Boarman D M, Allegra C J
Division of Cancer Treatment, National Cancer Institute, Bethesda, Maryland 20892.
Cancer Res. 1992 Jan 1;52(1):36-44.
This report describes the intracellular metabolism of 5-formyltetrahydrofolate into the various one-carbon substituted folate and polyglutamate pools in a human breast (MCF-7) and colon (HCT 116) carcinoma cell line. Metabolism into the one-carbon substituted pools was found to be time and dose dependent over a concentration range up to 50 microM. A 3-fold increase in total intracellular folate was noted over a 50-fold concentration range (1-50 microM) of 5-formyltetrahydrofolate tested in the colon cell line, while in the breast line, a 6-fold increase was detected over a 500-fold concentration range (0.1-50 microM). The level of 5, 10-methylenetetrahydrofolate, which was detectable only in the breast cell line, was found to increase by a factor of 10 (1.8 pmol/mg to 17.9 pmol/mg) over the concentration range studied. The majority of metabolism was into the 10-formyltetrahydrofolate and tetrahydrofolate pools in the breast cells and into the 5-methyltetrahydrofolate pool in the colon cells. Polyglutamation was also time and dose dependent, with a significant proportion of the total pool represented by the higher polyglutamate forms (Glu3-Glu5) after 24 h of continuous exposure to 5-formyl tetrahydrofolate. Pentaglutamate was the highest level noted in both cell lines. The intracellular half-life of the polyglutamate forms was inversely related to the length of the polyglutamate tail with half-lives of 71, 131, 143, 441, and 1167 min for the mono- through pentaglutamate, respectively. Finally, up to a 20:1 ratio of the biologically inactive (6R) isomer to active (6S) isomer of 5-formyltetrahydrofolate resulted in no effect on metabolism into the one-carbon substituted folate pools and only minimal decreases in metabolism to the polyglutamate forms. These studies suggest that prolonged exposure to even relatively low doses of 5-formyltetrahydrofolate may be optimal for intracellular metabolism to the most biologically relevant forms for ternary complex formation with thymidylate synthase and fluorodeoxyuradylate, since longer exposures result in a greater accumulation of the higher polyglutamates.
本报告描述了在人乳腺癌（MCF - 7）和结肠癌细胞系（HCT 116）中，5 - 甲酰四氢叶酸在细胞内代谢为各种一碳取代叶酸和多聚谷氨酸池的过程。在高达50微摩尔的浓度范围内，发现其代谢为一碳取代池的过程具有时间和剂量依赖性。在结肠癌细胞系中，对5 - 甲酰四氢叶酸进行50倍浓度范围（1 - 50微摩尔）测试时，细胞内总叶酸增加了3倍；而在乳腺癌细胞系中，在500倍浓度范围（0.1 - 50微摩尔）内检测到增加了6倍。仅在乳腺癌细胞系中可检测到的5, 10 - 亚甲基四氢叶酸水平，在所研究的浓度范围内增加了10倍（从1.8皮摩尔/毫克增至17.9皮摩尔/毫克）。在乳腺癌细胞中，大部分代谢产物进入10 - 甲酰四氢叶酸和四氢叶酸池，而在结肠癌细胞中则进入5 - 甲基四氢叶酸池。多聚谷氨酸化也具有时间和剂量依赖性，在持续暴露于5 - 甲酰四氢叶酸24小时后，总池中相当大比例由较高的多聚谷氨酸形式（Glu3 - Glu5）代表。五聚谷氨酸是两个细胞系中检测到的最高水平。多聚谷氨酸形式的细胞内半衰期与多聚谷氨酸尾巴的长度呈负相关，单聚至五聚谷氨酸的半衰期分别为71、131、143、441和1167分钟。最后，5 - 甲酰四氢叶酸的生物无活性（6R）异构体与活性（6S）异构体的比例高达20:1时，对其代谢为一碳取代叶酸池没有影响，对代谢为多聚谷氨酸形式的影响也很小。这些研究表明，即使长时间暴露于相对低剂量的5 - 甲酰四氢叶酸，可能对于细胞内代谢为与胸苷酸合成酶和氟脱氧尿苷酸形成三元复合物最具生物学相关性的形式而言是最佳的，因为更长时间的暴露会导致更高多聚谷氨酸的积累增加。
