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Abstract. Disease-associated strains of Helicobacter pylori produce a potent toxin that is believed to play a key role in peptic ulcer disease in man. In vitro the toxin causes severe vacuolar degeneration in target cells and has thus been termed VacA (for vacuolating c_.ytotoxin A). Cytotoxic activity is associated with a >600-kD protein consisting of several copies of a 95-kD polypeptide that undergoes specific proteolytic cleavage after release from the bacteria to produce 37-and 58-kD fragments. Quick freeze, deep etch electron microscopy has revealed that the native cytotoxin is formed as regular oligomers with either six-or sevenfold radial symmetry. Within each monomer, two domains can clearly be distinguished, suggesting that the 37-and 58-kD fragments derive from proteolytic cleavage between discrete subunits of the monomer. Analysis of preparations of the toxin that had undergone extensive cleavage into the 37-and 58-kD subunits supports this interpretation and reveals that after cleavage the subunits remain associated in the oligomeric structure. The data suggest a structural similarity with AB-type toxins.

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The crystal structure of normalAgO has been determined by means of neutron diffraction. The nuclear scattering of Ag and O atoms has made it possible to determine the positions of the oxygen atoms and to modify the space group which was reported for this compound from x‐ray data. No magnetic diffractions were observed even at liquid helium temperature; the absence of magnetic scattering confirms the diamagnetism of the compound. The lattice was found to be monoclinic, space group P21/normalc , with four formula weights of normalAgO in the unit cell. The silver atoms are not equivalent, and in the structure there are two Ag‐O distances of 2.18Aå and 2.03Aå; the first one corresponds to colinear Ag(I)‐O bonds, the second distance corresponds to square planar Ag(III)‐O bonds.