PH and Temperature Optima

The optimum pH for the activity of an enzyme preparation from any one source usually varies with different substrates and is characteristic of the substrate as well as the enzyme preparation. The optimum pH of PPO activity has a relative wide range, pH 5.0 and 7.0. Some preparations were reported to be inactive below 4.0. The type of buffer and the purity of the enzyme affect the pH optimum; also, isoenzymes have different pH optima. Enzyme preparations obtained from the same fruit or vegetable at various stages of maturity have been reported to differ in optimum pH of activity as well. The pH optimum of laccase activity depends on the enzyme source and on the substrate. The stability of the enzyme is reported to be dependent upon buffer and other factors in the medium. Optima between pH 4.5 and 7.5 have been reported for enzymes from various fungi and peaches (54).

The optimum temperature of PPO depends essentially on the same factors as the pH optimum. Generally, the activity of PPO increases as the temperature rises from cold to temperatures of maximum activity (25-35°C); activity declines rapidly at temperatures of 35 to 45°C, depending on the enzyme source and substrate (16). Like most enzymes, the application of heat (70-90°C) to plant tissues or enzyme solutions for a short period causes rapid inactivation. Exposures to temperatures below zero may also affect activity. Concentrated solutions of the enzyme in dilute phosphate buffer at a pH near neutrality can be held without detectable loss of activity for several months at 1°C or when frozen at — 25°C. However, the enzyme slowly losses activity even in the frozen state. Thermo-tolerance of PPO also depends on the source of the enzyme. It is difficult to compare heat stability data of purified enzyme preparations with that of the enzyme when in tissues or juice. Different molecular forms of PPO from the same source may have different thermostabilities. Laccase is usually more sensitive to inactivation by heat than is catechol oxidase.

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