Function
Catalyzes the production of D-threo-tetrahydrobiopterin (DH4), also known as dictyopterin, and in a much smaller amount that of it's stereoisomer L-erythro-tetrahydrobiopterin (BH4). Prefers 1-oxo-2-D-hydroxypropyl-tetrahydropterin to 6-pyruvoyltetrahydropterin as a substrate, thereby maintaining dominant production of DH4 over BH4 in sufficient supply of an aldose reductase-like enzyme. DH4 represents more than 95% of the total tetrahydrobiopterin production. It has a direct antioxidant function to protect mitochondria against oxidative stress.