Three polyphenol oxidases from red clover (Trifolium pratense) differ in enzymatic activities and activation properties.

Polyphenol oxidases (PPOs) oxidize o-diphenols to o-quinones, which cause browning reactions in many wounded fruits, vegetables, and plants including the forage crop red clover (Trifolium pratense L.). Production of o-quinones in red clover inhibits postharvest proteolysis during the ensiling process. The cDNAs encoding three red clover PPOs were expressed individually in alfalfa (Medicago sativa L.), which lacks detectable endogenous foliar PPO activity and o-diphenols. Several physical and biochemical characteristics of the red clover PPOs in alfalfa extracts were determined. In transgenic alfalfa extracts, red clover PPOs exist in a latent state and are activated (10-40-fold increase in activity) by long incubations (>2 days) at ambient temperature or short incubations (<10 min) at > or =65 degrees C. PPO1 appears to be more stable at high temperatures than PPO2 or PPO3. During incubation at ambient temperature, the molecular masses of the PPO enzymes were reduced by approximately 20 kDa. The apparent pH optima of latent PPO1, PPO2, and PPO3 are 5.5, 6.9, and 5.1, respectively, and latent PPO1 is slightly activated (~5-fold) by low pH. Activation of the PPOs shifts the pH optima to approximately 7, and the activated PPOs retain substantial levels of activity as the pH increases above their optima. The latent and activated PPOs were surveyed for ability to oxidize various o-diphenols, and activation of the PPOs had little effect on substrate specificity. Activation increases the V max but not the affinity of the PPO enzymes for caffeic acid. Results indicate red clover PPOs undergo structural and kinetic changes during activation and provide new insights to their effects in postharvest physiology.