Content and Availability of 5-Phosphoribosyl-1-pyrophosphate in Cultured Cells of Catharanthus roseus

Purine salvage in plants

Purine bases and nucleosides are produced by turnover of nucleotides and nucleic acids as well as from some cellular metabolic pathways. Adenosine released from the S-adenosyl-L-methionine cycle is linked to many methyltransferase reactions, such as the biosynthesis of caffeine and glycine betaine. Adenine is produced by the methionine cycles, which is related to other biosynthesis pathways, such those for the production of ethylene, nicotianamine and polyamines. These purine compounds are recycled for nucleotide biosynthesis by so-called "salvage pathways". However, the salvage pathways are not merely supplementary routes for nucleotide biosynthesis, but have essential functions in many plant processes. In plants, the major salvage enzymes are adenine phosphoribosyltransferase (EC 2.4.2.7) and adenosine kinase (EC 2.7.1.20). AMP produced by these enzymes is converted to ATP and utilised as an energy source as well as for nucleic acid synthesis. Hypoxanthine, guanine, inosine and guanosine are salvaged to IMP and GMP by hypoxanthine/guanine phosphoribosyltransferase (EC 2.4.2.8) and inosine/guanosine kinase (EC 2.7.1.73). In contrast to de novo purine nucleotide biosynthesis, synthesis by the salvage pathways is extremely favourable, energetically, for cells. In addition, operation of the salvage pathway reduces the intracellular levels of purine bases and nucleosides which inhibit other metabolic reactions. The purine salvage enzymes also catalyse the respective formation of cytokinin ribotides, from cytokinin bases, and cytokinin ribosides. Since cytokinin bases are the active form of cytokinin hormones, these enzymes act to maintain homeostasis of cellular cytokinin bioactivity. This article summarises current knowledge of purine salvage pathways and their possible function in plants and purine salvage activities associated with various physiological phenomena are reviewed.

Purine and pyrimidine nucleotide metabolism in higher plants

Purine and pyrimidine nucleotides participate in many biochemical processes in plants. They are building blocks for nucleic acid synthesis, an energy source, precursors for the synthesis of primary products, such as sucrose, polysaccharides, phospholipids, as well as secondary products. Therefore, biosynthesis and metabolism of nucleotides are of fundamental importance in the growth and development of plants. Nucleotides are synthesized both from amino acids and other small molecules via de novo pathways, and from preformed nucleobases and nucleosides by salvage pathways. In this article the biosynthesis, interconversion and degradation of purine and pyrimidine nucleotides in higher plants are reviewed. This description is followed by an examination of physiological aspects of nucleotide metabolism in various areas of growth and organized development in plants, including embryo maturation and germination, in vitro organogenesis, storage organ development and sprouting, leaf senescence, and cultured plant cells. The effects of environmental factors on nucleotide metabolism are also described. This review ends with a brief discussion of molecular studies on nucleotide synthesis and metabolism.

Purine metabolism during white spruce somatic embryo development: salvage of adenine, adenosine, and inosine

Contribution of the adenine, adenosine and inosine salvage to the purine nucleotide and nucleic acid biosynthesis during white spruce (Picea glauca) somatic embryo maturation was estimated by in situ assays using [8-(14)C]adenine, [8-(14)C]adenosine and [8-(14)C]inosine. The salvage of adenine and adenosine was high during the initial stages of embryo maturation, characterized by rapid cell proliferation, but it declined upon further embryo development. Inosine salvage activity was always much lower than that observed for adenine and adenosine. Consistent with these results, activities of adenine phosphoribosyltransferase (APRT) and adenosine kinase (AK) measured in the embryo extracts in vitro were much higher than the activity of inosine kinase (IK) during all stages of embryo development. Utilization of adenosine and inosine for nucleotide and nucleic acid synthesis was found to be regulated by the enzymes AK and IK, as the pattern of their activities was very similar to the activity of adenosine and inosine salvage, estimated with exogenously supplied precursors. However, little correlation between salvage of adenine and activity of APRT was found throughout somatic embryo maturation. As no adenosine nucleosidase activity was found in white spruce embryos, adenosine, but not adenine, seems to be the major end product of adenylate catabolism and becomes the predominant substrate for purine salvage in vivo. Thus, adenosine salvage appeared to have the most important role in white spruce embryos. Studies on the metabolic fate of [8-(14)C]adenine and [8-(14)C]adenosine suggest that turnover of adenine nucleotides is rapid, as some of them are utilized for nucleic acid synthesis. In contrast, most of [8-(14)C]inosine taken up by the embryos seems to be directly catabolized by the conventional purine catabolic pathway via ureides in all stages of embryo maturation.

Identification of non-equilibrium glycolytic reactions in suspension-cultured plant cells

Levels of all enzymes and metabolites involved in glycolysis were determined in suspension-cultured Catharanthus roseus cells. From both the maximum catalytic activities of the enzymes and comparisons of mass-action ratios with the apparent equilibrium constants for the reactions, it is concluded that the reactions catalysed by hexokinase, fructokinase, phosphofructokinase and pyruvate kinase are far from equilibrium, whereas the other reactions including that catalysed by pyrophosphate-fructose-6-phosphate 1-phosphotransferase, are close to equilibrium in vivo.

Phosphoribosyl pyrophosphate and the measurement on inorganic pyrophosphate in plant tissues

This work provides further evidence that plants contain appreciable amounts of inorganic pyrophosphate (PPi), and that breakdown of phosphoribosyl pyrophosphate (PPRibP) does not contribute significantly to the PPi detected in plant extracts. Inorganic pyrophosphate in extracts of the roots of Pisum sativum L., clubs of the spadices of Arum maculatum L., and the developing endosperm of Zea mays L. was assayed with pyrophosphate fructose 6-phosphate 1-phosphotransferase (EC 2.7.1.90), and with sulphate adenyltransferase (EC 2.7.7.4). The two different assays gave the same value for PPi content, and for recovery of added PPi. It was shown that PPRibP is converted to PPi during the extraction of PPi. However, the amounts of PPRibP in clubs of A. maculatum and the developing endosperm of Z. mays were negligible in comparison with the contents of PPi.