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Dihydropterins are intermediates of folate synthesis and products of folate breakdown that are readily oxidized to their aromatic forms. In trypanosomatid parasites, reduction of such oxidized pterins is crucial for pterin and folate salvage. We therefore sought evidence for this reaction in plants. Three lines of evidence indicated its absence. First, when pterin-6-aldehyde or 6-hydroxymethylpterin was supplied to Arabidopsis (Arabidopsis thaliana), pea (Pisum sativum), or tomato (Lycopersicon esculentum) tissues, no reduction of the pterin ring was seen after 15 h, although reduction and oxidation of the side chain of pterin-6-aldehyde were readily detected. Second, no label was incorporated into folates when 6-[^sup 3^H]hydroxy-methylpterin was fed to cultured Arabidopsis plantlets for 7 d, whereas [^sup 3^H]folate synthesis from p-[^sup 3^H]aminobenzoate was extensive. Third, no NAD(P)H-dependent pterin ring reduction was found in tissue extracts. Genetic evidence showed a similar situation in Escherichia coli: a GTP cyclohydrolase I (folE) mutant, deficient in pterin synthesis, was rescued by dihydropterins but not by the corresponding oxidized forms. Expression of a trypanosomatid pterin reductase (PTR1) enabled rescue of the mutant by oxidized pterins, establishing that E. coli can take up oxidized pterins but cannot reduce them. Similarly, a GTP cyclohydrolase I (fol2) mutant of yeast (Saccharomyces cerevisiae) was rescued by dihydropterins but not by most oxidized pterins, 6-hydroxymethylpterin being an exception. These results show that the capacity to reduce oxidized pterins is not ubiquitous in folate-synthesizing organisms. If it is lacking, folate precursors or breakdown products that become oxidized will permanently exit the metabolically active pterin pool.
Pterins have a wide range of metabolic roles, including as essential intermediates in folate biosynthesis, as products of folate breakdown (Scott et al., 2000; Suh et al., 2001), and as cofactors for aromatic hydroxylases (Zhao et al., 1994; Thony et al., 2000; Yamamoto et al., 2001). The oxidation state of the pterin ring (Fig. 1A) is crucial to biochemical function because only reduced (dihydro or tetrahydro) forms are metabolically active. The intermediates of folate synthesis are dihydro forms, as is the major pterin formed by folate breakdown (Fig. 1B). Pterin cofactors are tetrahydro forms.
Reduced pterins readily autoxidize to their fully oxidized (aromatic) state (Fig. 1B, mauve arrows; Pfleiderer, 1985). A capacity to reduce oxidized pterins to their bioactive forms would thus...