Inhibition of lysozyme by polyvalent metal ions

The rate of hen egg-white lysozyme (mucopeptide N-acetylmuramoylhydrolase, EC 3.2.1.17), catalysis was determined in the presence of various metal ions (CO²⁺, Zn²⁺ and eight of the trivalent lanthanide ions). In the assay system employed, the lanthanides were found to inhibit more strongly than either Zn²⁺ or Co²⁺. The inhibition data was fitted to several models of the interactions of the metal ion with the enzyme. These models ranged in complexity from a single inhibitory metal binding site on the enzyme (two-parameter fit) to the presence of two non-independent and non-equivalent inhibitory metal binding sites (five-parameter fit). The more complicated models did not fit the data more precisely than the simplest one-site model, suggesting that the adoption of the simpler model is warranted. The fact that association constants obtained from the simplest analysis for Co²⁺ (1.3 ± 1.9 · 10² M^-1) and Gd³⁺ (7.0 ± 2.6 · 10³ M^-1) are consistent with literature values determined from spectroscopic measurements further supports the validity of the simplest model.