Contrary to our findings, other work has shown that thiol containing compounds such as Cys and metabisulfite were effective in inhibiting persimmon PPO activity, suggesting an addition reaction taking place with the quinones to form stable colorless products, and/or a binding to the active center of the enzyme in the case of metabisulfite (Nú˜nez-Delicado et al., 2003; Özen et al., 2004).