1. The thermal denaturation and precipitation of β-lactamase I from Bacillus cereus 569/H/9 at 60°C are reversible, a soluble and almost fully active enzyme being obtained after solution of the precipitate in 5m-guanidinium chloride or 8m-urea and subsequent removal of the denaturing agent. 2. Inactivation of β-lactamase I occurs rapidly between 50° and 55°C and is shown by circular-dichroism spectra to be accompanied by an extensive conformational change. 3. A change to a different conformation occurs in 6m-urea. This change is also reversible; refolding with almost complete recovery of enzymic activity occurs within 5min of dilution of the denaturing agent. 4. Inactivation of β-lactamase I at pH3.0 and 11.0 is also associated with conformational changes, since a proportion of the lost activity is recovered within 5min of adjustment of the pH to 7.0.
