Mammalian inducible nitric oxide synthase (iNOS) catalyzes the production of L-citrulline and nitric oxide (NO) from L-arginine and O-2. The Soret peak in the spectrum of the iNOS heme domain (iNOS(oxy)) shifts from 423 to 390 nm upon addition of a sensitizer-wire, [Re-I-imidazole-(CH2)(8)-nitroarginine](+), or [ReC(8)argNO(2)](+), owing to partial displacement of the water ligand in the active site. From analysis of competitive binding experiments with imidazole, the dissociation constant (K-d) for [ReC(8)argNO(2)](+)-iNOS(oxy) was determined to be 3.0 +/- 0.1 mu M, confirming that the sensitizer-wire binds with higher affinity than both L-arginine (K-d = 22 +/- 5 mu M) and imidazole (K-d = 14 +/- 3 mu M). Laser excitation (355 nm) of [ReC(8)argNO(2)](+)-iNOS(oxy) triggers electron transfer to the active site of the enzyme, producing a ferroheme in less than similar to 1 mu s.