The effects of chemically synthesized saposin C on glucosylceramide-β-glucosidase

Objective: Saposin C (SAP-C) is an essential activator of glucosylceramide (GlcCer)-beta-glucosidase (GCase), the enzyme deficient in Gaucher's disease. In this study, we investigated the effects of chemically synthesized SAP-Cs (synthetic SAP-Cs) on GCase. Methods: Enzymatic assays and western blot analyses were carried out to evaluate the effects of two kinds of synthetic SAP-Cs, a non-glycosylated form and a N-glycosylated form bearing a complex type nonasaccharide, on GCase with respect to its activation, stabilization, and protection. Imiglucerase (Cerezyme) was used as the GCase. To mimic physiological conditions, GCase activity was assayed in the presence of 4-nitrobenzo-2-oxa-1,3-diazole-labeled GlcCer-containing liposomes composed of bis(monoacylglycero)phos.phate, L-alpha-phosphatidylcholine, and cholesterol. Results: GCase activities increased depending on the concentration of synthetic SAP-Cs. SAP-Cs at a concentration of 1 mu M increased GCase activities significantly, by 14- to 22-fold (non-glycosylated SAP-C: 22.9 +/- 0.16; nona-glycosylated SAP-C: 14.9 +/- 0.19; without SAP-C: 1.05 +/- 0.035 pmol/h/ng GCase). These values equaled or surpassed previously published values obtained using recombinant non-glycosylated SAP-C. Both synthetic SAP-Cs were as effective as bovine serum albumin in stabilizing GCase at 37 C Western blot analysis revealed that synthetic SAP-Cs specifically protected GCase from cathepsin D digestion. Conclusions: The results demonstrate that these novel, chemically synthesized SAP-Cs function as activators, stabilizers, and protectors of GCase, suggesting their utility in enzyme replacement therapy in patients with Gaucher's disease. (C) 2015 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.