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We investigated the functional and allosteric effects of the 4-amino analogue of tetrahydrobiopterin, (6R)-2,4-diamino- 5,6,7,8-tetrahydro-6-(L-erythro-1,2-dihydroxypropyl) pteridine (4-amino-H4biopterin) on pteridine-free rat neuronal nitric oxide synthase. In the presence of added (6R)-5,6,7,8-tetrahydro-L-erythrobiopterin (H4biopterin; 10 microM), 4-amino-H4biopterin completely inhibited the conversion of both L-arginine and NG-hydroxy-L-arginine with half-maximally effective concentrations of 1.1+/-0.09 and 1.3+/-0.09 microM, respectively. Inhibition was reversible, as shown by a time-dependent restoration of citrulline formation upon dilution of the inhibitor-treated enzyme (t1/2=3.0 min). Binding of 4-amino-H4biopterin led to a complete conversion of the haem from low-spin to high-spin state, and to the formation of stable homodimers which partially survived electrophoresis under denaturating conditions. These results show that oxidation of both L-arginine and NG-hydroxy-L-arginine is pteridine-dependent, and that the allosteric effects of H4biopterin do not fully explain the essential role of the pteridine cofactor in nitric oxide biosynthesis.

Allosteric modulation of rat brain nitric oxide synthase by the pterin-site enzyme inhibitor 4-aminotetrahydrobiopterin