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.
