Plant Nucleotide Metabolism. Hiroshi Ashihara

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
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