Reactivity of the heme–dioxygen complex of the inducible nitric oxide synthase in the presence of alternative substrates.

Lefèvre-Groboillot, David; Boucher, Jean-Luc; Mansuy, Daniel; Stuehr, Dennis J.

Single turnover reactions of the inducible nitric oxide synthase oxygenase domain (iNOSoxy) in the presence of several non α-amino acid N-hydroxyguanidines and guanidines were studied by stopped-flow visible spectroscopy, and compared with reactions using the native substratesl-arginine (l-arg) or Nω-hydroxy-l-arginine (NOHA). In experiments containing dihydrobiopterin, a catalytically incompetent pterin, and each of the studied substrates,l-arg, butylguanidine (BuGua), para-fluorophenylguanidine (FPhGua), NOHA, N-butyl- and N-( para-fluorophenyl)- N′-hydroxyguanidines (BuNOHG and FPhNOHG), the formation of a iron(II) heme–dioxygen intermediate (FeIIO2) was always observed. The FeIIO2 species then decayed to iron(III) iNOSoxy at rates that were dependent on the nature of the substrate. Identical reactions containing the catalytically competent cofactor tetrahydrobiopterin (BH4), iNOSoxy and the three N-hydroxyguanidines, all exhibited an initial formation of an FeIIO2 species that was successively converted to an FeIIINO complex and eventually to high-spin iron(III) iNOSoxy. The formation and decay kinetics of the FeIIINO complex did not vary greatly as a function of the N-hydroxyguanidine structure, but the formation of FeIIINO was substoichiometric in the cases of BuNOHG and FPhNOHG. Reactions between BH4-containing iNOSoxy and BuGua exhibited kinetics similar to those of the corresponding reaction withl-arginine, with formation of an FeIIO2 intermediate that was directly converted to high-spin iron(III) iNOSoxy. In contrast, no FeIIO2 intermediate was observed in the reaction of BH4-containing iNOSoxy and FPhGua. Multi-turnover reaction of iNOS with FPhGua did not lead to formation of NO or to hydroxylation of the substrate, contrary to reactions with BuGua orl-arg. Our results reveal how different structural and chemical properties of NOS substrate analogues can impact on the kinetics and reactivity of the FeIIO2 intermediate, and support an important role for substrate p Ka during NOS oxygen activation.

GUANIDINES; NITRIC oxide; FLOW injection analysis; CHEMICAL reactions; AMINO acids; INTERMEDIATES (Chemistry); ORGANIC compounds; BIOCHEMISTRY

FEBS Journal, 2006, Vol 273, Issue 1, p180

1742-464X

Academic Journal

10.1111/j.1742-4658.2005.05056.x