In this paper, the activity of horseradish peroxidase was further determined in the presence of several uracil derivatives. The rate of guaiacol peroxidation decreases in presence of 2-thiouracil and of 6-n-propyl-2-thiouracil, but is not changed by 6-n-propyluracil nor uracil. Thus, thiouracils inhibit horseradish peroxidase in a noncompetitive form. The binding of 6-n-propyl-2-thiouracil, 2-thiouracil, 6-n-propyluracil and uracil with horseradish peroxidase shows difference spectra due to changes in the environment of heme group in peroxidase. Then, the binding sites for these uracil derivatives are in an hydrophobic pocket at the heme periphery of peroxidase. The lesser binding rates were for uracil and propyluracil, which did not inhibit the peroxidase activity. These results point to the thiol group in uracils as responsible for the inhibition of peroxidase activity through interaction with an allosteric binding site, in peroxidase heme environment.
