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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Pterin and Folate Salvage. Plants and Escherichia coli Lack Capacity to Reduce Oxidized Pterins^1,[OA]

Horticultural Sciences Department (A.N., A.D.H.) and Food Science and Human Nutrition Department (V.N., J.F.G.), University of Florida, Gainesville, Florida 32611

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-[³H]hydroxymethylpterin was fed to cultured Arabidopsis plantlets for 7 d, whereas [³H]folate synthesis from p-[³H]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.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Andrew D. Hanson (adha{at}mail.ifas.ufl.edu).

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www.plantphysiol.org/cgi/doi/10.1104/pp.106.093633

^* Corresponding author; e-mail adha{at}mail.ifas.ufl.edu; fax 352–392–5653.

Received November 23, 2006; accepted December 20, 2006; published January 12, 2007.