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 代谢中的酶是维生素B、叶酸。维护和维护 二氢叶酸还原酶在四水水平上的叶酸是至关重要的 为了细胞的生存。以二氢叶酸为靶点的抗叶酸药物 还原酶继续被广泛用于治疗癌症、类风湿 关节炎，以及越来越多的自身免疫性疾病。 到目前为止，所有脊椎动物的二氢叶酸还原酶都有独特的 当它们的催化活性在以下情况下显著激发时 用某些已知可以扰乱三级结构的药物进行治疗 蛋白质。鸡肝二氢叶酸还原酶表现为五倍 在约5M存在的情况下进行检测，活性可提高6倍 尿素。胍盐是最有趣的蛋白质之一。 由于通常伴随着强烈的变性活性而产生扰动 使用GDNION。但是，使用以下命令时，ck DHFR仅显示最低限度的激活 相对较低的GdnHCl浓度，即在 0.2微米高浓度的GdnHCl会导致快速变性和 活动减少。由于这一活动的增长与 在相应浓度的Na或KCl存在下观察到，它 得出的结论是这种刺激是由于离子或盐的影响，而不是 比广为人知的GDN化合物的变色特性更重要。然而， GdN部分影响蛋白质，因为等量或更高 盐的浓度不会引起类似的变性，尽管 相应的激活。另一方面，重组人DHFR (RhDHFR)似乎确实表现出对GDNHC1的激活。在… 0.6M GdnHCl时，rhDHFR的活性比GdnHCl高约40% 用KC1观察到的最大活化度。同样，绵羊肝脏DHFR 0.65M GdnHCl的活性显著高于 离子强度引起的活化。其他研究与 硫氰酸盐、醋酸盐和硫酸盐GDN盐表现出类似的激活作用 与硫氰酸盐和醋酸盐反应。然而，GdnSO(4)被发现是 一种对所有DHFRs有效的抑制剂。与此类似的结果是 纳索(4)。这些结果的意义正在被解释为 尿素的活化特性和已知的优先选择 蛋白质与胍离子和盐类的相互作用 最近对激活的ck-DHFR进行x射线研究的结果。研究仍在继续 重点介绍β-胡萝卜素、其可能的抗氧化性及其 与维生素A的关系一种类胡萝卜素，包括β-胡萝卜素- 胡萝卜素，当喂给动物时，在体内没有表现出抗氧化特性 缺乏维生素E和硒。铜的缺乏对 从胡萝卜素代谢的角度看 酶，胡萝卜素环氧酶，需要铜。

This project continues to focus on dihydrofolate reductase, a critical enzyme in the metabolism of the B-vitamin, folic acid. The maintenance of folic acid at the tetrahydro-level by dihydrofolate reductase is critical to cellular survival. Antifolate drugs which target dihydrofolate reductase continue to be widely used in the treatment of cancer, rheumatoid arthritis, and an increasing number of autoimmune diseases. All vertebrate dihydrofolate reductases examined to date have the unique ability to have their catalytic activity significantly stimulated when treated with certain agents known to perturb the tertiary structure of proteins. Chicken liver (ckDHFR) dihydrofolate reductase shows a five-fold to six-fold increase in activity when assayed in the presence of about 5 M urea. Guanidinium (Gdn) salts are among the most interesting protein perturbants because of the strong denaturing activity usually associated with the Gdn ion. However, ckDHFR shows only minimal activation with relatively low concentrations of GdnHC1, i.e., approximately two-fold at 0.2 M. Higher concentrations of GdnHC1 results in rapid denaturation and loss in activity. Since this increase in activity is in the same range as observed in the presence of corresponding concentrations of Na or KC1, it is concluded that this stimulation is due to ionic or salt effects, rather than the well known chaetropic properties of Gdn compounds. However, the Gdn moiety is affecting the protein since equivalent or higher concentrations of salts do not cause similar denaturation despite corresponding activations. On the other hand, the recombinant human DHFR (rhDHFR) does appear to exhibit an activation in response to Gdn HC1. At 0.6 M GdnHC1, the rhDHFR shows about a 40% higher activation than the maximum activation observed with KC1. Similarly, the sheep liver DHFR exhibits a significantly higher activation with 0.65 M GdnHC1 than activation induced by ionic strength. Additional studies with the thiocyanate, acetate, and sulfate Gdn salts revealed similar activations with the thiocyanate and acetate salts. However, GdnSO(4) was found to be a potent inhibitor of all of the DHFRs. Similar results were obtained with NaSO(4). The significance of these results are being interpreted in terms of the activating properties of urea and the known preferential interactions of proteins with the guanidinium ion and salts as well as recent results from x-ray studies on an activated ckDHFR. Studies continue to focus on beta-carotene, its putative antioxidant properties and its relationship to vitamin A. A variety of carotenoids, including beta- carotene, showed no in vivo antioxidant properties when fed to animals deficient in vitamin E and selenium. A copper deficiency had no effect on carotene metabolism in view of the fact that the carotene-converting enzyme, carotene epoxidase, requires copper.