Читать книгу Plant Nucleotide Metabolism - Hiroshi Ashihara - Страница 68
5.2 Characteristics of Purine Salvage in Plants
ОглавлениеThe purine salvage mechanism in plants is essentially the same in microorganisms and animals but some of the participating enzymes are different. Profiles of in vitro activity of enzymes involved in purine salvage has been reported in cell-free preparations from young tuber tissues of potato (Solanum tuberosum) (Katahira and Ashihara 2006) and young leaves of tea (Camellia sinensis) (Deng and Ashihara 2010), see Table 5.1.
In plants, as shown in Figure 5.1, purine bases are salvaged by phosphoribosyltransferases (reactions 1, 2, and/or 2a), while purine nucleosides are salvaged by nucleoside kinases (reaction 3 and/or 4) and nucleoside phosphotransferase (NPT) (reaction 7). In some cases, purine nucleosides are hydrolysed to purine bases by nucleosidases (reaction 8 and/or 9) and then salvaged by the phosphoribosyltransferases mentioned above.
Possible salvage routes of four purine bases (adenine, hypoxanthine, guanine, and xanthine) and six purine nucleosides (adenosine, inosine, guanosine, xanthosine, deoxyadenosine, and deoxyguanosine) are illustrated in Figure 5.1. Activity profiles of enzymes involved in purine salvage in potato tubers and tea leaves are shown in Table 5.1.
Purine base salvage in plants is carried out by two distinct phosphoribosyltransferases, APRT and HGPRT. In mammalian tissues HGPRT has a central role in the purine salvage (Adams and Harkness 1976). However, in plant tissues activity of this enzyme is much lower (∼6%) than that of APRT (Table 5.1). In bacteria and animals purine bases can also be converted to their respective purine nucleosides, utilizing ribose-1-phosphate, by purine nucleoside phosphorylase (EC 2.4.2.1) (Bzowska et al. 2000). However, this phosphorylase was not detected in enzyme extracts from yellow lupin seeds and seedlings (Guranowski 1982), potato tubers (Katahira and Ashihara 2006), or tea leaves (Deng and Ashihara 2010). Hence, the production of ribonucleotides by purine nucleoside phosphorylase in plants would appear to have a very minor role in purine salvage.
Purine nucleosides, namely, adenosine, inosine, guanosine, deoxyadenosine, and deoxyguanosine are converted to their respective nucleoside monophosphates. In plants, as illustrated in Figure 5.1, there are three possible routes: (i) direct formation by purine nucleoside kinases (steps 3–6); (ii) interconversion by a non-specific NPT (step 7); and (iii) a two-step reaction, hydrolysis of nucleosides to bases (steps 8 or 9) and salvage by phosphoribosyltransferases (steps 1 or 2).
There are two distinct purine ribonucleoside kinases in plants, AK (EC 2.7.1.20) and inosine/guanosine kinase (IGK) (EC 2.7.1.73). Deoxyadenosine kinase (dAK) (EC 2.7.1.76) and deoxyguanosine kinase (dGK) (EC 2.7.1.113) activities have been detected in plant extracts, and they participate in phosphorylation of deoxyribonucleoside. However, a non-specific deoxyribonucleoside kinase (EC 2.7.1.145) may be the main contributor to deoxyribonucleoside salvage, at least in Arabidopsis thaliana.
In addition to nucleoside kinases, NPT (aka non-specific NPT, EC 2.7.1.77) also participates in purine nucleoside salvage. High activity is found with adenosine, inosine, guanosine, deoxyadenosine, and deoxyguanosine, but there is an absence of activity with xanthosine and xanthine (Table 5.1).
