Determination of binding affinity of poly-γ-glutamate to Shiga toxin

Abstract

We examined poly-γ-glutamate (PGA) from natto, a Japanese fermented food, in the ability to adsorb Shiga toxin 1 (Stx1) and Shiga toxin 2 (Stx2). The polymer was immobilized by direct coupling to EAH–SepharoseTM. The PGA–Sepharose (about 10 mg of ligand/mL of gel) adsorbed Stx2, but not Stx1: its dissociation constant (Kd) against Stx2 was calculated to be 14.0 μM. To analyze the binding site of PGA against Stx2, we similarly immobilized glutamate and glutarate. Glutamate– and glutarate–Sepharoses (each 7 μmol of ligand/mL of gel) similarly adsorbed Stx2, but not Stx1; Kd values against Stx2 were calculated to be 14.0 and 30.0 μM, respectively. The common structures of PGA–, glutamate–, and glutarate–Sepharoses were considered to be glutaryl groups. When we added the mixture of Stx2 and PGA–Sepharose to Caco-2 cells (a human colon epithelial cell line), PGA–Sepharose was found to reduce the cytotoxicity of Stx2. Practical applications Poly-γ-glutamate (PGA) is a component of a traditional Japanese food, and is believed to be absolutely safe. Present study revealed that immobilized PGA adsorbed Stx2 produced by enterohemorrhagic Escherichia coli. To our knowledge, this is the first report on adsorbents specific to Stx2. Our results are probably useful for development of new functional foods with the ability to adsorb Stx2.