Changes in pyrimidine nucleotide biosynthesis during germination of white spruce (Picea glauca) somatic embryos

Changes in pyrimidine metabolism were investigated in germinating white spruce somatic embryos by following the metabolic fate of [2-14C]uracil and [2-14C]uridine, intermediate metabolites of the salvage pathway and [6-14C]orotic acid, a central metabolite of the de novo nucleotide biosynthesis. An active uridine salvage was found to be responsible for the enlargement of the nucleotide pool at the inception of germination. Uridine kinase, which catalyzes the conversion of uridine to uridine monophosphate (UMP), was found to be very active in partially dried embryos and during the early phases of imbibition. The contribution of uracil to the nucleotide pool was negligible since a large amount of radioactivity from [2-14C]uracil was recovered in degradation products. As germination progressed, the decline of the uridine salvage pathway was concomitant with an increase of the de novo biosynthetic pathway. The central enzyme of the de novo pathway, orotate phosphoribosyltransferase, showed increased activity and contributed to the larger amount of orotate being anabolized. These results suggest that although both the salvage and de novo pathways operate in germinating white spruce somatic embryos, their contribution to the enlargement of the nucleotide pool appears tightly regulated as germination progresses.