1. A homogeneous preparation of penicillinase (penicillin amido-β-lactamhydrolase, EC 3.5.2.6) was isolated and purified from cultures of Staphylococcus aureus by a simple two-stage procedure. 2. The native protein contains 20-30% helix as determined by optical-rotatory-dispersion and circular-dichroism measurements. Some 54(+/-5)% of the 13 tyrosine residues are exposed to solvent molecules of diameter 0.44 and 0.94 nm. 3. Conditions that allow full recovery of enzymic activity and native conformation from the fully unfolded state in 4M-guanidinium chloride were defined. 4. Refolding of the protein was shown to be inhibited by intermolecular interaction, by small changes in ionization and by low concentrations (0.025 M) of phenol.
