Oxidation of salsolinol by banana pulp polyphenol oxidase and its kinetic synergism with dopamine

Salsolinol, a tethrahydroisoquinoline present in banana and biosynthesized from dopamine, was oxidized by banana pulp polyphenol oxidase to its corres ponding salsolinol-o-quinone. This oxidation was pH-dependent and showed a maximum at acidic pH values. At physiological pH of 5.0, the values obtaine d for the kinetic parameter (V-m and K-m) were 62.5 muM/min and 1.7 mM, res pectively. When dopamine was added to the reaction medium to imitate physio logical conditions, salsolinol was co-oxidized by dopamine-quinone. When th is phenomenon was studied oxygraphically, an unexpected activation of dopam ine oxidation was found in the presence of salsolinol. This activation was related with the enzyme's kinetic mechanism and was named "kinetic synergis m", because a bad substrate activated a good one. A possible physiological role is discussed.