Covalent binding sites of victorin in oat leaf tissues detected by anti-victorin polyclonal antibodies

Polyclonal antibodies against victorin, the host-specific toxin produced by Cochliobolus victoriae, were raised in rabbits immunized with a victorin-bovine serum albumin conjugate. The antibodies were purified from serum by protein A column chromatography and characterized by indirect and direct enzymelinked immunosorbent assays (ELISA). The concentration of victorin that inhibited anti-victorin antibody binding by 50% was 10 nanograms per milliliter in an indirect ELISA. The lowest concentration of victorin detectable was 10 picograms per milliliter. In a direct ELISA, 25 nanograms per milliliter of victorin inhibited binding of victorin-horseradish peroxidase conjugate by 50%. In vivo and in vitro covalent binding of victorin to proteins in susceptible and resistant oat (Avena sativa) tissue was examined by western blotting assays using anti-victorin antibody and a second antibody conjugated with ¹²⁵I or alkaline phosphatase. In vivo binding of victorin to proteins of 100 and 45 kilodaltons was observed in both susceptible and resistant cultivars of oats. Victorin also bound in vitro to proteins of 100, 65, and 45 kilodaltons in both susceptible and resistant oats. The data indicate that victorin binds covalently to the same sites in susceptible and resistant genotypes of oats.