Involved in tetrahydrobiopterin biosynthesis. Seems to both prevent the formation of 7-pterins and accelerate the formation of quinonoid-BH2 (By similarity). Regulates the dimerization of homeodomain protein HNF-1-alpha and enhances its transcriptional activity. (4aS,6R)-4a-hydroxy-L-erythro-5,6,7,8-tetrahydrobiopterin = (6R)-L-erythro-6,7-dihydrobiopterin + H2O Highest level found in the kidney, liver, heart and ovarian follicles. Belongs to the pterin-4-alpha-carbinolamine dehydratase family.