Studies on the chemical modification of tryptophan residues in thermolysin and in talopeptin (MKI) with N-bromosuccinimide.

Tryptophan residues in thermolysin (3 Trp/molecule) and in its specific inhibitor, talopeptin (1 Trp/molecule), were modified with N-bromosuccinimide (NBS). The decrease in the absorption at 280 nm and the fluorescence intensity above 310 nm (excited at 280 nm) accompanying the modification were followed by the stopped-flow method as a function of time. When the sole tryptophan residue of talopeptin was modified with NBS, its inhibitory activity against thermolysin was almost completely destroyed. For thermolysin, the decrease in molar absorptivity corresponds to the modification of one of its three tryptophan residues, and the enzyme activity does not decrease significantly with the modification (remaining activity was 96% at [NBS]/[E] = 6). The results obtained for the modification of EI complex suggested that the formation of EI complex remarkably reduces the rate constant for the modification of the tryptophan residue in talopeptin, but does not affect that of the tryptophan residue(s) in thermolysin.