Adenine Phosphoribosyltransferase of Catharanthus roseus Cells: Purification, Properties and Regulation

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.

Catabolism of adenine nucleotides in suspension-cultured plant cells

Profiles of the catabolism of adenine nucleotides in cultured plant cells were investigated. Adenine nucleotides, prelabelled by incubation of suspension-cultured Catharantus roseus cells with [8-14C]adenosine, were catabolized rapidly and most of the radioactivity appeared in 14CO2. Allantoin and allantoic acid, intermediates of the oxidative catabolic pathway of purines, were temporarily labelled. When the cells, prelabelled with [8-14C]adenosine, were incubated with high concentrations of adenosine, the rate of catabolism of adenine nucleotides increased. The results suggest that the relative rate of catabolism of adenine nucleotides is strongly dependent on the concentration of adenine nucleotides in the cells. Studies using allopurinol, coformycin and tiazofurin, inhibitors of enzymes involved in purine metabolism, suggest that participation of AMP deaminase and xanthine oxidoreductase in the catabolism of adenine nucleotides in plant cells. AMP deaminase was found in extracts from C. roseus cells and its activity increased significantly in the presence of ATP. In contrast, no adenosine deaminase or adenine deaminase activity was detected. Qualitative differences in the catabolic activity of AMP were observed between suspension-cultured cells from different species of plants.

Purification of adenine phosphoribosyltransferase from Brassica juncea

Adenine phosphoribosyltransferase was purified from Brassica juncea leaves approximately 4000-fold, to homogeneity. The native enzyme is a homodimer, with a Mr of 54,000. The purification involved (NH4)2SO4 fractionation, differential ultracentrifugation, and anion-exchange, hydrophobic, dye-ligand, and affinity chromatography. The purified enzyme has a pH optimum of 9.15 and a temperature optimum of 60 degrees C. Activity of the enzyme is stimulated by Mg2+ and is inhibited by sulfhydryl reagents. At the optimum pH and 37 degrees C, the apparent Km values for adenine and 5-phosphoribosyl-1-pyrophosphate were 3.8 and 15 microM, respectively. Analysis of the purified protein by isoelectric focusing revealed the presence of two isozymes with approximate isoelectric points of 5.3 and 5.4.