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.