Orotidine-5′-Phosphate Pyrophosphorylase of Wheat Embryos

Orotidine-5′-phosphate pyrophosphorylase was isolated from wheat embryos and purified 35-fold. The enzyme is inhibited by high concentrations of orotate. Cu++and Mn++ions can replace the Mg++ion requirement. The enzyme shows a primary and secondary optimum at pH 8.5 and 9.8. Dihydro-orotate:NAD dehydrogenase also was demonstrated in extracts of wheat embryos, but there was no evidence for an orotate decarboxylase. Embryo homogenates incubated with orotate-6-C14formed measurable amounts of radioactive OMP and UMP, which were increased in the presence of P-ribosyl-PP but were decreased by the addition of benzimidazole.