Pyrimidine metabolism during somatic embryo development in white spruce (Picea glauca)

Pyrimidine metabolism was investigated at various stages of somatic embryo development of white spruce (Picea glauca). The contribution of the de novo and the salvage pathways of pyrimidine biosynthesis to nucleotide and nucleic acid formation and the catabolism of pyrimidine was estimated by the exogenously supplied [6-14C]orotic acid, an intermediate of the de novo pathway, and with [2-14C]uridine and [2-14C]uracil, substrates of the salvage pathways. The de novo pathway was very active throughout embryo development. More than 80% of [6-14C]orotic acid taken up by the tissue was utilized for nucleotide and nucleic acid synthesis in all stages of this process. The salvage pathways of uridine and uracil were also operative. Relatively high nucleic acid biosynthesis from uridine was observed, whereas the contribution of uracil salvage to the pyrimidine nucleotide and nucleic acid synthesis was extremely limited. A large proportion of uracil was degraded as 14CO2, probably via beta-ureidopropionate. Among the enzymes of pyrimidine metabolism, orotate phosphoribosyltransferase was high during the initial phases of embryo development, after which it gradually declined. Uridine kinase, responsible for the salvage of uridine, showed an opposite pattern, since its activity increased as embryos developed. Low activities of uracil phosphoribosyltransferase and non-specific nucleoside phosphotransferase were also detected throughout the developmental period. These results suggest that the flux of the de novo and salvage pathways of pyrimidine nucleotide biosynthesis in vivo is roughly controlled by the amount of these enzymes. However, changing patterns of enzyme activity during embryo development that were measured in vitro did not exactly correlate with the flux estimated by the radioactive precursors. Therefore, other fine control mechanisms, such as the fluctuation of levels of substrates and/or effectors may also participate to the real control of pyrimidine metabolism during white spruce somatic embryo development.