Function
Catalyzes the conversion of 7,8-dihydroneopterin into 6-hydroxymethyl-7,8-dihydropterin, a biosynthetic precursor of the vitamin tetrahydrofolate. Can use L-threo-dihydroneopterin and D-erythro-dihydroneopterin as substrates for the formation of 6-hydroxymethyldihydropterin, but it can also catalyze the epimerization of carbon 2' of dihydroneopterin and dihydromonapterin.