Only sphingolipid activator protein B (SAP‐B or saposin B) stimulates the degradation of globotriaosylceramide by recombinant human lysosomal α‐galactosidase in a detergent‐free liposomal system

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Abstract
The degradation of globotriaosylceramide (GbOse3Cer) by insect-cell derived recombinant human α-galactosidase (EC 3.2.1.22) was carried out in a detergent-free liposomal system in order to mimic intralysosomal conditions. GbOse3Cer incorporated into unilamellar liposomes was used as the substrate, and naturally occurring sphingolipid activator proteins, rather than detergents, were used to stimulate the enzyme reaction. The degradation of GbOse3Cer was dependent on the presence of both α-galactosidase and sphingolipid activator protein B (SAP-B or saposin B). It proceeded optimally at pH 4.6, and was enhanced by increasing amounts of both α-galactosidase (0.24–24 mU/50 μl assay) and SAP-B (0–5 μg/50 μl assay). The enzyme reaction was not affected by SAP-A, SAP-C, or SAP-D. Therefore, our results indicate that only SAP-B is essential for the degradation of GbOse3Cer by α-galactosidase.

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