Nutritional Requirement for 4-Aminobenzoate Caused by Mutation of Dihydropteroate Synthetase

Rappold, H.; Bacher, Adelbert

doi:10.1515/znc-1976-5-612

Cited by 2 publications

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“…TLC analysis of methanol extracts of the harvested cells showed that Յ0.1% of the absorbed label was present as pABA-Glc. This negative result is consistent with the lack of reports of pABA-Glc in E. coli, whose pABA metabolism has been thoroughly studied (14,28).…”

Section: P-aminobenzoate Glucose Ester In Plantssupporting

confidence: 89%

The Folate Precursor p-Aminobenzoate Is Reversibly Converted to Its Glucose Ester in the Plant Cytosol

Quinlivan

Roje

Basset

et al. 2003

Journal of Biological Chemistry

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Plants synthesize p-aminobenzoate (pABA) in chloroplasts and use it for folate synthesis in mitochondria. It has generally been supposed that pABA exists as the free acid in plant cells and that it moves between organelles in this form. Here we show that fruits and leaves of tomato and leaves of a diverse range of other plants have a high capacity to convert exogenously supplied pABA to its ␤-D-glucopyranosyl ester (pABA-Glc), whereas yeast and Escherichia coli do not. High performance liquid chromatography analysis indicated that much of the endogenous pABA in fruit and leaf tissues is esterified and that the total pool of pABA (free plus esterified) varies greatly between tissues (from 0.2 to 11 nmol g ؊1 of fresh weight). UDP-glucose:pABA glucosyltransferase activity was readily detected in fruit and leaf extracts, and the reaction was found to be freely reversible. p-Aminobenzoic acid ␤-D-glucopyranosyl ester esterase activity was also detected in extracts. Subcellular fractionation indicated that the glucosyltransferase and esterase activities are predominantly if not solely cytosolic. Taken together, these results show that reversible formation of pABA-Glc in the cytosol is interposed between pABA production in chloroplasts and pABA consumption in mitochondria. As pABA is a hydrophobic weak acid, its uncharged form is membrane-permeant, and its anion is consequently prone to distribute itself spontaneously among subcellular compartments according to their pH. Esterification of pABA may eliminate such errant behavior and provide a readily reclaimable storage form of pABA as well as a substrate for membrane transporters.Tetrahydrofolate and its derivatives, collectively termed folates, are essential cofactors for one-carbon reactions in eukaryotes and most prokaryotes. Bacteria, fungi, and plants synthesize folates de novo, but humans and other higher animals do not and therefore need a dietary supply (1, 2). For humans, a major part of this supply comes from plants, and there is consequently interest in engineering food plants for enhanced folate content (3, 4). This engineering approach to human nutrition, termed biofortification, depends on understanding the synthesis, metabolism, and transport of folates and their precursors in plants.The folate molecule is tripartite, comprising pteridine, glutamate, and p-aminobenzoate (pABA) 1 moieties (Fig. 1A). In plants, biochemical and genomic data indicate that the pteridine moiety is made in the cytosol, that the pABA moiety is made in the chloroplast, and that the two are coupled together and glutamylated in the mitochondrion (Fig. 1B) (reviewed in Refs. 5 and 6). In the course of folate synthesis, pABA must therefore exit chloroplasts, cross the cytosol, and enter mitochondria. This elaborate trajectory is not seen in bacteria and fungi, in which the entire pathway is cytosolic (7,8).Recent reviews of plant folate synthesis have tacitly assumed that pABA exists as the free acid in plant cells and that it moves between organelles in this form (5, 6). This assum...

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Section: P-aminobenzoate Glucose Ester In Plantssupporting

confidence: 89%