Inhibition of lysozyme by imidazole and indole derivatives

Imidazole, 4(5)-imidazole acetic acid, histidine, histidine methyl ester, and histamine, at varying concentrations, were found to inhibit the lysis by lysozyme of Micrococcus lysodeikticus cells or cell walls. From the pH profile of the extent of inhibition it is deduced that the enzyme is inhibited mainly by the protonated imidazole ring. Inhibition of lysozyme by imidazole or by the imidazole derivatives investigated was accompanied by a decrease in fluorescence intensity of the enzyme. Lysozyme activity could also be inhibited by 3-indole derivatives such as 3-indole propionic acid and tryptamine. The formation in aqueous solution of complexes of the charge-transfer type between compounds containing an indole ring and compounds containing a protonated imidazole ring has been demonstrated by a fluorescence quenching method, and by two thermodynamic methods. Compounds containing a protonated imidazole ring were shown to quench the fluorescence of indoles and to increase their solubilities in aqueous solution. From the temperature dependence of the association constants, enthalpies of association, ΛH, of about −3 Kcal per mole were calculated for the complexes indole-imidazole·HClO 4, 3-methyl indole-imidazole·HClO 4, indole-histidine·HClO 4, and 3-methyl indole-histidine·HClO 4. In view of the results obtained with lysozyme and with the low molecular weight model systems, it is suggested that inhibition of lysozyme by compounds containing a protonated imidazole ring is at least partially due to the formation of a charge-transfer complex with the tryptophan residues of the enzyme.