Inhibition of horseradish peroxidase activity by thiol type inhibitors

The inhibition by cysteine and glutathione of 4-amino antipyrine oxidation by horseradish peroxidase was compared. The kinetics of the peroxidase inhibition by glutathione was investigated and compared to l-cysteine inhibition. IC50 for l-cysteine and glutathione was determined and used for the subsequent kinetics studies. The biological activity of the enzyme was measured in the presence of each inhibitor using 4-amino antipyrine as substrate. It was found that reduced glutathione was a more potent inhibitor on peroxidase activity than l-cysteine. Kinetic studies showed that inhibition was non-competitive and mixed type in both cases. The values of Km and Vmax in the presence of each inhibitor were also calculated.