A spectrophotometric study on the interaction of thermolysin with chloride and bromide ions, and the state of tryptophyl residue 115.

The activity of thermolysin is greatly enhanced in the presence of high concentrations of neutral salts [Holmquist, B. and Vallee, B.L. (1976) Biochemistry 15, 101-107; Inouye, K. (1992) J. Biochem. 112, 335-340]. NaBr and NaCl are the most effective for the activation. An absorption difference spectrum with a peak around 293 nm, which is characteristic of the red-shift of a tryptophyl residue caused by charge effects, was observed on mixing of thermolysin with NaCl. As the peak disappeared in the presence of competitive inhibitors of the enzyme (phosphoramidon and zincov), it was considered to be derived from a tryptophyl residue (Trp 115) located in the active site of the enzyme. On the other hand, this peak was not observed on the mixing of thermolysin and NaBr, indicating that the slight difference in size between chloride and bromide ions is critical for the interaction with the tryptophyl residue. NaCl and NaBr exhibit comparable effects on the activation of thermolysin regardless of the considerable discrepancy in their effects on the absorptivity difference around 293 nm. This suggests that the interaction of salts with Trp 115 is not necessarily correlated with the activation of thermolysin.