Orotidine-5′-Phosphate Decarboxylase and Pyrophosphorylase of Bean Leaves

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Abstract
This report includes results demonstrating the existence of orotidine-5′-phosphate decarboxylase and orotidine-5′-phosphate pyrophosphorylase in plant leaves. The decarboxylase enzyme, purified 8 fold from leaves of etiolated pinto beans (Phaseolus vulgaris L.), had a pH optimum of 6.3. It was strongly inhibited by 6-azauridine-5′-phosphate; a concentration of 12 μm decreased the reaction rate 60%. The enzyme was not dependent upon magnesium ions or inhibited by p-chloromercuribenzoate. It was present in other parts of the bean plant and was found in young leaves of tomato (Lycopersicon esculentum Mill.) and Canada thistle (Cirsium arvense L.) The enzyme orotidine-5′-phosphate pyrophosphorylase, which catalyzes the formation of orotidine-5′-phosphate from orotic acid and 5-phosphoribosyl-1-pyrophosphate, was found in the etiolated bean leaves, and was also present in the leaves of tomato and Canada thistle. It was stimulated by manganous or magnesium ions and had a pH optimum of 7.2. The Km value obtained by varying the concentrations of 5-phosphoribosyl-1-pyrophosphate was 75 μm, and when orotic acid was varied the resulting Km was 3.5 μm. The presence of these 2 enzymes in higher plants, combined with previous results with inhibitors and labeled metabolites, indicates that the normal pathway of pyrimidine nucleotide synthesis in higher plants proceeds through orotic acid and OMP.