IC50: 76-112 μM for GTP cyclohydrolase I

6,7-Dimethyltetrahydropterin is a GTP cyclohydrolase I inhibitor.

GTP cyclohydrolase I catalyzes the formation of D-erythro-7,8-dihydroneopterin (dihydroneopterin) triphosphate and formate from GTP. Dihydroneopterin triphosphate has been identified as a critical intermediate in the biosynthesis of folic acid, pteridines in insects and amphibians, and tetrahydrobiopterin. Tetrahydrobiopterin is the obligatory cofactor for tyrosine and tryptophan hydroxylase, which are rate-limiting enzymes for biogenic amine synthesis. Tetrahydrobiopterin is the cofactor for phenylalanine hydroxylase as well, which converts Lphenylalanine to L-tyrosine.

In vitro: Previous study identified 6,7-dimethyltetrahydropterin as a noncompetitive inhibitor of GTP cyclohydrolase. However, no substrate inhibition of the enzyme was detected 1mM GTP, which is about 8-fold the Km value of the enzyme [1]. Another study found that phenylalanine hydroxylase could be inhibited by 6,7-dimethyltetrahydropterin, its cofactor. The rate of inactivation, which was irreversible, increased with the concentration of 6,7-dimethyltetrahydropterin. Moreover, 6,7-dimethyltetrahydropterin was found to be unstable when the solution was exposed to air but was stabilized by dithiothreitol the aerobic oxidation of which was significantly accelerated by 6,7-dimethyltetrahydropterin [2].

In vivo: Up to now, there is no animal in vivo data reported.

Clinical trial: So far, no clinical study has been conducted.

References:
[1] Shen, R. ,Alam, A. and Zhang, Y. Inhibition of GTP cyclohydrolase I by pterins. Biochimica et Biophysica Acta 965, 9-15 (1988).
[2] Jakubovic A, Woolf LI, Chan-Henry E.  The inactivation of phenylalanine hydroxylase by 2-amino-4-hydroxy-6,7-dimethyltetrahydropteridine and the aerobic oxidation of the latter. The effects of catalase, dithiothreitol and reduced nicotinamide-adenine dinucleotide. Biochem J. 1971 Nov;125(2):563-8.