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2-oxypurine dehydrogenase from Micrococcus aerogenes. I. Isolation, specificity, and some chemical and physical properties.

Abstract Extracts of Micrococcus aerogenes, an anaerobe capable of fermenting purines, contain two enzymes which catalyze the oxidation of 2-oxypurine. These have been purified and separated from one another. One has the broad substrate specificity typical of xanthine dehydrogenase; the other is very specific with regard to the substrates oxidized and will only oxidize 2-oxypurine and closely related compounds. This new enzyme, 2-oxypurine dehydrogenase, catalyzes the oxidation of purines exclusively at position 6; other compounds are oxidized at the equivalent position. The following compounds, listed in order of decreasing reactivity when tested at 1 mm concentration, have been shown to serve as substrates: 2-oxypurine, 2-oxy-8-azaxanthine, 2,8-dioxypurine, 2-oxypyrimidine, and 2-oxypteridine. Compounds which are methylated at either of the nitrogen atoms adjacent to the required oxy group will neither serve as substrates nor inhibit the action of the enzyme. These and other results have been invoked to relate the activities observed with 2-oxypurine dehydrogenase to indices of chemical reactivity and to mechanisms proposed from studies of the substrate specificity of xanthine oxidase. In contrast to the marked specificity observed with electron donors, the enzyme uses a wide variety of artificial electron acceptors. However, molecular oxygen is very poorly utilized. The enzyme contains iron but not flavin or molybdenum. The protein has a visible absorption spectrum characteristic of certain non-heme iron proteins; the absorbance in the visible regions is decreased by incubation of the enzyme with 2-oxypurine. The enzyme displays cooperative saturation curves with substrates, suggesting the possibility of an allosteric response. The presence of a number of subunits in the protein is indicated by the observation that in acidic sucrose gradients the enzyme sediments at approximately one-fifth the rate found under alkaline conditions. Addition of substrate to the gradient specifically reverses the effect of an acid pH. Kinetic experiments suggest that the disaggregated enzyme is inactive but is capable of being activated by the substrate.