Adsorption of pepsin by aluminum hydroxide I: Adsorption mechanism.

Adsorption of pepsin by gibbsite and boehmite, non-acid-reactive forms of aluminum hydroxide, was observed and related to the surface area of the adsorbent. Adsorption was pH dependent, with maximum adsorption occurring between pH 2.7-3.3 for gibbsite and pH 2.7-4.3 for boehmite. Electrostatic attraction was an important adsorption mechanism at the pH conditions encountered in the GI tract; the isoelectric point of pepsin was approximately 1, giving it a negative charge, and the point of zero charge for the adsorbents was greater than 9, giving them a positive charge. However, the pH-adsorption profile can not be fully explained by electrostatic considerations. Desorption studies indicate the importance of specific adsorption because pepsin was not desorbed by washing with acidified water, but was partly desorbed by exchange with phosphate. The IR spectrum of adsorbed pepsin also suggested that specific adsorption of pepsin occurred through anionic ligand exchange involving carboxylate groups of pepsin and surface aluminum ions.