J.A.M. Fransen

alpha-Glucosidase deficiency (Pompe's disease).

alpha-Glucosidase is deficient (less than 30% of control) in Pompes disease, but the extent of the deficiency does not always correlate with the severity of the clinical symptoms. The defects that lead to a deficiency of alpha-glucosidase include synthesis of catalytically inactive protein, absence of mRNA for the enzyme, decreased synthesis of the precursor, lack of phosphorylation of the precursor, impaired conversion of the precursor to the mature enzyme and synthesis of unstable precursor. A single type of defect can lead to different clinical phenotypes. The precursor of alpha-glucosidase is present in the brush border of the polarized epithelial cells of small intestine and kidney and is secreted into urine.

Intracellular Transport of Brush-Border and Lysosomal Enzymes in the Human Enterocyte

The mechanisms underlying the biosynthesis, intracellular transport, and sorting of proteins destined for extra- and intracellular use have been extensively studied in polarized epithelial cells during the last few years. Much of our understanding about these mechanisms arose from studies on the intracellular pathways taken by newly synthesized viral membrane glycoproteins in cultured epithelial cells (K. Simons and Fuller 1985, N.L. Simons et al. 1985). However, little information is available about the synthesis and transport of endogenous glycoproteins (Danielsen et al. 1984). To study these processes the intestinal epithelial cell (enterocyte) is an attractive model for a number of reasons.