Partial purification and characterisation of polyphenol oxidase from celery root (Apium graveolens L.) and the investigation of the effects on the enzyme activity of some inhibitors

Polyphenol oxidase (PPO) of celery root was extracted and partially purified by (NH4)(2)SO4 fractionation and dialysis. Optimum pH and temperature were found at pH 7.0 and 30 degrees C, and K-m and V-max values were 29 mm and 5560 U mL(-1) min(-1) with catechol, respectively. The activation energy of the enzyme with catechol was 17.9 kJ mol(-1) at pH 7.0. In electrophoretic seperation, six isoenzymes were detected with DL-dopa substrate. PPO showed activity to catechol, 4-methylcatechol, pyrogallol, gallic acid, DL-dopa. l-Tyrosine was also tested but was not oxidised by celery root PPO. beta-Mercaptoethanol was found to be the most effective inhibitor. (NH4)(2)SO4, NaCl, KCl and sucrose appeared to be protective agents of celery root PPO against thermal denaturation. Metal ions (Cu2+, Zn2+, Mn2+) were poor inhibitors of the celery root PPO at 1 mm. PPO activity was also inhibited by CaCl2, NaCl, BaCl2, FeSO4 and NiCl2.