KINETICS OF FOLATE METABOLISM

在过去的十年中，研究表明，叶酸的抗叶酸作用， 药物甲氨蝶呤（MTX）的代谢是由其多聚谷氨酸代谢物介导的。 已知聚谷氨酸盐直接或间接地负责， 抑制胸苷酸和嘌呤合成。 最终， 有兴趣制定一个生物化学动力学模型， MTX抑制嘌呤的各种假说，但在此之前， MTX细胞转运及其多聚谷氨酸化的动力学 必须加以描述。 因此，在MCF-7中甲氨蝶呤（MTX）的多聚谷氨酸化动力学 人类乳腺癌细胞已经用数学公式表示。 模型 解释了谷氨酸化和水解动力学， 五谷氨酸水平，二氢叶酸还原酶合成增加 暴露于药物后，与还原酶可逆紧密结合，和 所有药物聚谷氨酸盐的膜转运。 谷氨酸化， 水解和外排参数已经从拟合中确定， 实验MTX聚谷氨酸摄取和流出数据。 优选的 完整细胞中叶多聚谷氨酰合酶的底物已被证明 是MTX二谷氨酸盐，平均反应性是MTX二谷氨酸盐的2至3倍， 母体药物或三谷氨酸盐。 水解速率常数显示没有 由于每个链长的不确定性很大， 参数估计 然而，MTX多聚谷氨酸盐从MCF-7的流出， 细胞确实显示出随着链长的增加而减少的趋势， 长度如预期。 该模型还显示了定量协议与 MTX聚谷氨酸盐的一部分仍被发现与还原酶结合， 流出24小时后的MCF-7细胞。

Over the last decade, research has shown that the antifolate effects of the drug, methotrexate (MTX), are mediated by its polyglutamate metabolites. The polyglutamates are known to be responsible, directly or indirectly, for inhibition of both thymidylate and purine synthesis. Ultimately we are interested in formulating a biochemical kinetic model useful for exploring various hypotheses of purine inhibition by MTX, but before this is possible, the kinetics of MTX cellular transport and of its polyglutamation must be described. Accordingly, the polyglutamation kinetics of methotrexate (MTX) in MCF-7 human breast cancer cells have been formulated mathematically. The model accounts for glutamation and hydrolysis kinetics up through the pentaglutamate level, increased synthesis of dihydrofolate reductase following exposure to drug, reversible tight-binding to reductase, and membrane transport of all the drug polyglutamates. The glutamation, hydrolysis, and efflux parameters have been determined from fits to experimental MTX polyglutamate uptake and efflux data. The preferred substrate for folypolyglutamyl synthase in the intact cell has been shown to be MTX diglutamate, on average being 2 to 3 times as reactive as either the parent drug or the triglutamate. Hydrolysis rate constants exhibit no clear trend with chain-length because of the large uncertainty of each parameter estimate. However, the efflux of MTX polyglutamates from MCF-7 cells does show a trend with chain-length, decreasing with increasing length as expected. The model also shows quantitative agreement with the fraction of MTX polyglutamates found still to be bound to reductase in MCF-7 cells following 24 hrs of efflux.