Protease degradation of Escherichia coli heat-stable, mouse-negative, pig-positive enterotoxin.

Enterotoxigenic Escherichia coli produces three enterotoxins: heat-labile toxin, a mouse-positive heat-stable toxin, and a mouse-negative heat-stable toxin (STb). The only species in which a response to STb has been documented is the pig, and this response is inconsistent. When STb was placed in 60 ligated jejunal segments (loops) in six pigs, a positive response (net secretion) was observed in only 40 loops. In contrast, when the jejunal lumen was pre-rinsed with 50 ml of saline, the same STb preparation induced net secretion in 60 of 60 loops. STb did not induce secretion in rinsed loops when jejunal luminal washings were collected and mixed with STb in vitro. The anti-STb activity of jejunal luminal washings was filterable through 0.45-micron-pore-size filters, was destroyed by heating at 100 degrees C for 15 min, and was blocked by soybean trypsin inhibitor. STb was inactivated when incubated with trypsin in vitro for 60 min at 37 degrees C. It is concluded that STb is susceptible to trypsin degradation and that variable amounts of trypsin-like activity in swine jejuna are responsible for inconsistent responses to STb in jejunal loops of swine. These results also suggest that the concept of species specificity of the STb response should be reexamined.