Metabolism of the folate precursor p-aminobenzoate in plants: glucose ester formation and vacuolar storage.
Plants produce p-aminobenzoate (pABA) in chloroplasts and use it for folate synthesis in mitochondria. In plant tissues, however, pABA is known to occur predominantly as its glucose ester (pABA-Glc), and the role of this metabolite in folate synthesis has not been defined. In this study, the UDP-glucose:pABA acyl-glucosyltransferase (pAGT) activity in Arabidopsis extracts was found to reside principally (95%) in one isoform with an apparent K(m) for pABA of 0.12 mm. Screening of recombinant Arabidopsis UDP-glycosyltransferases identified only three that recognized pABA. One of these (UGT75B1) exhibited a far higher k(cat)/K(m) value than the others and a far lower apparent K(m) for pABA (0.12 mm), suggesting its identity with the principal enzyme in vivo. Supporting this possibility, ablation of UGT75B1 reduced extractable pAGT activity by 95%, in vivo [(14)C]pABA glucosylation by 77%, and the endogenous pABA-Glc/pABA ratio by 9-fold. The K(eq) for the pABA esterification reaction was found to be 3 x 10(-3). Taken with literature data on the cytosolic location of pAGT activity and on cytosolic UDP-glucose/UDP ratios, this K(eq) value allowed estimation that only 4% of cytosolic pABA is esterified. That pABA-Glc predominates in planta therefore implies that it is sequestered away from the cytosol and, consistent with this possibility, vacuoles isolated from [(14)C]pABA-fed pea leaves were estimated to contain> or =88% of the [(14)C]pABA-Glc formed. In total, these data and the fact that isolated mitochondria did not take up [(3)H]pABA-Glc, suggest that the glucose ester represents a storage form of pABA that does not contribute directly to folate synthesis.
Eudes
Aymerick
A
Departments of Horticultural Sciences and Food Science and Human Nutrition, University of Florida, Gainesville, FL 32611, USA.
Bozzo
Gale G
GG
Waller
Jeffrey C
JC
Naponelli
Valeria
V
Lim
Eng-Kiat
EK
Bowles
Dianna J
DJ
Gregory
Jesse F
JF
Hanson
Andrew D
AD
D010129:4-Aminobenzoic Acid / Q000378:metabolism*; D017360:Arabidopsis / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000378:metabolism; D002384:Catalysis; D002736:Chloroplasts / Q000378:metabolism*; D004952:Esters / Q000378:metabolism*; D005492:Folic Acid / Q000096:biosynthesis*; D005947:Glucose / Q000378:metabolism*; D005964:Glucosyltransferases / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism; D018532:Pisum sativum / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism; D014617:Vacuoles / Q000378:metabolism*
D029681:Arabidopsis Proteins; D004952:Esters; D005492:Folic Acid; D005964:Glucosyltransferases; D005947:Glucose; D010129:4-Aminobenzoic Acid