STUDIES ON FOLIC ACID (DIHYDROFOLATE REDUCTASE) AND VITAMIN A (BETA-CAROTENE)

该项目继续关注二氢叶酸还原酶，这是一种关键的 B-维他命叶酸(一种蝶呤)代谢中的酶，以及 β-胡萝卜素，维生素A的前体。叶酸的维持 在完全还原的水平上，四氢叶酸，通过二氢叶酸还原酶 对细胞的生存至关重要。针对这一点的抗叶酸药物 酶在癌症治疗中继续被广泛使用，而且越来越多。 自身免疫性疾病的数量。β-胡萝卜素目前正在接受 作为生物抗氧化剂受到相当大的关注，特别是 对心血管疾病的尊重。 其他的蝶呤类化合物的酶还原也是由 二氢叶酸还原酶，尽管效率降低。的 特别的生物学兴趣是生物蝶呤的减少， 酪氨酸、苯丙氨酸和苯丙氨酸羟化酶的辅因子 色氨酸。特异地降低生物蝶呤的酶是蝶啶 还原酶。然而，我们已经证明，二氢叶酸还原酶可以 将二氢生物蝶呤从牛肝中的3%降至鸡肝中的50%， 与二氢叶酸还原速率进行比较。此外， 生物蝶呤与二氢叶酸还原酶紧密结合。鸡肉 肝脏二氢叶酸还原酶不仅以二元组分结晶 和与底物和缓蚀剂的三元络合物，我们现在报告 生物蝶呤-NADP络合物的结晶和X射线分析 既有底物又有辅因子的三元络合物。X-射线分析 这种结构进一步揭示了一种可能的机制 底物质子化及其可能的催化作用 叶酸底物的氨基苯甲酰谷氨酸部分。 在胡萝卜素代谢的持续研究中，铜的影响 缺乏β-胡萝卜素向维生素A(视黄醇)的转化 量过了。其基本原理是，这种酶假定可以转化为 肠壁中的胡萝卜素到视黄醇是一种双加氧酶，含有 铜。铜缺乏的大鼠被发现将视黄醇储存为 就像铜充足的大鼠一样高效。当β-胡萝卜素被喂给 缺铜大鼠，肝脏储存视黄醇，一种衡量胡萝卜素的指标 吸收和转化，与对照组没有区别。这些 结果令人怀疑双加氧酶在人体内的可能作用 胡萝卜素转化为视黄醇。

This project continues to focus on dihydrofolate reductase, a critical enzyme in the metabolism of the B-vitamin folic acid (a pteridine), and beta-carotene, a precursor of vitamin A. The maintenance of folic acid at the fully reduced level, tetrahydrofolate, by dihydrofolate reductase is essential to cellular survival. Anti-folate drugs which target this enzyme continue to be widely used in cancer treatment and an increasing number of autoimmune diseases. Beta-carotene is currently receiving considerable attention as a biological antioxidant, particularly with respect to cardiovascular disease. The enzymatic reduction of other pteridines is also catalyzed by dihydrofolate reductase, although with decreased efficiency. Of particular biological interest is the reduction of biopterin, the cofactor for the enzymes that hydroxylate tyrosine, phenylalanine and tryptophan. The enzyme that specifically reduces biopterin is pteridine reductase. However, we have shown that dihydrofolate reductase can reduce dihydrobiopterin from 3% in beef liver to 50% in chicken liver, as compared with the rate of dihydrofolate reduction. Furthermore, biopterin is tightly bound to dihydrofolate reductase. The chicken liver dihydrofolate reductase has been crystallized not only as a binary and ternary complex with substrates and inhibitors, we now report the crystallization and x-ray analysis of a complex with biopterin and NADP, a ternary complex with both substrate and cofactor. X-ray analysis of this structure has shed further light on a possible mechanism of substrate protonation and on the possible catalytic function of the p- aminobenzoylglutamate moiety of folate substrates. In continuing studies of carotene metabolism, the effect of copper deficiency on the conversion of beta-carotene to vitamin A (retinol) was measured. The rationale was that the enzyme postulated to convert carotene to retinol in the intestinal wall is a dioxygenase containing copper. Copper-deficient rats were found to store retinol as efficiently as copper-adequate rats. When beta-carotene was fed to copper-deficient rats, liver storage of retinol, a measure of carotene absorption and conversion, was not different from controls. These results bring into question the possible role of a dioxygenase in carotene conversion to retinol.