Effects of Phenolic Compounds on the Mitochondrial Phenol Oxidase of Spinach Leaves

When a solution of purified mitochondrial phenol oxidase from spinach leaves has been stored frozen, the Michaelis constant decreased with the lapse of time after thawing of the frozen enzyme solution, and reached a constant value, 4.7 × 10−3 m, in 2.5 hr. Immediately after thawing of the frozen enzyme solution, the oxidation of catechol catalyzed by the phenol oxidase was competitively inhibited by p-substituted phenols. The enzyme kept for 2.5 hr after thawing was allosterically inhibited by p-substituted phenols. These results suggest that the conformation of the enzyme changes with the lapse of time after thawing. However, a differential spectral study of the enzyme immediately after thawing and that 2.5 hr after thawing did not indicate any appreciable change of optical density in the range of 260 ~ 350 nm.