Clifford A J, Arjomand A, Dueker S R, Schneider P D, Buchholz B A, Vogel J S
Department of Nutrition, University of California, Davis 95616, USA.
Adv Exp Med Biol. 1998;445:239-51. doi: 10.1007/978-1-4899-1959-5_15.
Folate is an essential nutrient that is involved in many metabolic pathways, including amino acid interconversions and nucleotide (DNA) synthesis. In genetically susceptible individuals and populations, dysfunction of folate metabolism is associated with severe illness. Despite the importance of folate, major gaps exist in our quantitative understanding of folate metabolism in humans. The gaps exist because folate metabolism is complex, a suitable animal model that mimics human folate metabolism has not been identified, and suitable experimental protocols for in vivo studies in humans are not developed. In general, previous studies of folate metabolism have used large doses of high specific activity tritium and 14C-labeled folates in clinical patients. While stable isotopes such as deuterium and 13C-labeled folate are viewed as ethical alternatives to radiolabeled folates for studying metabolism, the lack of sensitive mass spectrometry methods to quantify them has impeded advancement of the field using this approach. In this chapter, we describe a new approach that uses a major analytical breakthrough, Accelerator Mass Spectrometry (AMS). Because AMS can detect attomole concentrations of 14C, small radioactive dosages (nCi) can be safely administered to humans and traced over long periods of time. The needed dosages are sufficiently small that the total radiation exposure is only a fraction of the natural annual background radiation of Americans, and the generated laboratory waste may legally be classified non-radioactive in many cases. The availability of AMS has permitted the longest (202 d) and most detailed study to date of folate metabolism in a healthy adult human volunteer. Here we demonstrate the feasibility of our approach and illustrate its potential by determining empirical kinetic values of folate metabolism. Our data indicate that the mean sojourn time for folate is in the range of 93 to 120 d. It took > or = 350 d for the absorbed portion of small bolus dose of 14C-folic acid to be eliminated completely from the body.
叶酸是一种必需营养素，参与许多代谢途径，包括氨基酸的相互转化和核苷酸（DNA）合成。在遗传易感性个体和人群中，叶酸代谢功能障碍与严重疾病相关。尽管叶酸很重要，但我们对人类叶酸代谢的定量理解仍存在重大差距。之所以存在这些差距，是因为叶酸代谢复杂，尚未找到能模拟人类叶酸代谢的合适动物模型，也未开发出适用于人体体内研究的合适实验方案。一般来说，以往关于叶酸代谢的研究在临床患者中使用了大剂量的高比活度氚和14C标记的叶酸。虽然诸如氘和13C标记的叶酸等稳定同位素被视为研究代谢时放射性标记叶酸的伦理替代物，但缺乏灵敏的质谱方法来定量它们阻碍了该领域采用这种方法的进展。在本章中，我们描述了一种利用重大分析突破——加速器质谱（AMS）的新方法。由于AMS能够检测到14C的阿托摩尔浓度，因此可以安全地向人体施用小剂量放射性物质（纳居里）并进行长时间追踪。所需剂量足够小，以至于总的辐射暴露仅为美国人自然年度背景辐射的一小部分，并且在许多情况下产生的实验室废物可合法归类为非放射性废物。AMS的可用性使得能够对一名健康成年人类志愿者进行迄今为止最长（202天）且最详细的叶酸代谢研究。在这里，我们通过确定叶酸代谢的经验动力学值来证明我们方法的可行性并说明其潜力。我们的数据表明，叶酸的平均停留时间在93至120天范围内。一小剂量14C - 叶酸吸收部分从体内完全消除需要≥350天。
