Most, if not all, L-lactamases reported to date are
irreversibly denatured at 60^70‡C. Here, we found that a halophilic
L-lactamase from the moderately halophilic bacterium
Chromohalobacter sp. 560 was highly stable against heat inactivation:
it retained V75% of its activity after boiling for 5 min
in the presence of 0.2 M NaCl, suggesting that the protein
either incompletely denatures during the boiling process or readily
renatures upon cooling to the assay temperature. Circular
dichroism showed a complete unfolding at 60‡C and a full reversibility,
indicating that the observed activity after boiling is
due to e⁄cient refolding following heat denaturation. The enzyme
showed optimal activity at 50^60‡C, indicating that an
increase in activity with temperature o¡sets the thermal denaturation.
The gene bla was cloned, and the primary structure of
the enzyme was deduced to be highly abundant in acidic amino
acid residues, one of the characteristics of halophilic proteins.
Despite its halophilic nature, the enzyme refolds in low salt
media after heat denaturation.