat

Circadian changes in activities and electrophoretic pattern of the intestinal brushborder enzymes alkaline phosphatase (EC 3.1.3.1), isomaltase (EC 3.2.1.10) and sucrase (EC 3.2.1.26) of normal rats and of rats with a lesion of the dorsomedial hypothalamus (DMH-rats) were studied. In contrast to the rhythm found in normal rats, there are no significant differences between the average enzyme activities in the light or dark period for the DMH-rats. The peak ratio in the electrophoretic pattern for sucrase-isomaltase (SI) over pro-sucrase-isomaltase (pro-SI) changes over 24 hr with higher values during the active (dark) period of the normal rat. In DMH-rats this ratio is much higher, but no significant difference could be demonstrated between the values near the acrophase in these rats.

Sucrase-isomaltase: a stalked intrinsic protein of the brush border membrane

The isolation and purification of sucrase-isomaltase from brush border membrane is described and the physicochemical properties of the pure enzyme are discussed. Our present understanding of the mode of association of the intrinsic membrane protein sucrase-isomaltase with the brush border membrane will be the central point of this contribution. The assembly of sucrase-isomaltase into phospholipid bilayers has been reported to result in a model membrane system which resembles the "native" brush border membrane as regards the mode of lipid-protein interaction. The physicochemical properties of this reconstituted model membrane will be compared to the in vivo situation as represented by brush border membrane vesicles routinely isolated from small intestinal brush borders. The biosynthetic mechanism will be discussed.