Thioredoxin-linked reductive inactivation of venom neurotoxins.

Thioredoxin, a 12-kDa protein with a catalytically active disulfide group, has recently been found to reduce intramolecular disulfide bonds in a variety of proteins. We now report that thioredoxin, reduced either enzymically with NADPH and NADP-thioredoxin reductase or chemically with dithiothreitol or lipoic acid, acts as a specific reductant of purified snake venom neurotoxins, a diverse group of disulfide proteins. Included were Bungarus multicinctus neurotoxins that act presynaptically (beta-bungarotoxin) or postsynaptically (alpha-bungarotoxin) as well as a postsynaptic neurotoxin from Laticauda semifasciata (erabutoxin b). We also observed a thioredoxin-specific reduction with other disulfide proteins of venom from Bungarus multicinctus, scorpion (Androctonus australis), and bee (Apis mellifera). Other cellular sulfhydryl agents, glutathione and glutaredoxin, were uniformly inactive. Thioredoxins from bacterial, plant, and animal sources were all active in neurotoxin reduction, but differed in effectiveness. Reduction of the neurotoxins by thioredoxin was accompanied by an increased susceptibility to tryptic proteolysis and a decrease of associated toxin activity: phospholipase A2 (beta-bungarotoxin, snake, and bee venoms) or acetylcholine receptor binding (alpha-bungarotoxin). These findings extend the function of thioredoxin to the reduction of a broad group of low-molecular-weight proteins, all containing intramolecular disulfide bonds. The loss of activity accompanying reduction raises the possibility that venoms may be detoxified by thioredoxin either as a defense mechanism or as a clinical antidote.