To check the stimulatory potency of the tetrahydro forms of the two major pteridines occurring in human tissues, neopterin and biopterin, NO synthase was purified 6000-fold from human cerebellum. Tetrahydrobiopterin stimulated the activity up to 4.5-fold in a concentration dependent manner with a maximum above 1 μM, whereas tetrahydroneopterin was completely inactive in concentrations up to 100 μM. Tetrahydrobiopterin, but not neopterin derivatives, were copurified with the NO synthase activity. Our results demonstrate that human cerebellum contains a tetrahydrobiopterin dependent NO synthase activity.
