sucraseisomaltase

Sucrase-isomaltase is a brush-border disaccharidase complex of the small intestine that catalyzes the hydrolysis of dietary sugars. The SI gene, located on chromosome 3, encodes a single type II transmembrane glycoprotein that is post-translationally cleaved into two catalytic subunits, sucrase and isomaltase, both anchored to the apical membrane of enterocytes.

The enzyme is predominantly expressed on the microvillar surface of enterocytes in the small intestine, with activity concentrated in the jejunum and ileum. Its two catalytic domains provide distinct but overlapping activities: sucrase hydrolyzes sucrose into glucose and fructose, while isomaltase cleaves alpha-1,6 linkages in isomaltose and in limit dextrins derived from starch.

Together with other brush-border disaccharidases, sucrase-isomaltase plays a key role in dietary carbohydrate digestion and the absorption of monosaccharides. Activity can be measured in intestinal mucosa or inferred from in vivo breath tests and genetic analysis in certain contexts.

Clinical relevance centers on congenital sucrase-isomaltase deficiency (CSID), a disorder caused by mutations in SI. CSID leads to impaired digestion of sucrose and isomaltose-containing carbohydrates, resulting in osmotic diarrhea, bloating, abdominal pain, and poor weight gain in infancy or childhood. Diagnosis combines enzymatic assays, genetic testing, and clinical presentation. Management includes dietary restriction of sucrose and starch-containing foods and, where appropriate, enzyme replacement therapy with sacrosidase (sucrase-isomaltase) to supplement intestinal digestion. CSID is typically inherited in an autosomal recessive pattern, with multiple mutations described in the SI gene.