Inhibition of GTP cyclohydrolase I by pterins.

Pterins inhibit rat liver GTP cyclohydrolase I activity noncompetitively. Reduced pterins, such as 7,8-dihydro-D-neopterin, (6R,S)-5,6,7,8-tetrahydro-D-neopterin, 7,8-dihydro-L-biopterin, (6R)-5,6,7,8-tetrahydro-L-biopterin, L-sepiapterin, and DL-6-methyl-5,6,7,8-tetrahydropterin are approximately 12-times more potent as inhibitors than are oxidized pterins, such as D-neopterin, L-biopterin, and isoxanthopterin. They are also 12-times more potent than folates, such as folic acid, dihydrofolic acid, (+/-)-L-tetrahydrofolic acid, and aminopterin. The Ki values for 7,8-dihydro-D-neopterin, 7,8-dihydro-L-biopterin, and (6R)-5,6,7,8-tetrahydro-L-biopterin are 12.7 microM, 14.4 microM, and 15.7 microM, respectively. These results suggest that mammalian GTP cyclohydrolase I may be regulated by its metabolic end products.