Synthesis and characterization of 3H-labelled tetrahydrobiopterin

We synthesized [3′-3H]-5,6,7,8-tetrahydrobiopterin from [8,5′-3H]guanosine 5′-triphosphate ([8,5′-3H]GTP) using GTP cyclohydrolase (EC 3.5.4.16), 6-pyruvoyltetrahydropterin synthase and sepiapterin reductase (EC 1.1.1.153). After purification by cation-exchange h.p.l.c. a solution of radiochemically pure (> 95%) [3′-3H]-5,6,7,8-tetrahydrobiopterin with a specific activity of 9.2 Ci/mmol was obtained. The product proved well suited for studying the binding of tetrahydrobiopterin to nitric-oxide synthase.