Sucrase Isomaltase (SI)

Congenital sucrase-isomaltase deficiency is an example of a disease in which mutation results in transport-incompetent molecules. sucrase-isomaltase is not transported to the brush border membrane but accumulates as a mannose-rich precursor in the endoplasmic reticulum, ER-Golgi intermediate compartment, and the cis-Golgi, where it is finally degraded.

The enzyme consists of 2 subunits which are synthesized as a single chain mannose-rich precursor. From studies of the cDNA, the human protein has 83% identity with the rabbit enzyme. In addition to the previously reported homology with lysosomal alpha-glucosidase, the sucrase and isomaltase subunits appear to be homologous to a yeast glucoamylase.