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

Changes in pyrimidine metabolism were investigated in germinating white spr uce somatic embryos by following the metabolic fate of [2-C-14]uracil and [ 2-C-14]uridine, intermediate metabolites of the salvage pathway and [6-C-14 ]orotic acid, a central metabolite of the de novo nucleotide biosynthesis. An active uridine salvage was found to be responsible for the enlargement o f the nucleotide pool at the inception of germination. Uridine kinase, whic h catalyzes the conversion of uridine to uridine monophosphate (UMP), was f ound to be very active in partially dried embryos and during the early phas es of imbibition. The contribution of uracil to the nucleotide pool was neg ligible since a large amount of radioactivity from [2-C-14]uracil was recov ered in degradation products. As germination progressed, the decline of the uridine salvage pathway was concomitant with an increase of the de novo bi osynthetic pathway, The central enzyme of the de novo pathway, orotate phos phoribosyltransferase, showed increased activity and contributed to the lar ger 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 po ol appears tightly regulated as germination progresses.