Table 5.1 Profile of activity of enzymes involved in purine salvage in growing potato tubers (Katahira and Ashihara 2006) and young tea leaves (Deng and Ashihara 2010).
| Enzyme (EC number) | Reaction (step number in Figure 5.1) (*14C-Labelled substrates and products) | Potato tubers | Tea leaves |
| 1) Phosphoribosyltransferases | |||
| Adenine phosphoribosyltransferase (2.4.2.7) | Adenine* + PRPP → AMP* + PPi (1) | 101.4 | 96.9 |
| Hypoxanthine/guanine phosphoribosyltransferase (2.4.2.8) | Hypoxanthine* + PRPP → IMP* + PPi (2) | 6.4 | 0.79 |
| Guanine* + PRPP → GMP* + PPi (2) | 4.8 | 1.26 | |
| Xanthine phosphoribosyltransferase (2.4.2.22) | Xanthine* + PRPP → XMP* + PPi | 0.22 | |
| 2) Nucleoside kinase | |||
| Adenosine kinase (2.7.1.20) | Adenosine* + ATP → AMP* + ADP (3) | 76.3 | 34.7 |
| Inosine/guanosine kinase (2.7.1.73) | Inosine* + ATP → IMP* + ADP (4) | 16.8 | 0.37 |
| Guanosine* + ATP → GMP* + ADP (4) | 15.5 | 0.13 | |
| Xanthosine* + ATP → XMP* + ADP | a) | a) | |
| Deoxyadenosine kinase (2.7.1.76) | Deoxyadenosine* + ATP → dAMP* + ADP (5) | 75.9 | |
| Deoxyguanosine kinase (2.7.1.113) | Deoxyguanosine* + ATP → dGMP* + ADP (6) | 53.8 | |
| 3) Nucleoside phosphotransferase | |||
| Nucleoside phosphotransferase (2.7.1.77) | Adenosine* + AMP → AMP* + Adenosine (1) | 39.7 | |
| Inosine* + AMP → IMP* + Adenosine (6) | 19.8 | ||
| Guanosine* + AMP → GMP* + Adenosine (7) | 20.9 | ||
| Xanthosine* + AMP → XMP* + Adenosine | a) | ||
| Xanthosine* + IMP → XMP* + Inosine | a) | ||
| Deoxyadenosine* + AMP → dAMP* + Adenosine (4) | 89.8 | ||
| Deoxyguanosine* + AMP → dGMP* + Adenosine (5) | 55.0 | ||
| 4) Nucleosidases | |||
| Adenosine nucleosidase (3.2.2.7) | Adenosine* → Adenine* + Ribose (8) | 9.5 | 84.9 |
| Deoxyadenosine* → Adenine* + Deoxyribose (8) | 13.2 | ||
| Inosine/guanosine nucleosidase (3.2.2.2) | Inosine* → Hypoxanthine* + Ribose (9) | 1.1 | 9.21 |
| Guanosine* → Guanine* + Ribose (9) | 0.1 | 8.53 | |
| Deoxyguanosine* → Guanine* + Deoxyribose (9) | <0.1 | ||
| Purine nucleosidase (3.2.2.1) | Xanthosine* → Xanthine* + Ribose (9) | 8.7 | 29.1 |
Enzyme activity is expressed as pkat mg−1 protein; a) no activity detected.
Figure 5.1 Outline of purine salvage reactions and related enzymes in plants. Phosphoribosyltransferases: adenine phosphoribosyltransferase (1), hypoxanthine/guanine phosphoribosyltransferase (2), xanthine phosphoribosyltransferase or side reaction of hypoxanthine/guanine phosphoribosyltransferase (2a). Nucleoside kinases: adenosine kinase (3), inosine/guanosine kinase (4), deoxyadenosine kinase (5), deoxyguanosine kinase (6). Steps 5 and 6 may be catalysed by deoxynucleoside kinase (see Section 5.3.3). Nucleoside monophosphate phosphotransferase: non-specific nucleoside monophosphate phosphotransferase (7). Nucleosidases: adenosine nucleosidase (8), inosine/guanosine nucleosidase (9). Steps 8 and 9 are also catalysed by non-specific purine nucleosidase. Nucleosides are sometimes hydrolysed to purine bases and salvaged by purine phosphoribosyltransferase.
Since activity of adenosine nucleosidase (EC 3.2.2.7), inosine/guanosine nucleosidase (EC 3.2.2.2), and/or purine nucleosidase (EC 3.2.2.1) occur in potato tubers and tea leaves (Table 5.1), simple hydrolysis of purine nucleosides to purine bases would appear to be the main route in plants. However, purine base formation from purine nucleosides catalysed by purine phosphorylase, or by the reverse reaction of phosphoribosyltransferases, appears not to participate in this hydrolysis in plants. Properties of nucleosidases are described with the interconversion of purines in Chapter 6.
