Comparative activity of rat liver dihydrofolate reductase with 7,8-dihydrofolate and other 7,8-dihydropteridines

Abstract

The various interactions of rat liver dihydrofolate reductase with two unconjugated 7,8-dihydropteridines, 7,8-dihydrobiopterin and 6-methyl-7,8-dihydropteridine, have been compared with those of 7,8-dihydrofolate and folate. Of particular interest was the reactivity demonstrated by 7,8-dihydrobiopterin because of the potential physiological significance of this reaction both in the regeneration of tetrahydrobiopterin, a cofactor for various biological hydroxylations, and as a step in the biosynthesis of this compound from GTP. Kinetic experiments gave Km values of 0.17, 6.42, and 10.2 μm for 7,8-dihydrofolate, 7,8-dihydrobiopterin, and 6-methyl-7,8-dihydropteridine, respectively, with Vmax = 6.22, 2.39, and 1.54 μmol min-1 mg-1. With folate the enzyme showed high affinity (Km = 0.88 μm) but low Vmax (0.20 μmol min-1 mg-1). The natural cofactor was NADPH and a Km of ~0.7 μm was measured with each substrate. The enzyme was activated by both p-hydroxymercuribenzoate and urea when assayed with 7,8-dihydrofolate but was inhibited when 7,8-dihydrobiopterin was the substrate. The pH optimum for dihydrofolate reduction was 4 with enhancement at pH ≥5.5 in the presence of 1 m NaCl. Peak activity with 7,8-dihydrobiopterin occurred at pH 4.8; this was shifted to pH 5.3 but was not enhanced by 1 m NaCl. Inhibition with methotrexate was similar whether the enzyme was assayed with either the conjugated or unconjugated 7,8-dihydro derivatives. The rat liver enzyme, highly unstable after purification, was stabilized in the presence of the nonionic detergent, Tween-20 (0.1%); however, the comparative properties toward the conjugated and unconjugated substrates were not altered by this treatment. © 1985.