The interaction of various N-alkyl- and N-aryl-N'-hydroxyguanidines with recombinant NOS containing or not containing tetrahydrobiopterin (BH(4)) was studied by visible, electronic paramagnetic resonance (EPR), and resonance Raman (RR) spectroscopy. N-Hydroxyguanidines interact with the oxygenase domain of BH(4)-free inducible NOS (BH(4)-free iNOS(oxy)), depending on the nature of their substituent, with formation of two types of complexes that are characterized by peaks around 395 (type I) and 438 nm (type II') during difference visible spectroscopy. The complex formed between BH(4)-free iNOS(oxy) and N-benzyl-N'-hydroxyguanidine 1 (type II') exhibited a Soret peak at 430 nm, EPR signals at g = 1.93, 2.24, and 2.38, and RR bands at 1374 and 1502 cm(-)(1) that are characteristic of a low-spin hexacoordinated Fe(III) complex. Analysis of its EPR spectrum according to Taylor's equations indicates that the cysteinate ligand of native BH(4)-free iNOS(oxy) is retained in that complex. Similar iron(III)-ligand complexes were formed upon reaction of 1 and several other N-hydroxyguanidines with BH(4)-free full-length iNOS and BH(4)-free nNOS(oxy). However, none of the tested N-hydroxyguanidines were able to form such iron(III)-ligand complexes with BH(4)-containing iNOS(oxy), indicating that a major factor involved in the mode of binding of N-hydroxyguanidines to NOS is the presence (or absence) of BH(4) in their active site. Another factor that plays a key role in the mode of binding of N-hydroxyguanidines to NOS is the nature of their substituent. The N-hydroxyguanidines bearing an N-alkyl substituent exclusively or mainly led to type II' iron-ligand complexes. Those bearing an N-aryl substituent mainly led to type II' complexes, even though some of them exclusively led to type I complexes. Interestingly, the K(s) values calculated for BH(4)-free iNOS(oxy)-N-hydroxyguanidine complexes were always lower when their substituents bore an aryl group (140-420 microM instead of 1000-3900 microM), suggesting the existence of pi-pi interactions between this group and an aromatic residue of the protein. Comparison of the spectral and physicochemical properties of the N-hydroxyguanidine complexes of BH(4)-free iNOS(oxy) (type II') with those of the previously described corresponding complexes of microperoxidase (MP-8) suggests that, in both cases, N-hydroxyguanidines bind to iron(III) via their oxygen atom after deprotonation or weakening of the O-H bond. The aforementioned results are discussed in relation with recent data about the transient formation of iron-product intermediates during the catalytic cycle of l-arginine oxidation by eNOS. They suggest that N-hydroxyguanidines could constitute a new class of good ligands of heme proteins.