Cassady I A, Budge M M, Healy M J, Nixon P F
Biochim Biophys Acta. 1980 Dec 1;633(2):258-68. doi: 10.1016/0304-4165(80)90411-0.
The relative concentrations of folylpolyglutamates of differing chain length in rat liver and the uptake of exogenous [3H]folic acid (20 microCi, 20 microgram) into liver folylpolyglutamates were examined in rats maintained on (a) standard and folate-supplemented standard diets and (b) semi-defined folate-sufficient and folate-deficient diets. Folylpolyglutamates extracted from liver were cleaved to p-aminobenzoylpolyglutamates which were separated by ion-exchange chromatography. The relative concentrations and ultimate radiolabeling of longer-chain folylpolyglutamates (six, seven and eight glutamate residues) were greatest in the livers of folate-deficient rats, whereas the intermediate-chain folylpolyglutamates (three, four and five glutamate residues) were the greatest portion of total liver folates of folate-supplemented rats. Thus, the length of the polyglutamate chain added to liver folates is inversely related to the total concentration of liver folates. These data suggest that folylpolyglutamate biosynthesis in the liver may be controlled by the liver folate concentrations. In folate insufficiency such a control mechanism would serve to enhance the affinity of folates for folate-dependent enzymes and to conserve the liver folate concentration.
在以（a）标准饮食和补充叶酸的标准饮食以及（b）半合成叶酸充足和叶酸缺乏饮食喂养的大鼠中，检测了大鼠肝脏中不同链长的叶酰聚谷氨酸的相对浓度以及外源性[³H]叶酸（20微居里，20微克）摄入肝脏叶酰聚谷氨酸的情况。从肝脏中提取的叶酰聚谷氨酸被裂解为对氨基苯甲酰聚谷氨酸，通过离子交换色谱法进行分离。在叶酸缺乏大鼠的肝脏中，较长链叶酰聚谷氨酸（六个、七个和八个谷氨酸残基）的相对浓度和最终放射性标记最高，而在补充叶酸大鼠的肝脏中，中间链叶酰聚谷氨酸（三个、四个和五个谷氨酸残基）占肝脏总叶酸的比例最大。因此，添加到肝脏叶酸上的聚谷氨酸链的长度与肝脏叶酸的总浓度呈负相关。这些数据表明，肝脏中叶酰聚谷氨酸的生物合成可能受肝脏叶酸浓度的控制。在叶酸不足的情况下，这种控制机制将有助于提高叶酸对叶酸依赖性酶的亲和力，并维持肝脏叶酸浓度。
