Properties of copper‐dependent o‐diphenol oxidase activity in the potato aphid Macrosiphum euphorbiae (thomas)

Potato aphid Macrosiphum euphorbiae (Thomas) was found to contain high amounts of o‐diphenol oxidase activity. Enzyme activity was largely distributed into the postmitochondrial supernatant from Brij‐35 extracted aphids and occurs in a latent form that was activated up to 45‐fold by pretreatment with isopropanol. The aphid enzyme has a broad pH optimum near 6, and utilized L‐dopa (K_m = 1.4 mM, V_max = 348 nmol/min‐mg protein), dopamine, and 4‐methylcatechol the best out of the twelve substrates tested. In addition, this activity is a typical copper‐dependent oxidase in that it is potently inhibited by phenylthiourea (50% inhibition at 30nM) and other copper chelators, including salicylhydroxamic acid. The above properties are common to most insect tyrosinases. However, the aphid enzyme lacked the o‐hydroxylase and laccase components and the optimal activity at higher temperatures that are typical of cuticular tyrosinases of other insects. The high levels of o‐diphenol oxidase in aphids compared to other insects is surprising, since the major function associated with these enzymes, that of melanization and sclerotization of cuticle, is of much less importance to aphids. The possibility that aphids use this enzyme to metabolize dietary phenolics is discussed.