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5 Salvage Pathways of Purine Nucleotide Biosynthesis
5.1 Introduction
Traditionally the term ‘salvage’ refers to the rescue of a wrecked or disabled ship or its cargo, but it also is used in a much narrower sense in biochemistry for the recovery of preformed bases and nucleosides for the regeneration of nucleotides and nucleic acids in nucleotide metabolism. Some salvage pathways, which recycle purine bases and nucleosides for nucleotide synthesis, function in plants and other organisms (Ashihara et al. 2018; Moffatt and Ashihara 2002). The net formation of purine nucleotides is performed by the de novo pathway, but rapid turnover of nucleic acids, especially RNA, is required for nucleotide production by the salvage pathways.
In contrast to de novo purine nucleotide synthesis, which consists of energy consuming multistep reactions when compounds with a purine skeleton are available, purine nucleotides can be synthesized by simpler savage pathways. In salvage pathways, 1 mol of 5-phosphoribosyl-1-pyrophosphate (PRPP) or adenosine-5′-monophosphate (AMP) is utilized for the synthesis of 1 mol of purine nucleotides from a purine base or a purine nucleoside, respectively, by purine phosphoribosyltransferases or purine nucleoside kinases.
In plant cells, purine bases and nucleosides originate from the intercellular breakdown of nucleic acids and nucleotides, as well as other reactions which release purine bases and nucleosides. These substrates can be transported from other tissues, such as storage organs and senescing leaves. It is also possible that they are taken up by the roots from the soil where the purines are breakdown products of fallen leaves, dead insects and bacteria. Salvage of purine bases and nucleosides is performed by phosphoribosyltransferases and nucleoside kinases. Reduced salvage activity inhibits the normal growth of plants and other organisms (Ashihara et al. 2018).
In mammals, genetic deficiency of the purine base salvage enzyme, hypoxanthine/guanine phosphoribosyltransferase (HGPRT, EC 2.4.2.8), causes Lesch–Nyhan syndrome which is characterized by neurological and behavioural abnormalities and the overproduction of uric acid (Nyhan 1997). In plants, adenine phosphoribosyltransferase (APRT, EC 2.4.2.7) appears to be a more important enzyme in purine salvage than HGPRT. In addition to producing adenine nucleotides for energy metabolism and nucleic acid biosynthesis, APRT and adenosine kinase (AK) are involved in the removal of adenine and adenosine, which act as inhibitors of several reactions. These salvage enzymes are also involved in caffeine biosynthesis and the activation and inactivation of cytokinins (see Part VI). In addition, deficiency of APRT causes male sterility in plants (Gaillard et al. 1998). Thus, purine salvage is not only the energy-saving route of nucleotide biosynthesis, but is an essential pathway in plants. For a comprehensive review on purine salvage in plants, see Ashihara et al. (2018).
5.2 Characteristics of Purine Salvage in Plants
The 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).
