Interaction of catalase with montmorillonite homoionic to cations with different hydrophobicity: effect on enzymatic activity and microbial utilization

The exchange sites of montmorillonite (M) were made homoionic to calcium (C a), hexadecyltrimethylammonium (HDTMA) or pyridinium (PY) cations. The clay s were used as adsorbents for the enzyme, catalase (CA). Equilibrium adsorp tion and binding isotherms (i.e., after washing of the clay-CA complexes af ter adsorption at equilibrium until no CA was desorbed) were of the L-3-typ e and fitted the Langmuir equation in the initial: but not in the later, po rtions of the isotherms. The amounts adsorbed and bound at the plateau, as well as the affinity, were higher for the hydrophobic clays (M-HDTMA and M- PY) as indicated by the Langmuir parameter, B-max and K-eq. In all three sy stems, there was additional adsorption after the initial plateau at higher concentrations of CA. In the case of M-Ca-CA this probably resulted from so me penetration of CA into the interlayer spaces of the clay, as shown by X- ray diffraction analysis. No penetration of the interlayers was observed in the M-HDTMA-CA and M-PY-CA systems. The additional adsorption that occurre d after the initial plateau in these systems may have resulted from the for mation of multilayers of CA or from a change in the orientation of CA on th ese clay-organic surfaces, which may also have occurred in the M-Ca-CA syst em in addition to intercalation. Most of the CA adsorbed at equilibrium was bound on the clays (85-90%). Fourier-transform infrared difference spectra showed a shift in the Amide I and Amide II frequencies for only M-Ca-CA an d M-PY-CA, which was consistent with the hypothesis of a conformational mod ification of the structure of CA on M-Ca and M-PY. The enzymatic activity o f CA adsorbed at equilibrium on the three clays was lower than that of free CA and decreased in the order of M-Ca-CA > M-PY-CA > M-HDTMA-CA As shown b y the values of the overall first-order rate constant, K-1, there was a fur ther reduction in activity when CA was bound on the clays, especially on M- Ca. The pH optimum for the activity of CA remained essentially unchanged wh en adsorbed or bound on all clays. CA bound on the clay systems, except on M-PY-CA, was poorly utilized in comparison with the free enzyme as a sole s ource of carbon or nitrogen. (C) 2000 Elsevier Science Ltd. All rights rese rved.