METABOLIC-FATE OF GUANOSINE IN HIGHER-PLANTS

The aim of the present study was to investigate the metabolic fate of guanine nucleotides in higher plants. The rate of uptake of [8-C-14]gu anosine by suspension-cultured Catharanthus roseus cells was more than 20 times higher than that of [8-C-14]guanine. The rate of uptake of [ 8-C-14]guanosine increased with the age of the culture. Pulse-chase ex periments with [8-C-14]guanosine revealed that some of the guanosine t hat had been taken up by the cells was converted to guanine nucleotide s and incorporated into nucleic acids. A significant amount of [8-C-14 ]guanosine was degraded directly to xanthine, allantoin and allantoic acid, with the generation of (CO2)-C-14 as the final product. The rate of salvage of [8-C-14]guanosine for the synthesis of nucleic acids wa s highest in young cells, while the rate of degradation increased with the age of the cells. In segments of roots from Vigna mungo seedlings , nearly 50% of the [8-C-14]guanosine that had been absorbed over the course of 15 min was recovered in guanine nucleotides. A significant a mount of the radioactivity in nucleotides became associated with nucle ic acids and ureides during 'chase' periods. In segments of young leav es of Camellia sinensis, [8-C-14]guanosine was initially incorporated into guanine nucleotides, nucleic acids, theobromine and ureides, and the radioactivity in these compounds was transferred to caffeine and C O2 during a 24-h incubation. Our results suggest that guanosine is an intermediate in the catabolism of guanine nucleotides and that it is r e-utilised for nucleotide synthesis by 'salvage' reactions. Guanosine was catabolised by the conventional degradation pathway via xanthine a nd allantoin. In some plants, guanosine is also utilised for the forma tion of ureide or the biosynthesis of caffeine.