STUDIES ON FOLIC ACID (DIHYDROFOLATE REDUCTASE) AND VITAMIN A

我们正在继续研究二氢叶酸还原酶（DHFR） 因为这种酶的功能不仅是必需的， 细胞增殖，它也是作用的网站， 甲氨蝶呤是治疗多种人类肿瘤的关键药物， 是的 理论上，DHFR还原二氢叶酸 应该产生线性反应动力学。 的情况并非如此 细菌还原酶，其中初始速率表征为 一个明显的滞后。 这种现象被称为慢 瞬时动力学和酶被认为显示滞后，如果 用底物预孵育消除了滞后。 但根据 我们的分析条件DHFR从各种动物来源，鸡 和牛肝以及某些肿瘤细胞， 反应率在85%以内完成。 另一方面 绵羊肝脏DHFR的特征一直很复杂 在纯化的所有阶段，通过非线性初始反应滞后。 虽然这种行为暗示了滞后，但初始滞后 不能通过与任一底物预孵育来消除。 pH或缓冲液组成均不影响滞后。 然而，在这方面， 增加离子强度减小这种效应，在0.03 M KCl 速率基本上变成线性的。 增加盐 浓度也显著提高了催化活性。 因此，在0.8 M KCl时，绵羊肝脏DHFR的速率增加6- 折 在高浓度的 促凝剂如尿素和胍盐。 对维生素A的研究表明，个体在维生素A水平上存在很大差异。 类胡萝卜素的吸附效率。 此外，高 基线血浆β-胡萝卜素与 纯β-胡萝卜素剂量后的最大百分比增加 同时，观察到， 血浆浓度在较高剂量水平下较低，表明 类胡萝卜素吸收和转运具有限制性， 特色 个体有一个相对稳定的- 血浆类胡萝卜素状态模式。

We are continuing our studies on dihydrofolate reductase (DHFR) since the function of this enzyme is not only essential for cellular proliferation, it is also the site of action of Methotrexate, a key drug in the treatment of a variety of human scourges. Theoretically, the reduction of dihydrofolate by DHFR should yield linear reaction kinetics. This is not the case with the bacterial reductases where the initial rate is characterized by a significant lag. This phenomenon is designated as slow transient kinetics and the enzyme is said to exhibit hysteresis if preincubation with substrate eliminates the lag. However, under our analytic conditions DHFR from various animal sources, chicken and beef liver as well as certain tumor cells, exhibit linear reaction rate to within 85% of completion. On the other hand, the characterization of sheep liver DHFR is consistently complicated at all stages of purification by non-linear initial reaction lags. Although this behavior is suggestive of hysteresis, the initial lag cannot be eliminated by preincubation with either substrate. Neither pH or buffer composition affects the lag. However, increasing ionic strength diminishes this effect and at 0.03 M KCl the rate becomes essentially linear. Increasing the salt concentration also markedly increases the catalytic activity. Thus, at 0.8 M KC1, the rate of sheep liver DHFR is increased 6- fold. Similar effects are noted with high concentrations of chaeotropic agents such as urea and guanidinium salts. Studies on vitamin A have shown that individuals vary widely in the efficiency of their carotenoid adsoption. In addition, a high negative correlation between the baseline plasma beta carotene and the maximum percentage increase after a dose of pure beta carotene together with the observation that the proportionate increase in plasma concentration is lower at the higher dosage level suggest that carotenoid absorption and transport have limiting characteristics. Individuals have a relatively constant steady- state pattern of plasma carotenoids.