Suppression of Bacillus thuringiensis δ-endotoxin activity by low alkaline pH

Abstract

Reducing the alkaline pH (from 10.5 to 8) of enzyme-activated δ-endotoxin preparations from Bacillus thuringiensis subsp. kurstaki suppressed toxicity. The effect was reversible and readily observed in lawn assays against IPRICF-1 cells (a system in which alkaline injury to cells was prevented). Suppression of toxicity was also observed, though to a lesser extent, in force-feed assays against Bombyx mori. Doses in both types of bioassay were based on measurements of the total dissolved protein in activated toxin preparations. Inactivation caused by reducing the pH occurred in the presence of a chaotropic agent and was not affected by ultracentrifugation of toxin samples, indicating that molecular aggregation and precipitation of protein were probably not the immediate cause. A computer analysis of toxin proteins indicated that the effect might be related to the protonation of tyrosine residues when the pH is reduced or possibly to an overall change in molecular charge caused mostly by the protonation of these residues. The pH effect suggests that the highly alkaline environment of the lepidopteran midgut is important not only for solubilization and digestion of B. thuringiensis crystal protein, but for full expression of toxin activity as well.