Interaction of tylophorinidine with lysozyme

Abstract

The interaction of tylophorinidine (TPD) with lysozyme-a model protein-with biological activity, was investigated by determining its fluorescence and by assessing its activity under various conditions. The results indicated that TPD associated with lysozyme at pH 9.2 efficiently with an association constant K,, of 3.3 X lo4 M-' at 26OC. K,, increased with the increasing temperature in the range 26 to 5 5 T ; the calculated enthalpy change AH was found to be 2.3 kcal/mol. Under the same conditions as above TPD also associated with the free amino acid tryptophan with a K,, of 1.7 X lo4 M-' indicating half the efficiency of its association with protein lysozyme. TPD associated lysozyme was less active than the uncomplexed enzyme in the above temperature range although beyond 45OC the inhibition was more significant. The results imply that TPD binds lysozyme outside the cleft region in the temperature range studied here. However, with increasing temperature the cleft region is gradually widened and/or the whole molecule is expanded such that the accommodation of whole or part of the TPD molecule is facilitated leading to the blockage of lytic activity.