Nitric oxide mediates vascular relaxing effects of endothelial cells, cytotoxic actions of macrophages and neutrophils, and influences of excitatory amino acids on cerebellar cyclic GMP. Its enzymatic formation from arginine by a soluble enzyme associated with stoichiometric production of citrulline requires NADPH and Ca2 . We show that nitric oxide synthetase activity requires calmodulin. Utilizing a 2',5'-ADP affinity column eluted with NADPH, we have purified nitric oxide synthetase 6000-fold to homogeneity from rat cerebellum. The purified enzyme migrates as a single 150-kDa band on SDS/PAGE, and the native enzyme appears to be a monomer.Endothelium-derived relaxing factor, a labile substance formed by endothelial cells, which mediates vasodilation, has been shown to be identical to nitric oxide (NO) (1-3). In addition to relaxing blood vessels, NO has multiple messenger functions as it has been demonstrated in macrophages (4) and in brain tissue (5-7). NO appears responsible for the cytotoxic effects of macrophages and neutrophils (8). We have obtained direct evidence for NO as a messenger for the influences of the excitatory amino acid glutamate on cGMP in the cerebellum (7). We showed a striking enhancement by glutamate and other excitatory amino acids of the conversion of arginine to NO and the associated formation of citrulline. Moreover we observed that NV-monomethyl-L-arginine (MeArg), an inhibitor ofthe enzymatic conversion of arginine to NO, inhibits glutamate-elicited cGMP formation, an influence selectively reversed by excess arginine.Evidence that NO mediates functions of tissues as diverse as the brain, endothelium, and blood cells suggests a widespread role for NO as a messenger molecule. Localizing NO formation at a cellular level throughout the body would be greatly facilitated by immunohistochemical identification of NO synthetase, the NO-forming enzyme. The mechanism of conversion of arginine to NO, presently unclear (9) 220C, assays were terminated with 2 ml of 20 mM Hepes, pH 5.5/2 mM EDTA, and were applied to 1-ml columns of Dowex AG50WX-8 (Na+ form), which were eluted with 2 ml of water.[3H]Citrulline was quantified by liquid scintillation spectroscopy of the 4-ml flow-through. Purification of NO Synthetase. Eighteen rat cerebella were homogenized in 100 ml of ice-cold buffer A [50 mM Tris HCl, pH 7.4/1 mM EDTA/antipain (10 mg/liter)/leupeptin (10 mg/liter)/soybean trypsin inhibitor (10 mg/liter)/pepstatin (10 mg/liter)/chymostatin (10 mg/liter)/phenylmethylsulfonyl fluoride (100 mg/liter)], and all subsequent procedures were carried out at 4°C. The homogenate was centrifuged at 20,000 x g for 15 min, and the supernatant was loaded at 2 ml/min onto a 20-ml column of diethylaminoethyl (DEAE) equilibrated with buffer A. The column was washed with 50 ml of buffer A and eluted with a 100-ml linear gradient of 0-400 mM NaCl in buffer A. Fractions (2.5 ml) were assayed for enzyme activity. Fractions containing the first peak of activity from the DEAE column were pooled and added to ...

Neuronal nitric oxide synthase (nNOS) is concentrated at synaptic junctions in brain and motor endplates in skeletal muscle. Here, we show that the N-terminus of nNOS, which contains a PDZ protein motif, interacts with similar motifs in postsynaptic density-95 protein (PSD-95) and a related novel protein, PSD-93.nNOS and PSD-95 are coexpressed in numerous neuronal populations, and a PSD-95/nNOS complex occurs in cerebellum. PDZ domain interactions also mediate binding of nNOS to skeletal muscle syntrophin, a dystrophin-associated protein. nNOS isoforms lacking a PDZ domain, identified in nNOSdelta/delta mutant mice, do not associate with PSD-95 in brain or with skeletal muscle sarcolemma. Interaction of PDZ-containing domains therefore mediates synaptic association of nNOS and may play a more general role in formation of macromolecular signaling complexes.

Nitric oxide (NO), apparently identical to endothelium-derived relaxing factor in blood vessels, is also formed by cytotoxic macrophages, in adrenal gland and in brain tissue, where it mediates the stimulation by glutamate of cyclic GMP formation in the cerebellum. Stimulation of intestinal or anococcygeal nerves liberates NO, and the resultant muscle relaxation is blocked by arginine derivatives that inhibit NO synthesis. It is, however, unclear whether in brain or intestine, NO released following nerve stimulation is formed in neurons, glia, fibroblasts, muscle or blood cells, all of which occur in proximity to neurons and so could account for effects of nerve stimulation on cGMP and muscle tone. We have now localized NO synthase protein immunohistochemically in the rat using antisera to the purified enzyme. We demonstrate NO synthase in the brain to be exclusively associated with discrete neuronal populations. NO synthase is also concentrated in the neural innervation of the posterior pituitary, in autonomic nerve fibres in the retina, in cell bodies and nerve fibres in the myenteric plexus of the intestine, in adrenal medulla, and in vascular endothelial cells. These prominent neural localizations provide the first conclusive evidence for a strong association of NO with neurons.

No abstract