Folate synthesis in plants: the last step of the p-aminobenzoate branch is catalyzed by a plastidial aminodeoxychorismate lyase

A nudix enzyme removes pyrophosphate from dihydroneopterin triphosphate in the folate synthesis pathway of bacteria and plants

Removal of pyrophosphate from dihydroneopterin triphosphate (DHNTP) is the second step in the pterin branch of the folate synthesis pathway. There has been controversy over whether this reaction requires a specific pyrophosphohydrolase or is a metal ion-dependent chemical process. The genome of Lactococcus lactis has a multicistronic folate synthesis operon that includes an open reading frame (ylgG) specifying a putative Nudix hydrolase. Because many Nudix enzymes are pyrophosphohydrolases, YlgG was expressed in Escherichia coli and characterized. The recombinant protein showed high DHNTP pyrophosphohydrolase activity with a K(m) value of 2 microM, had no detectable activity against deoxynucleoside triphosphates or other typical Nudix hydrolase substrates, required a physiological level (approximately 1 mM) of Mg(2+), and was active as a monomer. Essentially no reaction occurred without enzyme at 1 mM Mg(2+). Inactivation of ylgG in L. lactis resulted in DHNTP accumulation and folate depletion, confirming that YlgG functions in folate biosynthesis. We therefore propose that ylgG be redesignated as folQ. The closest Arabidopsis homolog of YlgG (encoded by Nudix gene At1g68760) was expressed in E. coli and shown to have Mg(2+)-dependent DHNTP pyrophosphohydrolase activity. This protein (AtNUDT1) was reported previously to have NADH pyrophosphatase activity in the presence of 5 mM Mn(2+) (Dobrzanska, M., Szurmak, B., Wyslouch-Cieszynska, A., and Kraszewska, E. (2002) J. Biol. Chem. 277, 50482-50486). However, we found that this activity is negligible at physiological levels of Mn(2+) and that, with 1 mM Mg(2+), AtNUDT1 prefers DHNTP and (deoxy) nucleoside triphosphates.

Free and total para-aminobenzoic acid analysis in plants with high-performance liquid chromatography/tandem mass spectrometry

para-Aminobenzoic acid (PABA), a precursor in the synthesis of folates in plants, is determined by liquid chromatography/tandem mass spectrometry (LC/MS/MS). In plants PABA can be converted into its beta-D-glucopyranosyl ester (PABA-Glc) and can also exist in its free form. In this work, we developed and validated a quantitative method to study free and total PABA in plants. The total PABA (free PABA plus PABA-Glc) can be evaluated after acid hydrolysis at 80 degrees C for 2 hours. The plant material is homogenized and the PABA content is quantified using the standard addition procedure. The validated method is selective, sensitive, simple, accurate, has a recovery between 99.6 to 102.5%, is reproducible (RSD between 1.4 and 4.4%), and is linear between 2.5 and 1538 ng/mL. Free and total PABA determinations in five vegetables showed that different plant species had different amounts of free and total PABA, and that the ratios of total versus free PABA were also variable. This new method could be valuable for studies of folate synthesis in plants.