Metabolism of the folate precursor p-aminobenzoate in plants -: Glucose ester formation and vacuolar storage
Metabolism of the folate precursor p-aminobenzoate in plants -: Glucose ester formation and vacuolar storage
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DOI：
10.1074/jbc.m709591200
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发表时间：
2008-05-30
影响因子：
4.8
通讯作者：
Hanson, Andrew D.
中科院分区：
文献类型：
--
作者：
Eudes, Aymerick;Bozzo, Gale G.;Hanson, Andrew D.
植物在叶绿体中产生对氨基苯甲酸（pABA），并将其用于线粒体中的叶酸合成。然而，在植物组织中，已知pABA主要以其葡萄糖酯（pABA-Glc）的形式存在，并且该代谢物在叶酸合成中的作用尚未确定。在这项研究中，UDP-葡萄糖：pABA酰基-葡萄糖基转移酶（pAGT）在拟南芥提取物中的活性被发现主要（95%）驻留在一个异构体与表观K-m的pABA为0.12 mM. Screening重组拟南芥UDP-糖基转移酶确定只有三个承认pABA。其中一个（UGT 75 B1）表现出比其它酶高得多的k（cat）/K-m值和比其它酶低得多的pABA表观K-m（0.12 mm），表明其与体内主要酶相同。支持这种可能性的是，UGT 75 B1的消融使可提取的pAGT活性降低了95%，体内[C-14]pABA葡糖基化降低了77%，内源性pABA-Glc/pABA比率降低了9倍。发现pABA酯化反应的K-eq为3 × 10（-3）。与文献数据的pAGT活性的胞质位置和胞质UDP-葡萄糖/UDP比率，该K-eq值允许估计只有4%的胞质pABA被酯化。因此，pABA-Glc在植物中占优势意味着它与细胞溶质隔离，并且与这种可能性一致，估计从[C-14] pABA喂养的豌豆叶中分离的液泡含有>= 88%的所形成的[C-14]pABA-Glc。总之，这些数据和分离的线粒体不摄取[H-3]pABA-Glc的事实表明，葡萄糖酯代表pABA的储存形式，其不直接促进叶酸合成。
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.