Enzymes, an important group of macromolecules, are involved in most processes in the human body. Because of their involvement in both physiological and pathological processes, regulation of enzyme activity is of the utmost importance. Enzymes are often overly active in various pathological conditions. Inhibitors of enzyme activity therefore represent an important therapeutical implement for both managing and treating the disease. Different types of molecules, such as proteins, peptides, organic molecules, metals, or complex compounds can regulate enzyme activity. The lysosomal, cysteine proteases called cathepsins, which are expressed in all cell types, where they non-specifically degrade various protein components of cells are among the most often studied enzymes. Their pathological role is usually associated with their extracellular localization and degradation of the components of extracellular matrix. They are involved in different diseases, such as cardiovascular, autoimmune, and neurological diseases as well as osteoporosis, atherosclerosis and last but not least, in the progression and metastasis of malignant tumours.
Previous studies have already examined the effect of selected heavy metal ions (Cd(II), Ce(III), Ce(IV), Ga(III), La(III), Pb(II) and Zn(II)) on the activity of cathepsin K, an important enzyme in bone degradation and repair. Metal ions offer a good basis for the development of complex compounds that can selectively act on a specific enzyme and partially or fully inhibit its activity.
The aim of this study was to examine the effect of these heavy metal ions on activity of other cathepsins, namely cathepsins B, L and S. We tested the effects of selected metal ions by means of kinetic measurement of enzyme activity, which in turn enabled us to determine the Ki or EC50 values and mechanism of action for each metal cation. When using a synthetic substrate Cd(II), Zn(II) and Pb(II) ions act as linear competitive inhibitors of all three enzymes, of these the Zn(II) ions were the most effective inhibitor, while Pb(II) ions were the least effective inhibitor. The remaining cations (Ce(III), Ce(IV), Ga(III) ter La(III)) acted differently on different cathepsins. All cations except Ce(III) ions acted as partial inhibitors of cathepsin B, whilst low inhibition occurred only at very high concentrations of Ce(III) ions. All 4 cations acted as partial inhibitors of cathepsin S activity and as full inhibitors of cathepsin L activity. In all cases Ga(III) and Ce(IV) ions proved to be better inhibitors, which is most likely due to their smaller ionic radii. The results of macromolecular degradation assay the results were slightly different, with inhibition being lower in all cases. In the elastin degradation assay, metal ions proved to inhibit the activity of cathepsin S slightly better than that of cathepsin L. On the other hand, in the azocasein assay, only Zn(II) ions proved to be an effective inhibitor of both cathepsin L and S, while the effect of other metal ions was either non-existent or negligible.
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