The sphingolipid activator protein, saposin C (also termed SAP 2), was chemically synthesized, purified, and characterized. The fully protected 82-residue protein was synthesized by automated solid-phase methods, with multiple recoupling steps resulting in a high average coupling efficiency of 98.8%. The overall yield was estimated to be approx 40%. Deprotection and cleavage of the custom peptide synthesis from the resin was followed by folding in the absence of chaotropic agents at pH 8.5.
The protein was purified by reversed-phase high pressure liquid chromatography (HPLC) and its purity determined by capillary electrophoresis and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The composition of the synthetic saposin C was determined by amino acid analysis. Its sequence was verified by Edman sequence analysis of custom peptide synthesis fragments generated by chymotryptic andStaphylococcus aureus V8 digestions. The sequence at the C-terminus was determined by digestion with carboxypeptidase P, followed by phenylthiohydantoin (PTH) derivitization and HPLC analysis of the released amino acid residues. Deglycosylated native saposin C appeared as a lower molecular-weight species than synthetic saposin C on SDS-PAGE.
This has been explained by amino acid and C-terminal analysis showing native saposin C to be two amino acids shorter at the C terminus than a deduced sequence (from cDNA) previously published. Synthetic saposin C displayed 85% of full biological activity as determined by its ability to stimulate glucocerebrosidase activity in vitro: Synthetic and native saposin C increased glucocerebrosidase catalyzed hydrolysis of 4-methylumbelliferyl β-D-glucoside by factors of 6.0 and 7.1, respectively. Furthermore, synthetic and native saposin C share similar Kact values (0.5 and 1.5 μM respectively) indicating that they bind to glucocerebrosidase with similar