Kinetic characteristics of purified tyrosinase from different species of Dioscorea (yam) in aqueous and non-aqueous systems

Yam tubers could serve as a cost-effective source of tyrosinase for various applications. Here a novel but cheap method of purification, and some properties of the enzyme from four species of yam tubers having high tyrosinase activity are described. The native and subunit molecular weights of each of the purified tyrosinase were between 41–58 kDa and 24–27 kDa respectively. Optimum pH and temperature were 6.5 and 50 °C respectively. Tyrosinase from tubers of Colocasia esculenta retained more than 50% activity in ethanol (≤60%). All the purified enzymes were activated in 40% ether by between 120 and 170%, and maintained 100% residual activity at up to 65% ether for 17 h. Both the Km and Vmax increased in 40% ether, leading to a corresponding increase in kcat/Km. All the enzymes had both monophenolase and diphenolase activities. β-Mercaptoethanol and to a lesser extent, glutathione were good inhibitors. In aqueous systems, the purified tyrosinase catalyzed formation of coloured products in the presence of some substrates such as catechol, pyruvic acid and ammonia. The catalytic properties of these enzymes especially in organic solvents suggest that they may find uses in some biotechnological applications.