STUDIES ON FOLIC ACID (DIHYDROFOLATE REDUCTASE) AND VITAMIN A

我们正在继续对二氢叶酸还原酶(DHFR)的研究 因为这种酶的功能不仅对 细胞增殖，它也是作用的部位 甲氨蝶呤是治疗多种人类疾病的关键药物 天灾。理论上，DHFR对二氢叶酸的还原 应该会产生线性反应动力学。情况并非如此。 以初始速率为特征的细菌还原酶 有很大的滞后。这种现象被称为慢 瞬时动力学和酶被认为在以下情况下表现出滞后 与底物的预培养消除了这种滞后。然而，在 我们的分析条件来自各种动物来源的DHFR，鸡肉 和牛肝以及某些肿瘤细胞，表现出线性 反应率达到85%以内。另一方面， 绵羊肝脏DHFR的特征一直是复杂的 在提纯的各个阶段都受到非线性初始反应的滞后。 尽管这一行为暗示着滞后，但初始滞后 不能通过与任何一种底物预先孵育来消除。 PH和缓冲液成分都不影响滞后。然而， 增加离子强度可减弱这种影响，在0.03M KCl时 这个比率基本上变成了线性的。增加盐分 浓度也显著提高了催化活性。 因此，在0.8M Kc1时，绵羊肝脏DHFR的比率增加了6- 收牌。高浓度的氯仿也有类似的影响。 共沸剂，如尿素和胍盐。 关于维生素A的研究表明，不同的人在 它们吸收类胡萝卜素的效率。此外，一个高 基线血浆β-胡萝卜素与 一剂纯β胡萝卜素后的最大百分比增加 再加上观察到的按比例增加的 建议的较高剂量水平的血浆浓度较低 类胡萝卜素的吸收和运输具有局限性 特点。个体有一个相对恒定的稳定性- 血浆类胡萝卜素的状态模式。

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.