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Saposin A: second cerebrosidase activator protein.
S Morimoto1, B M Martin, Y Yamamoto
1Department of Neurosciences, University of California, San Diego, School of Medicine, La Jolla 92093.
Summary
Saposin A, derived from prosaposin, enhances the activity of specific enzymes involved in sphingolipid breakdown, similar to saposin C. Its structure, including glycosylation and disulfide bridges, is crucial for this function.
Area of Science:
- Biochemistry
- Molecular Biology
- Glycosphingolipid Metabolism
Background:
- Saposin A is a glycoprotein isolated from Gaucher disease spleen.
- It is derived from the proteolytic processing of prosaposin, which also yields saposins B, C, and D.
Purpose of the Study:
- To characterize saposin A's biochemical properties and its role in enzyme activation.
- To investigate the structural basis for saposin A's function.
Main Methods:
- Protein isolation and purification from Gaucher disease spleen.
- Chemical sequencing and peptide mapping using Staphylococcus aureus V-8 protease.
- Enzyme activity assays with various substrates and hydrolases.
- Deglycosylation, reduction, and alkylation experiments.
- Three-dimensional modeling and sequence homology analysis.
Main Results:
- Saposin A stimulates beta-glucosylceramidase and beta-galactosylceramidase activity, similar to saposin C.
- Activation is primarily achieved by increasing the maximal reaction velocity.
- Deglycosylation did not abolish saposin A activity, but reduction and alkylation did.
- Structural analysis revealed significant homology with saposin C, including conserved residues and a hydrophobic pocket.
Conclusions:
- Saposin A is a functional activator of specific sphingolipid hydrolases.
- Its activity is dependent on its three-dimensional structure, particularly disulfide bridges.
- Saposin A and C share structural and functional similarities, suggesting a common mechanism of action.