Study by carbon-13 NMR and by EPR of the reactions between the nitroprusside ion and haemoglobins

Release of NO from Reduced Nitroprusside Ion. Iron-Dinitrosyl Formation and NO-Disproportionation Reactions

The kinetics and mechanism of the thermal decomposition of the one-electron reduction product of [Fe(CN)(5)NO](2-) (nitroprusside ion, NP) have been studied by using UV-vis, IR, and EPR spectroscopy and mass-spectrometric and electrochemical techniques in the pH range of 4-10. The reduction product contains an equilibrium mixture of [Fe(CN)(4)NO](2-) and [Fe(CN)(5)NO](3-) ions. The first predominates at pH <8 and is formed by the rapid release of trans-cyanide from [Fe(CN)(5)NO](3-), which, in turn, is the main component at pH >9-10. Both nitrosyl complexes decay by first-order processes with rate constants around 10(-5) s(-1) (pH 6-10) related to the dissociation of NO. The decomposition is enhanced at pH 4 by 2 orders of magnitude with protons (and also metal ions) favoring the release of cyanides from the [Fe(CN)(4)NO](2-) ions and the ensuing rapid delivery of NO. At pH 7, an EPR-silent intermediate I(1) is detected (nu(NO), 1695 and 1740 cm(-1)) and assigned to the trans-[Fe(II)(CN)(4)(NO)(2)](2-) ion, an {Fe(NO)(2)}(8) species. At pH 6-8, I(1) induces a disproportionation process with formation of N(2)O and the regeneration of nitroprusside in a 1:2 molar ratio. At lower pHs, I(1) leads, competitively, to a second paramagnetic (S = 1/2) dinitrosyl intermediate I(2), [Fe(CN)(2)(NO)(2)](1-), a new member of a series of four-coordinate {Fe(L)(2)(NO)(2)} complexes (L = thiolates, imidazole, etc.), described as {Fe(NO)(2)}(9). Other decomposition products are hexacyanoferrate(II) or free cyanide, depending on the pH, and precipitates of the Prussian-Blue type. This study throws light on the conditions favoring rapid release of NO, to promote vasodilatory effects upon NP injection, and describes new processes related to dinitrosyl formation and NO disproportionation, which are also relevant to the diverse biological processes associated with NO and N(2)O processing.

FTIR investigation of the intermediates formed in the reaction of nitroprusside and thiolates

We have studied the outer-sphere reduction of [Fe(CN)5(NO)]2- by several reagents including dithionite and have for the first time measured the IR spectra of [Fe(CN)5(NO)]3- and [Fe(CN)4(NO)]2- in aqueous media. The spectra of [Fe(CN)5(NO)]3- and [Fe(CN)4(NO)]2- are consistent with bent six-coordinate {MNO}7 and linear five-coordinate {MNO}7 species, respectively. We have measured the UV-visible and IR spectra that evolve after [Fe(CN)5(NO)]2- is reacted with thiolate. These spectra permit us to assign the molecular structure of the so-called "red product" as [Fe(CN)5(eta1-N-RSNO)]3-. We have followed the decomposition of the [Fe(CN)5(eta1-N-RSNO)]3- by IR. Importantly, there is a 1:1 correspondence between the disappearance of the [Fe(CN)5(eta1-N-RSNO)]3- and the formation of [Fe(CN)5(NO)]3-. Thus, we conclude under the conditions of this study, reduction of [Fe(CN)5(NO)]2- by thiolate takes place via a (dark) inner-sphere mechanism that yields [Fe(CN)5(NO)]3- via homolytic N-S bond cleavage.

Cyanonitrosylmetallates as potential NO-donors

The [M(CN)xNOy]n- complexes (where M = Cr(I), Mn(I), Mn(II), Fe(I), Fe(II), Fe(III)) were studied as potential NO-donors using both pharmacological and theoretical semi-empirical methods. Only iron complexes appeared to be pharmacologically active. The quantum chemical calculations indicated that these complexes have the highest predisposition to undergo a nucleophilic attack followed by the NO+ release. The results allowed us to interpret the metabolism of the [M(CN)xNOy]n- complexes in terms of the NO(+)-donation.

Reductive metabolism of nitroprusside in rat hepatocytes and human erythrocytes

The metabolism of nitroprusside by hepatocytes or subcellular fractions involves a one-electron reduction of nitroprusside to the corresponding metal-nitroxyl radical. Thiol compounds also reduced nitroprusside to the metal-nitroxyl radical apparently via a thiol adduct. The nitroprusside reduction by microsomes was shown to be due to cytochrome P450 reductase as an antibody to cytochrome P450 reductase inhibits the microsomal reduction of nitroprusside, and the inhibitors of cytochrome P450 such as carbon monoxide or metyrapone had no effect. The reduction of nitroprusside by mitochondria in the presence of NADH or NADPH also produced the metal-nitroxyl radical. In hepatocytes, both mitochondria and the cytochrome P450 reductase are involved in the reduction of nitroprusside. The reductive metabolism of nitroprusside was found to produce toxic by-products, namely, free cyanide anion and hydrogen peroxide. We have also detected thiyl radicals formed in the thiol compound reduction of NP. We propose that cyanide and hydrogen peroxide are important toxic species formed in the metabolism of nitroprusside. The rate of reductive metabolism of nitroprusside by rat hepatocytes was much higher than with human erythrocytes. Therefore the major site of nitroprusside metabolism in vivo may be liver and not blood as originally proposed.

Decomposition and specific exchange of the trans-cyanide ligand on nitroprusside is facilitated by hemoglobin

Most investigators agree that sodium nitroprusside (SNP) undergoes biotransformation in vivo to release free cyanide (CN-). Despite a demonstrated reactivity of SNP toward oxyhemoglobin (HbO2) in vitro, that reaction is probably not the major cause for CN- release in vivo. An unknown reaction in various vascular beds inactivates SNP more rapidly than the reaction in blood, but both result in the release of free CN-. Recently it has been claimed that SNP is stable in blood and that the apparent CN- release is an artifact due to photodecomposition of SNP during CN- analyses. In this study the release of free CN- from SNP was followed over 3 hr of incubation in plain buffer, in suspensions of red blood cells (RBC), in lysates, and in solutions of HbO2 purified by isoelectric focusing and shown to be free of methemoglobin (MetHb), valency hybrid species, and reduced glutathione (GSH). In each case the mixtures containing HbO2 released significantly more CN- than plain buffer where CN- release was at the limit of sensitivity of the method. Moreover, in the order of RBC, purified HbO2 solutions and lysates, each preparation released significantly more CN- in 1 hr than the preceding one. In lysates the reaction had gone to completion by 3 hr. Using 13C nuclear magnetic resonance (NMR) spectroscopy, SNP was shown to be inert to CN- exchange in solutions of Na13CN, but when GSH or MetHb was added, an exchange reaction occurred between the trans-CN- of SNP and 13CN- in solution. The same reaction proceeded even more rapidly in the presence of HbO2. The results of this study show that in the presence of GSH, MetHb and particularly in the presence of HbO2, SNP readily exchanges its trans-CN- ligand with excess free CN-. This reaction is believed to represent an obligatory precursor step in the total decomposition of SNP which occurs in the absence of free CN-, but in the presence of RBC, solutions of HbO2, and lysates. It appears that excess free CN- halts total decomposition at the stage of trans-CN- labilization.