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
