Tetrahydrobiopterin, a cofactor for rat cerebellar nitric oxide synthase, does not function as a reactant in the oxygenation of arginine

Induction of nitric oxide synthase in glial cells

Primary astrocyte cultures, C6 glioma cells, and N18 neuroblastoma cells were assayed for nitric oxide synthase (NOS) activity with a bioassay of cyclic GMP production in RFL-6 fibroblasts. Treatment of astrocyte cultures for 16-18 h with lipopolysaccharide (LPS) induced NOS-like activity that was L-arginine and NADPH dependent, Ca2+ independent, and potentiated by superoxide dismutase. Induction was evident after 4 h, was dependent on the dose of LPS, and required protein synthesis. Treatment of astrocyte cultures with leucine methyl ester reduced microglial cell contamination from 7 to 1%, with a loss of 44% of NOS-like activity. C6 cells treated with LPS also showed Ca(2+)-independent and L-arginine-dependent NOS-like activity. N18 cells demonstrated constitutive Ca(2+)-dependent NOS-like activity that was not enhanced by LPS induction. These data indicate that NOS-like activity can be induced in microglia, astrocytes, and a related glioma cell line as it can in numerous other cell types, but not in neuron-like N18 cells.

A novel neuronal messenger molecule in brain: the free radical, nitric oxide

Understanding of the organization and function of a newly identified neuronal messenger molecule, nitric oxide, has progressed rapidly. Nitric oxide synthase has been purified and molecularly cloned from brain. Its localization is exclusively neuronal and endothelial. The catalytic activity of nitric oxide synthase accounts for the NADPH diaphorase staining of neurons that are uniquely resistant to toxic insults and neurodegenerative disorders. Nitric oxide has diverse functions. In platelets it inhibits their aggregation, in macrophages it mediates cytotoxicity, and in blood vessels it acts as a vasodilator. In the nervous system nitric oxide may be the retrograde transmitter in long-term potentiation. It is the "neurotransmitter" of cerebral vasodilator nerves and the inhibitory "neurotransmitter" of the motor neurons of the intestines. Nitric oxide in situations of excessive production may function as a neurotoxin, suggesting a role for nitric oxide in neurodegenerative disorders.

Mechanistic probes of N-hydroxylation of L-arginine by the inducible nitric oxide synthase from murine macrophages

NG-Hydroxy-L-arginine, [15N]-NG-hydroxy-L-arginine, and NG-hydroxy-NG- methyl-L-arginine were used as mechanistic probes of the initial step in the reaction catalyzed by nitric oxide synthase isolated from murine macrophages. NG-Hydroxy-L-arginine was found to be a substrate for nitric oxide synthase with a Km equal to 28.0 microM, yielding nitric oxide and L-citrulline. NADPH was required for the reaction and (6R)-tetrahydro-L-biopterin enhanced the initial rate of nitric oxide formation. The stoichiometry of NG-hydroxy-L-arginine loss to L-citrulline and nitric oxide (measured as nitrite and nitrate) formation was found to be 1:1:1. NG-Hydroxy-L-arginine was also observed in small amounts from L-arginine during the enzyme reaction. Studies with [15N]-NG-hydroxy-L-arginine indicated that the nitrogen in nitric oxide is derived from the oxime nitrogen of [15N]-NG-hydroxy-L- arginine. NG-Hydroxy-NG-methyl-L-arginine was found to be both a reversible and an irreversible inhibitor of nitric oxide synthase, displaying reversible competitive inhibition with K(i) equal to 33.5 microM. As an irreversible inhibitor, NG-hydroxy-NG-methyl-L-arginine gave kinact equal to 0.16 min-1 and KI equal to 26.5 microM. This inhibition was found to be both time- and concentration-dependent as well as showing substrate protection against inactivation. Gel filtration of an NG-hydroxy-NG-methyl-L-arginine-inactivated nitric oxide synthase failed to recover substantial amounts of enzyme activity.

Identification and characterization of a calmodulin-dependent nitric oxide synthase from GH3 pituitary cells

A nitric oxide synthase activity stimulated more than 30-fold by the concurrent presence of Ca2+ and calmodulin (CaM), and inhibited by trifluoperazine (50 microM), has been identified in extracts of GH3 pituitary cells. The CaM-dependent nitric oxide synthase of the crude extract was stimulated more than 9-fold by (6R)-5,6,7,8-tetrahydro-L-biopterin with half-maximal stimulation occurring at a concentration of 300 nM. Fractionation of the extract on DEAE-cellulose enhanced nitric oxide synthase specific activity up to 300-fold and provided a preparation which on Western blot analysis possessed a 152 kDa protein which cross-reacted with antibodies to homogeneous bovine brain nitric oxide synthase. The DEAE-cellulose-purified enzyme exhibited apparent Km values of 4.3 microM, 0.4 microM, 0.3 microM and 4 nM for L-arginine, NADPH, Ca2+ and CaM respectively. The CaM-dependent nitric oxide synthase of GH3 extract bound to 2',5'-ADP-agarose and was eluted by NADPH with a 500-fold increased specific activity. Citrulline formation by the ADP-agarose-purified enzyme was inhibited by NG-nitro-L-arginine, NG-methyl-L-arginine and Nitro Blue Tetrazolium with apparent Ki values of 0.2, 1.8 and 7 microM respectively. The ADP-agarose-purified enzyme displayed cytochrome c reductase activity which was stimulated more than 18-fold by the concurrent presence of Ca2+ and CaM and inhibited by trifluoperazine. NG-Nitro-L-arginine and NG-methyl-L-arginine did not inhibit the cytochrome c reductase activity.

Stimulation of human nitric oxide synthase by tetrahydrobiopterin and selective binding of the cofactor

To check the stimulatory potency of the tetrahydro forms of the two major pteridines occurring in human tissues, neopterin and biopterin, NO synthase was purified 6000-fold from human cerebellum. Tetrahydrobiopterin stimulated the activity up to 4.5-fold in a concentration dependent manner with a maximum above 1 microM, whereas tetrahydroneopterin was completely inactive in concentrations up to 100 microM. Tetrahydrobiopterin, but not neopterin derivatives, were copurified with the NO synthase activity. Our results demonstrate that human cerebellum contains a tetrahydrobiopterin dependent NO synthase activity.

Purification of a distinctive form of endotoxin-induced nitric oxide synthase from rat liver

An endotoxin-induced form of nitric oxide synthase (EC 1.14.23) was purified to homogeneity from rat liver by sequential anion-exchange chromatography and affinity chromatography using 2',5'-ADP-Sepharose. The enzyme has a subunit molecular mass of 135 kDa as determined by SDS/PAGE, a maximum specific activity of 462 nmol of citrulline formed from arginine per min per mg, and a Km for arginine of 11 microM. The enzyme was strongly stimulated by the addition of calmodulin with an EC50 of 2 nM, but removal of free calcium from the assay medium only reduced activity by 15%. Calmodulin inhibitors significantly reduced the enzyme activity. Tetrahydrobiopterin, FAD, and FMN were all required for full enzyme activity. This form of endotoxin-induced nitric oxide synthase from liver differs from the inducible enzyme found in macrophages and is unusual in that it is stimulated by calmodulin with little dependence on the calcium ion concentration.