Opposing effects of NaCl on reversibility and thermal stability of halophilic β-lactamase from a moderate halophile, Chromohalobacter sp. 560

β-lactamase from a moderately halophilic organism is expected to show salt-dependent stability. Here we examined the temperature-dependence of stability at different salt concentrations using circular dichroism (CD) and enzyme activity. NaCl showed opposing effects on melting temperature and reversibility of the thermal melting. Increasing NaCl concentration greatly increased the melting temperature from, e.g., 41 °C in the absence of NaCl to 61 °C in 3 M NaCl. Conversely, reversibility decreased from 92% to 0% in the corresponding NaCl solutions. When β-lactamase was heated at different temperatures and NaCl concentrations, the activity recovery followed the reversibility, not the melting temperature. Heating β-lactamase at 63 °C, slightly above the onset temperature of melting in 2 M NaCl and far above the melting in 0.2 M NaCl, showed a much greater recovery of activity in 0.2 M NaCl than in 2 M NaCl, again consistent with the reversibility of melting.