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， B族维生素叶酸（一种蝶啶）代谢中的酶，以及 β-胡萝卜素，维生素A的前体。 叶酸的摄入量 通过二氢叶酸还原酶，在完全还原水平下， 是细胞生存的关键 抗叶酸药物靶向这一点 酶继续广泛用于癌症治疗， 自身免疫性疾病的数量。 β-胡萝卜素目前正在接受 作为一种生物抗氧化剂，特别是与 关于心血管疾病。 其它蝶啶的酶促还原也由 二氢叶酸还原酶，但效率降低。 的 生物学上特别感兴趣的是生物蝶呤的减少， 羟化酪氨酸、苯丙氨酸和 色氨酸 特异性还原生物喋呤的酶是蝶啶 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%降低到鸡肝中的50%， 与二氢叶酸还原速率相比。 此外，委员会认为， 生物蝶呤与二氢叶酸还原酶紧密结合。 鸡 肝脏二氢叶酸还原酶不仅被结晶为二元 以及与底物和抑制剂的三元复合物，我们现在报告 生物蝶呤和NADP复合物的结晶和X射线分析， 具有底物和辅因子的三元复合物。 x线分析 这种结构进一步揭示了 底物质子化和对可能的催化功能的p- 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， 缺乏β-胡萝卜素转化为维生素A（视黄醇）， 测定了 基本原理是，假设酶转化为 胡萝卜素到视黄醇在肠壁是一个双加氧酶， 铜 发现缺铜大鼠将视黄醇储存为 像铜充足的老鼠一样高效。 当β-胡萝卜素被喂给 铜缺乏大鼠，肝脏储存的视黄醇，衡量胡萝卜素 吸收和转化，与对照组无差异。 这些 结果提出了双加氧酶在 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.