The NOmediated Oxidation of Oxymyoglobin and Oxyhemoglobin

The rapid reaction of nitrogen monoxide with oxyhemoglobin (oxyHb) is of particular interest, because it significantly reduces the half-life of NO' in vivo and is the cause of increased blood pressure observed when extracellular hemoglobin-based blood substitutes are administered [16].

We have recently shown that, in analogy to the reaction between nitrogen monoxide and superoxide, the reactions of oxymyoglobin (oxyMb) and oxyHb with NO' generate intermediate iron(iii)peroxynitrito complexes that were characterized by rapid-scan UV-visible spectroscopy [17, 18]. The intermediate complexes MbFe-IIIOONO and HbFeIIIOONO can be observed at alkaline pH, but rapidly decay to nitrate and the aquoiron(m) form of the proteins (metMb and metHb, respectively) under neutral or acidic conditions. The best spectrum obtained for HbFeIIIOONO in the visible region, shown as the first trace in Figure 2.6.1, has two absorption maxima at 504 nm (e504 = 8.7 mM-1 cm-1) and at 636 nm (e636 = 5.4 mM-1 cm-1) [18]. These maxima are characteristic of high-spin metHb and metMb derivatives with anionic ligands (Table 2.6.1). Almost identical absorption features were obtained for the myoglobin intermediate MbFe IIIOONO (Table 2.6.1), despite its greater instability, that made it impossible to obtain an accurate spectrum.

The second-order rate constants, obtained from the linear plots of the observed pseudo-first-order rate constants against oxyMb or oxyHb concentrations, are pH

500 550 600 650

Wavelength (nm)

Fig. 2.6.1. Rapid-scan UV-visible spectra of each s). The decay of the HbFemOONO (see the reaction of oxyHb (13.5 |im) with NO. (ca. arrows, spectrum 1) to HbFemOH (spectrum 7)

50 |m) in 0.1 m borate buffer at pH 9.5, 5 °C. is presented. Traces 1-7 were recorded 0, 160,

Averaged spectra were collected (62 scans 320, 480, 640, 800, and 1280 ms after mixing.

500 550 600 650

Wavelength (nm)

Fig. 2.6.1. Rapid-scan UV-visible spectra of each s). The decay of the HbFemOONO (see the reaction of oxyHb (13.5 |im) with NO. (ca. arrows, spectrum 1) to HbFemOH (spectrum 7)

50 |m) in 0.1 m borate buffer at pH 9.5, 5 °C. is presented. Traces 1-7 were recorded 0, 160,

Averaged spectra were collected (62 scans 320, 480, 640, 800, and 1280 ms after mixing.

Tab. 2.6.1. Selected spectroscopic data for different methmyoglobin and methemoglobin complexes.

Complex

Soret

Visible

0 0

Post a comment