(4aS,6R)-4a-hydroxy-L-erythro-5,6,7,8-tetrahydrobiopterin <=> (6R)-L-erythro-6,7-dihydrobiopterin + H2O
(4as,6r)-4a-hydroxy-l-erythro-5,6,7,8-tetrahydrobiopterin <=> (6r)-l-erythro-6,7-dihydrobiopterin + h2o