Analysis of the mechanism by which tryptophan analogs inhibit human myeloperoxidase

Myeloperoxidase (MPO) catalyzes the formation of oxidants that have been implicated in the pathogenesis of various diseases, including cardiovascular and pulmonary diseases and cancer. Inhibition of MPO oxidant-generating activity represents an attractive target for preventing the progression of inflammatory conditions. Peroxidation and chlorination catalytic activity were utilized to screen for the most effective tryptophan analog that inhibits MPO. Rapid kinetic measurements were performed to determine the mechanisms through which these compounds inhibit the catalytic activity of MPO. Substituents on the amino and carboxyl termini of tryptophan enhance its affinity toward MPO compared to a substituent on the indole ring. Hydrogen-bond donor capabilities and a positive charge on the amino group are not required for MPO inhibition. Hydroxyl-containing indoles did not inhibit MPO H₂O₂-consumption activity. Elimination of the negative charge from the carboxyl terminus by introducing a hydrophobic character significantly enhanced tryptophan analog affinity for MPO and improved its inhibitory properties. Further mechanistic studies indicated that indole compounds inhibited MPO activity through the accumulation of compound II, an inactive MPO intermediate. Our results show that specific structural features of tryptophan analogs are involved in increasing the affinity for MPO and providing a significantly greater inhibition of MPO's catalytic activities.