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 [C-14]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 [C-14]pABA-fed pea leaves were estimated to contain >= 88% of the [C-14]pABA-Glc formed. In total, these data and the fact that isolated mitochondria did not take up [H-3]pABA-Glc, suggest that the glucose ester represents a storage form of pABA that does not contribute directly to folate synthesis.

Metabolism of the folate precursor p-aminobenzoate in plants: glucose ester formation and vacuolar storage / Eudes, Aymerick; Bozzo, Gale G; Waller, Jeffrey C; Naponelli, Valeria; Lim, Eng-Kiat; Bowles, Dianna J; Gregory, Jesse F; Hanson, Andrew D. - In: THE JOURNAL OF BIOLOGICAL CHEMISTRY. - ISSN 0021-9258. - 283:22(2008), pp. 15451-9-15459. [10.1074/jbc.M709591200]

Metabolism of the folate precursor p-aminobenzoate in plants: glucose ester formation and vacuolar storage

Naponelli, Valeria;
2008

Abstract

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 [C-14]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 [C-14]pABA-fed pea leaves were estimated to contain >= 88% of the [C-14]pABA-Glc formed. In total, these data and the fact that isolated mitochondria did not take up [H-3]pABA-Glc, suggest that the glucose ester represents a storage form of pABA that does not contribute directly to folate synthesis.
Metabolism of the folate precursor p-aminobenzoate in plants: glucose ester formation and vacuolar storage / Eudes, Aymerick; Bozzo, Gale G; Waller, Jeffrey C; Naponelli, Valeria; Lim, Eng-Kiat; Bowles, Dianna J; Gregory, Jesse F; Hanson, Andrew D. - In: THE JOURNAL OF BIOLOGICAL CHEMISTRY. - ISSN 0021-9258. - 283:22(2008), pp. 15451-9-15459. [10.1074/jbc.M709591200]
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11381/2860005
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