Changes in mean arterial pressure were monitored in rats following 50% isovolemic exchange transfusion with solutions of chemically modified hemoglobins. Blood pressure responses fall into three categories: 1) an immediate and sustained increase, 2) an immediate yet transient increase, or 3) no significant change either during or subsequent to exchange transfusion. The reactivities of these hemoglobins with nitric monoxide ( ⅐ NO) were measured to test the hypothesis that different blood pressure responses to these solutions result from differences in ⅐ NO scavenging reactions. All hemoglobins studied exhibited a value of 30 M ؊1 s ؊1 for both ⅐ NO bimolecular association rate constants and the rate constants for ⅐ NO-induced oxidation in vitro. Only the ⅐ NO dissociation rate constants and, thus, the equilibrium dissociation constants varied. Values of equilibrium dissociation constants ranged from 2 to 14 pM and varied inversely with vasopressor response. Hemoglobin solutions that exhibited either transient or no significant increase in blood pressure showed tighter ⅐ NO binding affinities than hemoglobin solutions that exhibited sustained increases. These results suggest that blood pressure increases observed upon exchange transfusion with cell-free hemoglobin solutions can not be the result of ⅐ NO scavenging reactions at the heme, but rather must be due to alternative physiologic mechanisms.Control of blood pressure and resistance to blood flow is achieved by a dynamic constriction and relaxation of smooth muscle tissue which surrounds all blood vessels except capillaries. Vascular smooth muscle tension is continually adjusted by a complex system that causes either vasoconstriction or vasodilation, depending on metabolic need (1). Research performed over the last decade has established that endotheliumderived nitric oxide ( ⅐ NO) 1 can cause vasodilation. ⅐ NO is produced by endothelial cells that lie between the intravascular space and the surrounding smooth muscle. Among the findings was the demonstration that ⅐ NO donors (e.g. nitroprusside, nitroglycerin) lead to vasorelaxation through activation of guanylate cyclase, whereas inhibitors of ⅐ NO synthesis (e.g. N Gmonomethyl-L-arginine) or scavengers (e.g. hemoglobin) cause vasoconstriction (for reviews, see Refs. 2 and 3).Since cell-free hemoglobin is being developed as a red cell substitute (4), reactions between hemoglobin and ⅐ NO are of potential importance in maintenance of microvascular blood flow and O 2 delivery. Despite the wide variation that exists in the physical properties (O 2 affinity, molecular mass, and solution properties) of different cell-free hemoglobins, it appears vasoconstriction is a feature common to many hemoglobin solutions (for reviews, see Refs. 2, 3, and 5). It is tempting to conclude that ⅐ NO scavenging is the principal, if not sole mechanism for vasoconstriction associated with cell-free hemoglobin. However, it is well established that multiple factors contribute to the physiological control of vascular smooth muscle to...

Indium tin oxide (ITO) reacts with tetra(tert-butoxy)tin to give surface bound alkoxytin species. Ligand exchange occurs in these surface bound species by reaction with substituted phenols. The speciation of surface phenoxides was measured in ultrahigh vacuum by X-ray photoelectron spectroscopy, as was the work function for the surface modified ITO. It is shown that the molecular dipole moment of the phenol correlates strongly with the measured ITO work function change; no such correlation exists between the acidity of the phenol and this measured change in work function. Results are consistent therefore with an electrostatic model and not with one involving electronegativities of the ligated phenoxide oxygens.

Nitric-oxide synthase (NOS) catalyzes conversion of L-arginine to nitric oxide, which subsequently stimulates a host of physiological processes. Prior work suggests that NOS is inhibited by NO, providing opportunities for autoregulation. This contribution reports that NO reacts rapidly (k a Х 2 ؋ 10 7 M ؊1 s ؊1 ) with neuronal NOS in both its ferric and ferrous oxidation states. Association kinetics are almost unaffected by L-arginine or the cofactor tetrahydrobiopterin. There is no evidence for the distinct two phases previously reported for association kinetics of CO. Small amounts of geminate recombination of NO trapped in a protein pocket can be observed over nanoseconds, and a much larger amount is inferred to take place at picosecond time scales. Dissociation rates are also very fast from the ferric form, in the neighborhood of 50 s ؊1 , when measured by extrapolating association rates to the zero NO concentration limit. Scavenging experiments give dissociation rate constants more than an order of magnitude slower: still quite fast. For the ferrous species, extrapolation is not distinguishable from zero, while scavenging experiments give a dissociation rate constant near 10 ؊4 s ؊1. Implications of these results for interactions near the heme binding site are discussed. Nitric-oxide synthases (NOS)1 constitute a family of heme proteins that catalyze conversion of L-arginine to citrulline and nitric oxide (1). The production of nitric oxide in specific cell types fulfills certain physiological roles for which each isoform is suited according to its structure and regulation (2). Neuronal NOS ϩ (nNOS) (1) isoforms are localized in brain and in skeletal myotendinous junctions for the production of NO as a neurotransmitter. Inducible NOS (iNOS), first found in macrophages and induced by cytokines, produces NO for cytotoxic action. Endothelial NOS (eNOS) produces NO as a vasodilator. It was reported some years ago that NOS activity is inhibited by diatomic ligands such as carbon monoxide (3-5), cyanide (3), and nitric oxide (3). Although CO inhibition of NOS would be due to ligand binding with the ferrous heme intermediate, the situation with NO is more complex. Griscavage et al. demonstrated that NO inhibits and probably provides negative feedback for all three isoforms: iNOS (6), eNOS (7), and nNOS (8). The last of those studies made a particularly convincing case that NO is a powerful inhibitor and acts by interacting directly with the ferric heme intermediate (8).We previously reported the kinetics of CO association with nNOS and its expressed heme domain as well as dissociation from the nNOS-CO complex (9). From the rates, we estimated an equilibrium constant suggesting that in vivo inhibition of NOS by CO is unlikely for plausible physiological concentrations, unless there is great local enhancement of CO concentration, as could possibly be created by heme oxygenase activity. Nevertheless, in vitro studies of inhibition of NOS activity by CO are important for elucidating the reaction mechanism of NOS activ...

Kinetics data were collected for the palladium-catalyzed reduction of nitric oxide (NO) to nitrous oxide (N2O) with cuprous chloride reductant in 2 M hydrochloric acid (2NO + 2CuCl + 2HCl → N2O + 2CuCl2 + H2O). The rate-determining step was first order in the palladium concentration and NO partial pressure. The cuprous chloride dependence was first order below 0.1 M; at higher concentrations saturation kinetics were observed. The rate of reaction was independent of H+ and Cl- concentrations. Kinetics results were consistent with the initial, reversible attack (k 1/k -1) of free NO on the bound nitrosyl of [PdCl3NO]2- yielding [PdCl3(N2O2)]2-, which is then reduced by Cu(I) (k 2) to generate products and recycle the palladium. A k 1 value of (6.0 ± 0.4) × 10-6 (PNO)-1 s-1 at 20 °C was calculated, with a k -1/k 2 ratio of 0.116 ± 0.004 Μ. Rate measurements show that NO reduction by Cu(I) is the rate-limiting step in the Wacker-style catalysis of the CO + 2NO → CO2 + N2O reaction. The current mechanism resembles the nitric oxide reductase activities of cytochrome c oxidases, which proceed by Cu(I) reduction of a heme bound nitrosyl, and cytochrome P450nor.

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