Papain, either modified with polyethylene glycol (PEG) or in the form of a suspended powder, was used to catalyze the digestion of protein substrates in organic solvents to gain insights into protein-protein interactions in media of this kind. Bovine serum albumin was efficiently hydrolyzed in toluene containing 1-2% added water (v/v). The extent of proteolysis, as estimated by amino group determination, depended on the water concentration and, under the optimal condition, was 20% higher than that obtained in aqueous media using the same enzyme/substrate ratio. The highest rate of hydrolysis was obtained in the most hydrophobic solvent, but the attachment of polyethylene glycol chains to the enzyme did not enhance the catalysis. Proteolysis in organic solvents can be said to constitute an interesting approach to the enzymatic digestion of proteins and amino acid incorporation involved in aminolysis. In the case of the hydrophobic protein zein, more than 10% of the peptide bonds were hydrolyzed by PEG-papain in toluene, and in the presence of a potent nucleophile such as Lys-OMe or Lys-NH2 the transpeptidation reaction resulted in an increase in the lysine content of zein from 0. 15% to 3.5% and 10.4%, respectively. The use of organic media therefore provides a potentially useful approach to the enzymatic modification of food protein.
