Method to maintain the activity in polyethylene glycol-modified proteolytic enzymes

A method for preparing protease-polyethylene glycol adducts is presented wherein the immobilized reversible inhibitor, benzamidine, prevents reaction of activated polyethylene glycol with the active site of the protease. Improved activity against macromolecular substrates is obtained compared to when the benzamidine is in solution during the conjugation reaction.