Monophenolase Activity of Latent Banana Pulp Polyphenol Oxidase

Abstract

The monophenolase activity of latent banana polyphenol oxidase (PPO) partially purified by using a combination of two aqueous two-phase systems based on Triton X-114 and PEG 8000 (Sojo et al. J. Agric. Food Chem. 1998, 46, 4924−4930) has been characterized in the presence of sodium dodecyl sulfate (SDS). The purification method used avoids the loss of monophenolase activity described for other banana pulp PPOs obtained by acetone powders. The activity presented a lag period before the steady-state rate was reached. The activity and lag period depended on the SDS, substrate, and enzyme concentration, the pH, and the presence of catalytic amounts of o -diphenol. The enzyme was characterized with an enzyme activation constant, K act, of 2.4 μM. On the basis of these results we discuss why monophenolase activity is less studied (or detected) in plants based on the existence of two enzymatic forms, Eoxy and Emet, and the method used to purify the plant enzyme. Keywords: Polyphenol oxidase; monophenolase; monophenols; banana; tyramine; enzyme kinetics