A comparative study of polyphenoloxidases from taro (Colocasia esculenta) and potato (Solanum tuberosum var. Romano)

Taro (C. esculenta) is a staple food in many tropical regions. A comparative study of crude polyphenoloxidases from tare (tPPO) and potatoes (pPPO) was carried out to provide information useful for guiding food processing operations. Crude PPO was prepared by cold acetone precipitation using ascorbic acid as antioxidant. The PPO content of tare acetone powder was 770 +/- 17 units (mg protein)(-1) as compared with 3848 +/- 180 units (mg protein)(-1) in potato acetone powder. The pH-activity optimum was pH 4.6 for tPPO and pH 6.8 for pPPO. Both enzymes retained > 80% activity after incubation at pH 4.5-8 but there was rapid activity loss at pH < 4. The temperature-activity optimum (T-opt) was 30 degrees C for tPPO and 25 degrees C for pPPO with 75 and 27% of their respective maximum activity retained at 60 degrees C. Both tPPO and pPPO were irreversibly inactivated by 10 min heating at 70 degrees C. The activation enthalpy (Delta H-#) and activation entropy (Delta S-#) for tPPO heat-inactivation were 87.4 (+/-0.1) kJ mol(-1) and -56.2 (+/-4) J mol(-1) K-1, respectively. For pPPO, Delta H-# was 59.1 (+/- 0.1) kJ mol(-1) whilst Delta S-# was -141 (+/- 4) J mol(-1) K-1. The apparent substrate specificity was established from values V-max/K-m, as: 4-methylcatechol > chlorogenic acid > DL-dopa > catechol > pyrogallol > dopamine > > caffeic acid for tPPO. There was no detectable activity towards caffeic acid. The substrate specificity for pPPO was: 4-methylcatechol > caffeic acid > pyrogallol > catechol > chlorogenic acid > DL-dopa > dopamine. According to the order of inhibitor effectiveness (sodium metabisulphite > ascorbic acid > NaCl approximate to (EDTA), there was a significant lag-phase before increases occurred in the absorbance at 420 nm. Preincubation of PPO with inhibitors increased the extent of inhibition, indicating a direct effect on the structure of the enzyme. (C) 1998 Elsevier Science Ltd. All rights reserved.