- Differences in the biological properties of the Clostridium perfringens phospholipase C (KK-toxin) and the C.bifermentans phospholipase C (Cbp) have been attributed to differences in their carboxy-terminal domains. Three residues in the carboxy-terminal domain of KK-toxin, which have been proposed to play a role in membrane recognition (D269,Y331and F334), are not conserved in Cbp (Y, L and I respectively). We have characterised D269Y, Y331L and F334I variant forms of KK-toxin. Variant D269Y had reduced phospholipase C activity towards aggregated egg yolk phospholipid but increased haemolytic and cytotoxic activity. Variants Y331L and F334I showed a reduction in phospholipase C, haemolytic and cytotoxic activities indicating that these substitutions contribute to the reduced haemolytic and cytotoxic activity of Cbp.