Which toxin was first isolated in pure form?

Tabtoxin was first isolated in pure form.
Tabtoxin, also known as wildfire toxin, is a simple monobactam phytotoxin produced by Pseudomonas syringae. It is the precursor to the antibiotic tabtoxinine β-lactam (TBL).
Tabtoxin is a dipeptide precursor to the biologically active form of TBL, differing by having an extra threonine attached by a peptide bond to the C terminus.
Tabtoxin resistance protein (TTR) is an enzyme that catalyzes the acetylation of TBL, rendering tabtoxin-producing pathogens tolerant to their phytotoxins.