Cook J D, Cichowicz D J, George S, Lawler A, Shane B
Biochemistry. 1987 Jan 27;26(2):530-9. doi: 10.1021/bi00376a027.
The regulation of folate and folate analogue metabolism was studied in vitro by using purified hog liver folylpolyglutamate synthetase as a model system and in vivo in cultured mammalian cells. The types of folylpolyglutamates that accumulate in vivo in hog liver, and changes in cellular folate levels and folylpolyglutamate distributions caused by physiological and nutritional factors such as changes in growth rates and methionine, folate, and vitamin B12 status, can be mimicked in vitro by using purified enzyme. Folylpolyglutamate distributions can be explained solely in terms of the substrate specificity of folylpolyglutamate synthetase and can be modeled by using kinetic parameters obtained with purified enzyme. Low levels of folylpolyglutamate synthetase activity are normally required for the cellular metabolism of folates to retainable polyglutamate forms, and consequently folate retention and concentration, while higher levels of activity are required for the synthesis of the long chain length derivatives that are found in mammalian tissues. The synthesis of very long chain derivatives, which requires tetrahydrofolate polyglutamates as substrates, is a very slow process in vivo. The slow metabolism of 5-methyltetrahydrofolate to retainable polyglutamate forms causes the decreased tissue retention of folate in B12 deficiency. Although cellular folylpolyglutamate distributions change in response to nutritional and physiological modulations, it is unlikely that these changes play a regulatory role in one-carbon metabolism as folate distributions respond only slowly. 4-Aminofolates are metabolized to retainable forms at a slow rate compared to folates. Although folate accumulation by cells is not very responsive to changes in folylpolyglutamate synthetase levels and cellular glutamate concentrations, cellular accumulation of anti-folate agents would be highly responsive to any factor that changes the expression of folylpolyglutamate synthetase activity.
通过使用纯化的猪肝叶酰聚谷氨酸合成酶作为模型系统，在体外研究了叶酸和叶酸类似物的代谢，并在培养的哺乳动物细胞中进行了体内研究。猪肝中体内积累的叶酰聚谷氨酸类型，以及由生长速率、蛋氨酸、叶酸和维生素B12状态变化等生理和营养因素引起的细胞叶酸水平和叶酰聚谷氨酸分布的变化，可以通过使用纯化的酶在体外模拟。叶酰聚谷氨酸的分布可以仅根据叶酰聚谷氨酸合成酶的底物特异性来解释，并且可以通过使用用纯化酶获得的动力学参数进行建模。叶酸细胞代谢为可保留的聚谷氨酸形式通常需要低水平的叶酰聚谷氨酸合成酶活性，因此需要叶酸保留和浓缩，而哺乳动物组织中发现的长链长度衍生物的合成则需要更高水平的活性。以四氢叶酸聚谷氨酸为底物合成非常长链的衍生物在体内是一个非常缓慢的过程。5-甲基四氢叶酸代谢为可保留的聚谷氨酸形式的缓慢导致维生素B12缺乏时叶酸的组织保留减少。尽管细胞叶酰聚谷氨酸分布会随着营养和生理调节而变化，但这些变化不太可能在一碳代谢中起调节作用，因为叶酸分布的反应非常缓慢。与叶酸相比，4-氨基叶酸代谢为可保留形式的速率较慢。尽管细胞对叶酸的积累对叶酰聚谷氨酸合成酶水平和细胞谷氨酸浓度的变化反应不太敏感，但抗叶酸剂的细胞积累对任何改变叶酰聚谷氨酸合成酶活性表达的因素都会高度敏感。
