In enzymology, 6,7-dihydropteridine reductase is an enzyme that catalyzes the chemical reaction 5,6,7,8-tetrahydropteridine + NAD+ ⇌ {\displaystyle \rightleftharpoons } 6,7-dihydropteridine + NADH + H+The four substrates for this enzyme are a 6,7-dihydropteridine, NADH, NADPH, and H+ and its three products are 5,6,7,8-tetrahydropteridine, NAD+, and NADP+ This enzyme participates in folate biosynthesis. Wikipedia