Difference Spectrophotometric Measurement of Catecholamine Oxidative Activity of Edible Yam(Dioscorea) Polyphenol Oxidase.

Abstract

Polyphenol oxidase was partially purified from aerial tubers of edible yam (Dioscorea opposita THUNB.). The enzyme strongly oxidized catecholamines such as dopamine and DL-noradrenaline. The optimum pH of the enzyme was 6.0 and its temperature was 30°C. The difference spectra produced a high positive peak at 300 nm during the enzymatic dopamine oxidation. Peak height (ΔA300) in the difference spectra increased in proportion to the reaction time for about 10 min at 30°C and pH 6. The ΔA300 value was found to be directly proportional to the dopamine concentration. A linear relationship was seen between the rate of dopamine oxidation (ΔA300/5 min) and the enzyme concentration. These results indicate that the difference spectrophotometric method is a useful way of rapidly and accurately determining catecholamine oxidase activity which is closely related to the enzymatic browning in edible yam.