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 nucl eic acid formation and the catabolism of pyrimidine was estimated by the ex ogenously supplied [6-C-14]erotic acid, an intermediate of the de novo path way, and with [2-C-14]uridine and [2-C-14]uracil, substrates of the salvage . pathways. The de novo pathway was very active throughout embryo developme nt. More than 80% of [6-C-14]orotic acid taken up by the tissue was utilize d 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 contr ibution of uracil salvage to the pyrimidine nucleotide and nucleic acid syn thesis was extremely limited. A large proportion of uracil was degraded as (CO2)-C-14, probably via beta -ureidopropionate. Among the enzymes of pyrim idine metabolism, orotate phosphoribosyltransferase was high during the ini tial phases of embryo development. after which it gradually declined. Uridi ne kinase,responsible for the salvage of uridine, showed an opposite patter n, since its activity increased as embryos developed. Low activities of ura cil phosphoribosyltransferase and non-specific nucleoside phosphotransferas e were also detected throughout the developmental period. These results sug gest that the flux of the de novo and salvage pathways of pyrimidine nucleo tide biosynthesis in vivo is roughly controlled by the amount of these enzy mes. However, changing patterns of enzyme activity during embryo developmen t that were measured in vitro did not exactly correlate with the flux estim ated by the radioactive precursors. Therefore, other fine control mechanism s, such as the fluctuation of levels of substrates and/or effecters may als o participate to the real control of pyrimidine metabolism during white spr uce somatic embryo development.