Abstract
The endogenous formation of nitric oxide (NO) has become an area of intense interest as evidence for its biological functions has been obtained in three distinct tissues: circulating macrophages, in which it exerts cytotoxic effects; blood vessels, in which it has been identified as endothelium-derived relaxing factor; and neuronal cells, in which it functions as a neurotransmitter. The formation of NO in brain extracts has been shown to be catalyzed by an enzyme, termed NO synthase, which generates the NO responsible for stimulation of cGMP formation, the highest levels of which occur in the cerebellum. NO synthase catalyzes the formation of citrulline from arginine with the coincident production of NO and has been shown to be a flavoprotein, containing 1 mol each of FAD and FMN, tetrahydrobiopterin, and iron. It is also reported to contain an alpha-helical, calmodulin-binding consensus sequence consistent with its stimulation by calmodulin in the presence of Ca2+. The formation of NO requires incorporation of one of the atoms of molecular oxygen into one of the guanidinium nitrogen atoms of arginine with the coincident formation of citrulline. This communication reports that rat cerebellar NO synthase, cloned and stably expressed in human kidney 293 cells, contains heme in amounts stoichiometric with the flavins FAD and FMN as evidenced by the appearance of a pyridine hemochrome and a reduced CO difference spectrum with an absorbance maximum at approximately 445 nm. The finding of a CO-binding heme moiety explains the presence of iron in the enzyme and suggests a role for prosthetic heme as an oxygenase reaction center. This report also presents evidence for incorporation of delta-[14C]aminolevulinate specifically into immunoprecipitable NO synthase in stably transfected human kidney 293 cells but not in nontransfected cells. Simultaneously, K. A. White and M. A. Marletta [(1992) Biochemistry 31, 6627-6631] have demonstrated a CO-binding heme prosthetic group in purified murine macrophage NO synthase and have suggested the identity of these reaction centers in both the constitutive (cerebellar) and inducible (macrophage) forms of NO synthase.