Abstract
The catalytic properties of a new type of dihydropteridine reductase, NADPH-specific dihydropteridine reductase [EC 1.6.99.10], from bovine liver, were studied and compared with those of the previously characterized enzyme, NADH-specific dihydropteridine reductase [EC 1.6.99.7]. With quinonoid-dihydro-6-methylpterin, approximate Km values of NADPH-specific dihydropteridine reductase for NADPH and NADH were estimated to be 1.4 μM and 2, 900 μM, respectively. The Vmax values were 1.34μmol/min/mg with NADPH and 1.02 μmol/min/mg with NADH. With NADPH, the Km values of the enzyme for the quinonoid-dihydro forms of 6-methylpterin and biopterin were 1.4 μM and 6.8 μm, respectively. The enzyme was inhibited by its reaction product, NADP+, in a competitive manner, and the inhibition constant was determined to be 3.2 μM. The enzyme was severely inhibited by L-thyroxine and by 2, 6-dichlorophenolindophenol.
