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.

5.3 Properties of Purine Phosphoribosyltransferases

Phosphoribosyltransferases are enzymes which mediate nucleotide formation through the transfer of nucleobases to PRPP with the release of PPi as shown in reaction 1 in Figure 5.2.

      5.3.1 Adenine Phosphoribosyltransferase

Adenine phosphoribosyltransferase (APRT) (EC 2.4.2.7) catalyses the formation of AMP from adenine and PRPP (reaction 1 in Figure 5.1). APRT, and PRPP, the substrate of this enzyme, was first isolated from yeast by Arthur Kornberg, the American Nobel Prize Laureate, and subsequently APRT was found in bacteria, animals and plants (Kornberg et al. 1955a,b).

Kinetic properties of the native and recombinant APRT and HGPRT with the Arabidopsis Information Resource (TAIR) locus of the genes encoding these phosphoribosyltransferases in A. thaliana are listed in Table 5.2. Studies on APRT from several plants indicate high affinity with adenine; the Km values are usually low (1–10 μM). In contrast, the Km values for PRPP can fluctuate between 1 and 300 μM. APRT activity is inhibited by AMP, but no significant effect has been observed with other nucleotides (Hirose and Ashihara 1983).

      The Arabidopsis genome has five APRT sequences (Table 5.2), three of which, APT1, APT2, and APT3, have been cloned, overexpressed, and their catalytic properties characterized (Moffatt et al. 1992). All three isozymes bind adenine (Km: <3 μM). One of the isoenzymes (APT1) is 40–80 times more efficient, judged by the Vmax/Km ratio, in metabolizing adenine than the other isozymes. Since none of the predicted amino acid sequences for the adenine phosphoribosyltransferases (APTs) of A. thaliana appear to contain transit signalling peptides, APTs are assumed to be located in the cytosol. However, the findings of Zybailov et al. (2008) suggest that an isoform of APT1 has a transit peptide for chloroplasts. Traditional biochemical analysis indicates that in addition to the cytosol, APRT also occurs in plastids and mitochondria in some plants, (see Ashihara et al. 2018). Biochemical analysis indicates that large quantities of APRT activity are located in cytosol, but substantial amounts are also found in chloroplasts of spinach (Ashihara and Ukaji 1985) and tea leaves (Koshiishi et al. 2001), as well as mitochondria of Catharanthus roseus cells (Hirose and Ashihara 1982) and tubers of Jerusalem artichoke (Helianthus tuberosus) (Le Floc'h and Lafleuriel 1983). Although the occurrence of APRT in mitochondria has not yet been confirmed at the molecular level, purified intact mitochondria from C. roseus are capable of synthesizing adenine nucleotides from ¹⁴C-labelled adenine (Ukaji et al. 1986).

      5.3.2 Hypoxanthine/Guanine Phosphoribosyltransferase

      HGPRT (IMP/GMP): diphosphate phospho-D-ribosyltransferase, EC 2.4.2.8) catalyses the formation of IMP (or GMP) from inosine (or guanine) and PRPP. Although HGPRT activity is generally high in mammalian tissues (Adams and Harkness 1976), in plants it is usually much lower than that of APRT (Table 5.1).