Conversion of Helminthosporium sacchari Toxin to Toxoids by beta-Galactofuranosidase from Helminthosporium

Abstract

Helminthosporium sacchari produces a host-selective toxin and structurally related nontoxic compounds, here referred to as ;toxoids.' Toxin and the three toxoids were each isolated to a high level of purity and were hydrolyzed under acidic conditions. The released galactose was measured by a galactose oxidase/peroxidase assay. Toxin was found to contain four units of galactose per molecule, as previously reported. Toxoids I, II, and III contained one, two, and three units of galactose, respectively. In cultures of the fungus, toxin concentration peaked at 3 weeks, followed by a rapid decline; as toxin levels fell, the total amount of toxoids increased. An enzyme with beta-galactofuranosidase activity was found in small amounts in the cultures of H. sacchari; the enzyme converted toxin to the toxoids in vitro. beta-Galactofuranosidase was previously known from very few micro-organisms; therefore, several pathogenic Helminthosporia and other fungi were tested for production. beta-Galactofuranosidase activity in culture filtrates and mycelia of H. victoriae, H. maydis, H. carbonum, and H. turcicum was much greater than in filtrates and mycelium of H. sacchari. More work is needed to determine the significance of enzyme production by these fungi. No beta-galactofuranosidase was evident from Fusarium oxysporum and Cladosporium cucumerinum.