Saposin B-dependent Reconstitution of Arylsulfatase A Activity in Vitro and in Cell Culture Models of Metachromatic Leukodystrophy

J Biol Chem. 2009 Apr 3;284(14):9372-81. doi: 10.1074/jbc.M809457200. Epub 2009 Feb 18.

Abstract

Arylsulfatase A (ASA) catalyzes the intralysosomal desulfation of 3-O-sulfogalactosylceramide (sulfatide) to galactosylceramide. The reaction requires saposin B (Sap B), a non-enzymatic proteinaceous cofactor which presents sulfatide to the catalytic site of ASA. The lack of either ASA or Sap B results in a block of sulfatide degradation, progressive intralysosomal accumulation of sulfatide, and the fatal lysosomal storage disease metachromatic leukodystrophy. We studied the coupled Sap B-ASA reaction in vitro using detergent-free micellar and liposomal assay systems and in vivo using cell culture models of metachromatic leukodystrophy. Under in vitro conditions, the reaction had a narrow pH optimum around pH 4.3 and was inhibited by mono- and divalent cations, phosphate and sulfite. Bis(monoacylglycero) phosphate and phosphatidic acid were activators of the reaction, underscoring a significant role of acidic phosphoglycerolipids in sphingolipid degradation. Desulfation was negligible when Sap B was substituted by Sap A, C, or D. Up to a molar ratio between Sap B and sulfatide of 1:5, an elevation of Sap B concentrations caused a sharp increase of sulfatide hydrolysis, indicating the requirement of unexpected high Sap B levels for maximum turnover. Feeding of ASA-deficient, sulfatide-storing primary mouse kidney cells with ASA caused partial clearance of sulfatide. Co-feeding of Sap B or its precursor prosaposin resulted in the lysosomal uptake of the cofactor but did not promote ASA-catalyzed sulfatide hydrolysis. This suggests that Sap B is not a limiting factor of the coupled Sap B-ASA reaction in mouse kidney cells even if sulfatide has accumulated to unphysiologically high levels.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cells, Cultured
  • Cerebroside-Sulfatase / deficiency
  • Cerebroside-Sulfatase / genetics
  • Cerebroside-Sulfatase / metabolism*
  • Disulfides / metabolism
  • Enzyme Activation
  • Humans
  • Hydrolysis
  • Leukodystrophy, Metachromatic / enzymology*
  • Leukodystrophy, Metachromatic / genetics
  • Lipid Metabolism
  • Liposomes
  • Mice
  • Mice, Knockout
  • Models, Biological*
  • Saposins / metabolism*
  • Substrate Specificity
  • Swine

Substances

  • Disulfides
  • Liposomes
  • Saposins
  • Cerebroside-Sulfatase