Screening and analysis of edible seaweeds in the ability to adsorb Shiga toxin

We screened edible seaweeds in the ability to adsorb Shiga toxin (Stx) by an equilibrated dialysis method. Although water insoluble fractions of 14 dry seaweeds did not adsorb Stx, most water soluble fractions were found to adsorb it to one degree or another. Among the seaweed tested, the extract of the Ulva linza Linnaeus [ Enteromorpha linza (Linnaeus) J. Agardh] was found to well adsorb both Stx1 and Stx2. We purified the Stx-adsorbing substance from the U. linza extract by DEAE-Toyopearl column chromatography and gel filtration with HiPrep 16/60 Sephacryl S-300 HR column. The purified substance showed an average molecular mass of about 800 kDa by polyacrylamide gel electrophoresis. Analyses of its components indicated that the substance was a highly rhamnose-containing polysaccharide with sulfate esters of 18%. Apparent dissociation constants ( K d) of the polysaccharide to Stx1 and Stx2 were calculated to be 1.9 and 3.5 μM, respectively. To our knowledge, this is the first report on Stx-adsorbing dietary fibers.