I-cell disease, also known as mucolipidosis (type II), shares similarities with mucopolysaccharidoses and sphingolipidoses. Unfortunately, there is currently no cure for this condition, and it often leads to cardiopulmonary complications and death in early childhood.

When it comes to the degradation of glycoproteins, the process is similar to that of glycosaminoglycans (GAGs). Lysosomal acid hydrolases play a crucial role in breaking down glycoproteins, with each enzyme specializing in removing a specific component of the glycoprotein. These enzymes work as exoenzymes, removing their respective groups in the reverse order of their incorporation (last on, first off). If any one of these degradative enzymes is missing, the degradation process cannot proceed. In rare autosomal-recessive diseases known as glycoprotein storage diseases (oligosaccharidoses), which result from a deficiency in any of the degradative enzymes, there is an accumulation of partially degraded structures in the lysosomes. For instance, α-mannosidosis type 3 is a severe and fatal deficiency of the enzyme α-mannosidase. Its presentation is similar to Hurler syndrome, but immune deficiency is also observed. The urine of affected individuals contains mannose-rich oligosaccharide fragments. Diagnosis is typically made through an enzyme assay.

In summary, I-cell disease falls under the category of mucolipidosis (type II) due to its similarities with mucopolysaccharidoses and sphingolipidoses. Glycoprotein degradation follows a similar process to glycosaminoglycan degradation. Lysosomal acid hydrolases, acting as exoenzymes, remove specific components of glycoproteins in reverse order. Deficiencies in these degradative enzymes lead to glycoprotein storage diseases (oligosaccharidoses) where partially degraded structures accumulate in lysosomes. α-mannosidosis type 3 is an example of such a disease, characterized by a severe deficiency in α-mannosidase enzyme and presenting with similarities to Hurler syndrome, along with immune deficiency. Mannose-rich oligosaccharide fragments can be detected in the urine, and diagnosis is typically confirmed through an enzyme assay.
Because I-cell disease has features in common with the mucopolysaccharidoses and sphingolipidoses (see p. 210), it is termed a mucolipidosis (type II).] Currently, there is no cure, and death from cardiopulmonary complications usually occurs in early childhood.
IX. LYSOSOMAL GLYCOPROTEIN DEGRADATION
Degradation of glycoproteins is similar to that of the GAG (see p. 163). The lysosomal acid hydrolases are each generally specific for the removal of one component of the glycoprotein. They are primarily exoenzymes that remove their respective groups in the reverse order of their incorporation (last on, first off). If any one degradative enzyme is missing, degradation by the other exoenzymes cannot continue. A group of very rare autosomal-recessive diseases called the glycoprotein storage diseases (oligosaccharidoses), caused by a deficiency of any one of the degradative enzymes, results in accumulation of partially degraded structures in the lysosomes. For example, α-mannosidosis type 3 is a severe, progressive, fatal deficiency of the enzyme α-mannosidase. Presentation is similar to Hurler syndrome, but immune deficiency is also seen. Mannose-rich oligosaccharide fragments appear in the urine. Diagnosis is by enzyme assay.
X. CHAPTER SUMMARY