Liposome-Papain Conjugates for Catalytic Digestion of Antibody Producing Fab Fragments

Papain is useful for the enzymatic digestion of various proteins to produce functional peptides or protein fragments. Immobilized papain being reactive toward proteins and easily removable from a reaction mixture is worth developed. In the present work, liposomes were applied as colloidal carriers of papain for the catalytic digestion of polyclonal immunoglobulin G (IgG). Papain was covalently conjugated at pH = 7.0 via tris-succinimidyl aminotriacetate (TSAT) to liposomes incorporated with 5 mol % poly(ethylene glycol)-tethered lipid with a reactive amino group. The papain-conjugated liposome (liposome-papain) catalyzed the hydrolysis of N_α-benzoyl-l-arginine 4-nitroanilide hydrochloride (BAPNA) at pH = 5.0-7.0. The activity of liposome-papain significantly increased with increasing temperature from 25 to 50 °C. The Michaelis constant K_m was determined with respect to the liposome-papain- and free papain-catalyzed reactions with BAPNA at 37 °C as K_m = 1.11 ± 0.13 and 11.6 ± 2.9 mM, respectively. Liposome-papain was applied to the catalytic digestion of 10 mg·mL^-1 IgG at 37 °C for 24 h at pH = 5.0-7.0. The reaction mixture could be analyzed without pretreatment by using the affinity columns immobilized with the protein A or protein L ligand because colloidal liposome-papain quickly flowed through the chromatographic stationary phase, exhibiting little proteolytic effect on the proteinaceous ligands. The analysis clearly demonstrated the catalytic production of antigen-binding fragments (Fab) from IgG in an enzyme concentration- and pH-dependent manner. Liposome-papain with 15 or 50 mol % anionic lipids also catalyzed the formation of Fab from IgG. The above results demonstrated that liposome-papain was useful to digest IgG and to purify Fab formed with the affinity chromatography.