Baugh C M, Krumdieck C L, Baker H J, Butterworth C E
J Clin Invest. 1971 Oct;50(10):2009-21. doi: 10.1172/JCI106694.
Folic acid absorption was studied in anesthetized dogs by determining the amount and chemical nature of folate in venous blood emerging from isolated intestinal segments containing free folic acid and/or pteroylpolyglutamates of a known chain length. Chromatographically pure test materials placed in the lumen were prepared by unambiguous solid phase synthetic methods. This synthetic procedure not only yields compounds of known structure, it also provides a means by which glutamic acid residues at any given position in the gamma glutamyl chain can be made radioactive. For example, teropterin (pteroyltriglutamate) was synthesized in such a way that (14)C was present only in the middle glutamic acid unit. Suitable placement of label permitted assessment of the extent of peptide cleavage. The action of plasma conjugase was inhibited by copper chloride. Plasma samples were analyzed by Lactobacillus casei and Streptococcus faecalis assay, by column chromatography, and by quantitative measurement of pteridine-bound radioactivity. It was observed that biologically active folate appeared in the mesenteric vein with either pteroylmono-, di-, tri-, penta-, or heptaglutamate in the lumen. Generally speaking the absorption rate appeared to be inversely related to the length of the gamma glutamyl side chain. Roughly twice as much folic acid appeared in the circulation from (3)H-labeled pteroylmonoglutamate as from (14)C-labeled pteroylpentaglutamate when equimolar amounts of each were placed simultaneously in a single intestinal segment. Pteroylmonoglutamate appeared to be the predominant form entering the blood from each of the precursors tested. However, evidence was obtained that pteroyldiglutamate may enter the mesenteric vein soon after placing pteroyldi-, or triglutamate in the lumen, but not with the higher polyglutamates. Comparison of radioactivity and biological activity patterns suggests little conversion, if any, to reduced or methylated forms during the first 30 min of passage through the intestinal mucosa. We conclude that both pteroylmonoglutamates and pteroyldiglutamates may across the intestinal mucosa of the dog, and that reduction and methylation are not essential to the absorption process.
通过测定从含有游离叶酸和/或已知链长的蝶酰多聚谷氨酸的离体肠段流出的静脉血中叶酸的含量和化学性质,对麻醉犬的叶酸吸收情况进行了研究。通过明确的固相合成方法制备置于肠腔内的色谱纯测试材料。这种合成方法不仅能产生已知结构的化合物,还提供了一种使γ-谷氨酰链中任何给定位置的谷氨酸残基具有放射性的手段。例如,三蝶酰谷氨酸(蝶酰三谷氨酸)就是以仅使中间谷氨酸单元含有¹⁴C的方式合成的。标记的合适位置有助于评估肽链裂解的程度。血浆结合酶的活性受到氯化铜的抑制。通过干酪乳杆菌和粪链球菌检测、柱色谱法以及对蝶啶结合放射性的定量测量对血浆样本进行分析。观察到,肠腔内存在蝶酰单、二、三、五或七谷氨酸时,具有生物活性的叶酸会出现在肠系膜静脉中。一般来说,吸收速率似乎与γ-谷氨酰侧链的长度呈负相关。当将等摩尔量的³H标记的蝶酰单谷氨酸和¹⁴C标记的蝶酰五谷氨酸同时置于单个肠段中时,循环中出现的³H标记的蝶酰单谷氨酸中的叶酸量大约是¹⁴C标记的蝶酰五谷氨酸中的两倍。蝶酰单谷氨酸似乎是从所测试的每种前体进入血液的主要形式。然而,有证据表明,将蝶酰二或三谷氨酸置于肠腔后不久,蝶酰二谷氨酸可能会进入肠系膜静脉,但更高的多聚谷氨酸则不会。放射性和生物活性模式的比较表明,在通过肠黏膜的最初30分钟内,即使有转化,转化为还原型或甲基化形式的情况也很少。我们得出结论,蝶酰单谷氨酸和蝶酰二谷氨酸都可能穿过犬的肠黏膜,并且还原和甲基化对于吸收过程并非必不可少。