Multiple forms of soluble monophenol, dihydroxyphenylalanine: oxygen oxidoreductase (EC 1.14.18.1) from potato tubers (Solanum tuberosum). IV. Association and dissociation phenomena.

The soluble phenol oxidase of various potato juices (adjusted from physiological pH to pH 4.5, 7.0 and 7.8) was separated by gel chromatography into multiple molecular forms. In acid or neutral and alkaline potato juices, low-mol.-wt. (less than 150,000 daltons) or high-mol.-wt. (greater than 150,000 daltons) enzyme forms predominate, respectively. Conversion of the low-mol.-wt. enzyme forms into high-mol.-wt. enzyme forms, and vice versa, was achieved by changing the pH values from acidic to neutral or alkaline pH, and vice versa. This substantiated our previous idea that the enzyme multiplicity arises from association of various subunits. In alkaline potato juice, considerable loss of monophenol oxidase activity (assayed at pH 6.0) occurred. This confirmed our previous findings that o-diphenol oxidase is more alkali-stable than monophenol oxidase.