Meta-Biol

• Pathways

Mutations that disrupt the catalytic activity or strongly interfere with proper folding, glycosylation and transport of SI (sucrase-isomaltase) block the cleavage of maltose (Mal) to glucose in the gut lumen. Affected individuals can develop severe diarrhea; this symptom is managed by excluding indigestible sugars from the diet (Gray et al. 1976; Ritz et al. 2003; Semenza et al. 1965). A variety of SI mutant alleles have been described. Three missense mutations that are associated with severe loss of SI activity in vivo are annotated here (Alfalah et al. 2009;,Ouwendijk et al. 1996; Sander et al. 2006; Spodsberg et al. 2001). All missense mutant alleles that have been characterized to date encode proteins that fail to reach the lumenal plasma membrane or cannot associate stably with it; one missense mutation not annotated here encodes a polypeptide that undergoes an intracellular cleavage and is secreted as the active enzyme (Jacob et al. 2000).