Factors Affecting Activity and Stability of the Kluyveromyces lactis Killer Toxin.

A number of physical parameters determining the activity and stability of the killer toxin produced by the yeast Kluyveromyces lactis have been investigated. The toxin was active over a relatively narrow pH range of 4.4 to 5.8, with a maximum at the lower end of the range. However, it was stable up to at least pH 8.0 but appeared to be irreversibly inactivated below pH 4.4. The toxin was stable at 40 degrees C but rapidly inactivated at 50 degrees C. Strong agitation caused the inactivation of the toxin in one medium but not another; this seemed to be due to oxygen-mediated disulfide bond formation, which could be prevented by sulfhydryl protecting agents.