(6R)-L-erythro-5,6,7,8-tetrahydrobiopterin + NADP(+) <=> 6-lactoyl-5,6,7,8-tetrahydropterin + NADPH + H(+)
(6r)-l-erythro-5,6,7,8-tetrahydrobiopterin + nadp(+) <=> 6-lactoyl-5,6,7,8-tetrahydropterin + nadph + h(+)