are converted rapidly to trigonelline (7, N-methylnicotinate) or nicotinate glucosides (Ashihara et al. 2015). Typically, when the concentration of niacin, which comprises nicotinamide, nicotinate, and vitamin B3, is analysed in foodstuffs, the contribution of these individual components is not considered. Common extraction treatments, such as alkaline or acid hydrolysis, mean that any NAD, NADP, nicotinamide, and other nicotinate-related metabolites present in the sample are converted to nicotinate, which is the form that is then measured as niacin (Noctor et al. 2011).
2.3 Concentration of Cytokinins
Cytokinins are a class of phytohormones (aka growth substances) that promote cell division in plants. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. Cytokinins, such as isopentenyladenine (8), trans-zeatin (9), and benzyladenine (10) are adenine-derived plant hormones. The major sites of cytokinin biosynthesis are roots, cambium, and other actively dividing tissues. Cytokinins play vital roles in the regulation of multiple physiological processes in plants including cell proliferation and differentiation, nutrient allocation, leaf senescence, and responses to external biotic and abiotic signals (Bishop et al. 2015; Crozier et al. 2000).
Compared to purine nucleotides, the concentration of cytokinins is extremely low. Cytokinins are now routinely analysed using ultra-performance HPLC–high resolution mass spectrometry. The concentration of isopentenyladenine cytokinins, isopentenyladenine (8), isopentenyladenine ribotide (11), and isopentenyladenine riboside (12) in A. thaliana seedlings has been reported to be 0.4, 2.4, and 5.7 pmol g−1 f.w., respectively. While those of trans-zeatin cytokinins, trans-zeatin (9), trans-zeatin riboside (13) and trans-zeatin ribotide (14) were 0.6, 2.8, and 1.6 pmol g−1 f.w., respectively. Levels of dihydrozeatin (15) and benzyladenine cytokinins were <20 fmol g−1 f.w. (Novák et al. 2008).
2.4 Alkaloids Derived from Nucleotides
Some alkaloids, derived from purine and pyridine nucleotides, accumulate in specific plant species. Caffeine and nicotine (16) are typical nucleotide-derived alkaloids. For example, high concentrations of caffeine (∼2% d.w.) are found in tea (Ca. sinensis) and coffee (C. arabica) (Ashihara et al. 2017), while nicotine (∼1% d.w.) accumulates in tobacco (N. tabacum) (Saitoh et al. 1985). These alkaloids occur in more than 100 plant species. Detailed distribution and concentration of these alkaloids are described in purine and pyridine alkaloids in Parts V and VI.
2.5 Summary
Occurrence of major purine, pyrimidine, and pyridine nucleotides in plants are summarized. For comparison, cytokinins and nucleotide alkaloids are also noted. In most plants, the concentration of nucleotides is higher than nucleosides and bases. Cytokinins occur in much lower amounts than purine nucleotides. A much higher concentration of caffeine is found in tea and coffee plants.
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