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 comparativ e study of crude polyphenoloxidases from tare (tPPO) and potatoes (pPPO) wa s carried out to provide information useful for guiding food processing ope rations. Crude PPO was prepared by cold acetone precipitation using ascorbi c acid as antioxidant. The PPO content of tare acetone powder was 770 +/- 1 7 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 an d 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-acti vity optimum (T-opt) was 30 degrees C for tPPO and 25 degrees C for pPPO wi th 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 (Delt a 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) k J mol(-1) whilst Delta S-# was -141 (+/- 4) J mol(-1) K-1. The apparent sub strate specificity was established from values V-max/K-m, as: 4-methylcatec hol > chlorogenic acid > DL-dopa > catechol > pyrogallol > dopamine > > caf feic acid for tPPO. There was no detectable activity towards caffeic acid. The substrate specificity for pPPO was: 4-methylcatechol > caffeic acid > p yrogallol > 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 inc reases occurred in the absorbance at 420 nm. Preincubation of PPO with inhi bitors increased the extent of inhibition, indicating a direct effect on th e structure of the enzyme. (C) 1998 Elsevier Science Ltd. All rights reserv ed.