The Destruction of Epinephrine by the DOPA-Oxidase System of Ocular Tissue

Abstract

Since mydriatics are more effective in light than dark eyes, assays for enzymes possibly involved in the destruction of cholinergic and adrenergic mediators were undertaken. DOPA (dihydroxyphenylalanine) was oxidized by pigmented, but not by albino, rabbit iris and ciliary body homogenates. When mixtures of epinephrine plus DOPA were used as substrates, the O2-uptake by the pigmented homogenates was considerably greater than the sum of the values of the 2 substances alone and the rate was dependent upon the concn. of each (lower augmentation by autoxidation, and none in albino homogenates ). Addition of 0.001 [image] alpha-naphthothiourea (ANTU[long dash]a relatively selective inhibitor of plant tyrosinase) reduced homogenate O2-uptake by 50%, but did not affect the rate of autoxidation. No augmentation occurred when DOPA and norepinephrine mixtures were used as substrate. Two possible modes of action for the increased O2-uptake are suggested.