POLYACRYLONITRILE ENZYME ULTRAFILTRATION MEMBRANES PREPARED BY ADSORPTION, CROSS-LINKING, AND COVALENT BINDING

Amyloglucosidase (AG) and invertase (INV) were immobilized into polyac rylonitrile (PAN) and carboxyl-modified polyacrylonitrile (PAN-AA) ult rafiltration membranes. The effects of membrane pore size and immobili zation method on total enzymatic activity, enzyme distribution across the membrane structure, and long-term stability were investigated. Imm obilization by adsorption did not yield stable enzyme membranes. Enzym e cross-linking with glutaraldehyde and covalent binding after carboxy l activation with water-soluble carbodiimide were proven to be suitabl e methods to yield enzyme membranes with stable activities (about I U cm(-2) AG bound to the high specific surface area, low permeability me mbranes and 0.055 U cm(-2) AG and 2.3 U cm(-2) INV bound to the low su rface, high flux membranes). Covalent immobilization yielded AG UF mem branes with no activity drop within one year. The hydrophilic surface of PAN-AA membranes I-educed unspecific enzyme adsorption. The immobil ization reaction was driven by the reactivity of the activated surface . The accessibility of the membranes and the substrate size (starch vs . maltose for AG) had a major impact on observed activities. UF membra nes with apparently ''symmetric'' enzyme distribution were observed fo r moderate total loadings if sufficient substrate access on both outsi de surfaces is provided. The Michaelis-Menten constant K-m with the sm all substrate maltose were 3.5 mmol(-1). l for free and 3.2 mmol(-1). l for covalently bound AG whereas V-max values with maltose and starch , dropped to 32% and 22%, respectively, for the immobilized versus fre e enzyme. Under ultrafiltration conditions with a transmembrane pressu re of 0.3 MPa, the INV membrane activity rose to 250% compared with te sts where substrate transport occurred by diffusion exclusively. (C) 1 997 by Elsevier Science Inc.