4. Patients with Hunter\'s syndrome or Hurler\'s syndrome rarely live beyond the

Question

4. Patients with Hunter's syndrome or Hurler's syndrome rarely live beyond their teens. Analysis indicates that patients accumulate glycosaminoglycans (GAGs) in lysosomes due to the lack of specific lysosomal enzymes necessary for their degradation. When cells from patients with the two syndromes are fused, GAGs are degraded properly, indicating that the cells are missing different degradative enzymes. Even if the cells are just cultured together, they still correct each others defects. Most surprising of all, the medium from a culture of Hurler's cells corrects the defect in Hunter's cells (and vice versa). The corrective factors in the media are inactivated by treatment with proteases (which destroys proteins), by treatment with periodate (which destroys carbohydrates), and by treatment with alkaline phosphatase which removes phosphate groups). a. What do you think the corrective factors are, and how do you think they correct the lysosomal defects? b. Why do you think the treatments with protease, periodate, and alkaline phosphatase inactivate the corrective factors?

Explanation / Answer

Answer a) Lysosomal enzymes.

The process of protein sorting is not perfect. Instead of always being sent to the lysosome, sometimes the enzymes are exported from the cell by constitutive secretion

Answer b) Yes. The periodate and phosphatase treatments inactivate the corrective factors, so the secreted lysosomal proteins only correct the other cell's defects when they possess their original sugar and phosphate configurations. This must be the M-6-P tag that normally directs a protein to the lysosme. Some of the M-6-P receptors must also be missorted to the plasma membrane where they can bind to the secreted enzymes A or B and cause their endocytosis. Then the endocytic pathway carries them ultimately to the lysosome

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