RELATIONSHIP BETWEEN A LOW TOXICITY OF THE MUTANT A-SUBUNIT OF ENTEROTOXIGENIC ESCHERICHIA-COLI ENTEROTOXIN AND ITS STRONG ADJUVANT ACTION

In the work described here it was determined if and how unnicking in t he A subunit of Escherichia coli enterotoxin at Arg192 or nearby resid ues affected biological activities of the toxin. The mutant toxin was constructed to lack the nick site in the A subunit by deleting the tri peptide Arg192-Thr193-Ile194, which is essential for toxicity. The mut ant toxin did not exhibit agmatine ADP-ribosyltransferase activity in the presence or absence of the ADP-ribosylation factor and had less di arrhoeal activity and lower induction of cyclic AMP than did LT. The m utant toxin exhibited a much stronger adjuvant action on antibody resp onses to measles virus, keyhole limpt haemocyanin, bovine immunoglobul in and ovalbumin compared with LT. The altered toxicity of the mutant toxin might be closely related to the potent adjuvant action on antibo dy responses to antigens. The relationship between two activities is d iscussed.