Appearance of a lag period dependent on pH in the expression of the catecholase activity of a polyphenoloxidase extracted in a latent state from Airen grape (Vitis vinifera L.) berries, is revealed, suggesting the hysteretic nature of the enzyme. The lag time was independent of enzyme concentration, indicating that slow pH-induced conformational changes in the protein must occur during assay. Results obtained by varying substrate concentration show that the system presents hyperbolic or cooperative kinetics depending on the pH of the assay.