Factors affecting the line-shape of the EPR signal of high-spin Fe(III) in soybean lipoxygenase-1

EPR spectroscopy and electrospray ionization mass spectrometry reveal distinctive features of the iron site in leukocyte 12-lipoxygenase

The procedure for the expression and purification of recombinant porcine leukocyte 12-lipoxygenase using Escherichia coli [K.M. Richards, L.J. Marnett, Biochemistry 36 (1997) 6692-6699] was updated to make it possible to produce enough protein for physical measurements. Electrospray ionization tandem mass spectrometry confirmed the amino acid sequence. The redox properties of the cofactor iron site were examined by EPR spectroscopy at 25K following treatment with a variety of fatty acid hydroperoxides. Combination of the enzyme in a stoichiometric ratio with the hydroperoxides led to a g4.3 signal in EPR spectra instead of the g6 signal characteristic of similarly treated soybean lipoxygenase-1. Native 12-lipoxygenase was also subjected to electrospray ionization mass spectrometry. There was evidence for loss of the mass of an iron atom from the protein as the pH was lowered from 5 to 4. Native ions in these samples indicated that iron was lost without the protein completely unfolding.

Inhibition of lipoxygenase by soy isoflavones: evidence of isoflavones as redox inhibitors

Hydroperoxides, the products of lipoxygenase mediated pathways, play a major role in the manifestation of chronic inflammatory diseases. Soy isoflavones act as antioxidants due to their ability to scavenge free radicals. Isoflavones inhibit the activity of soy lipoxygenase-1 and 5-lipoxygenase, from human polymorph nuclear lymphocyte in a concentration dependent manner. Spectroscopic and enzyme kinetic measurements have helped to understand the nature and mechanism of inhibition. Genistein is the most effective inhibitor of soy lipoxygenase 1 and 5-lipoxygenase with IC(50) values of 107 and 125 microM, respectively. Genistein and daidzein are noncompetitive inhibitors of soy lipoxygenase 1 with inhibition constants, K(i), of 60 and 80 microM, respectively. Electron paramagnetic resonance and spectroscopic studies confirm that isoflavones reduce active state iron to ferrous state and prevent the activation of the resting enzyme. A model for the inhibition of lipoxygenase by isoflavones is suggested.

X-ray spectroscopy of the iron site in soybean lipoxygenase-1: changes in coordination upon oxidation or addition of methanol

Iron K-edge X-ray spectroscopy (XANES and EXAFS) was used to study iron coordination in frozen solutions of soybean lipoxygenase-1 (SLO). The intensity of the 1s-->3d pre-edge transition of native iron(II) lipoxygenase is greater than what was found for six-coordinate high-spin iron(II) model complexes, but comparable to that of a five-coordinate model. This and a relatively short average bond length determined by EXAFS (2.13 A) indicate that the native lipoxygenase in our frozen samples is five-coordinate, excluding possible bonds longer than 2.5 A. The coordination of the iron(II) in native lipoxygenase changes when methanol (as low as 0.1%) or glycerol (20%) is added to the buffer prior to freezing. The addition of methanol diminishes the pre-edge transition and increases EXAFS-derived bond lengths by 0.04 A, indicating a change to six-coordination. The small pre-edge feature in active iron(III) lipoxygenase suggests six-coordination. EXAFS indicates a short, 1.88 A Fe-O bond, which, given other spectroscopic and crystallographic evidence, is assigned to coordinated hydroxide. The average of the remaining bond lengths is 2.11 A. The iron coordination in iron(III) lipoxygenase is less affected by the presence of alcohols than is the site in the iron(II) enzyme. Bond valence sums indicate that the bond lengths for lipoxygenase derived from our EXAFS analyses are comparable to those of crystallographically characterized model complexes. The flexibility of the coordination number in SLON (native SLO) and the presence of an [FeIIIOH]2+ unit in SLOA (active SLO) are of possible mechanistic importance.

Characterization of the non-heme iron center of human 5-lipoxygenase by electron paramagnetic resonance, fluorescence, and ultraviolet-visible spectroscopy: redox cycling between ferrous and ferric states

Purified human 5-lipoxygenase, a non-heme iron containing enzyme, has been characterized by electron paramagnetic resonance, (EPR), ultraviolet (UV)-visible and fluorescence spectroscopy. As isolated, the enzyme is largely in the ferrous state and shows a weak X-band EPR signal extending from 0 to 700 G at 15 K, tentatively ascribed to integer spin Fe(II). Titration of the protein with 13-HPOD (13-hydroperoxyoctadecadienoic acid) generates a strong multicomponent EPR signal in the g' approximately 6 region, a yellow color associated with an increased absorption between 310 and 450 nm (epsilon 330nm = 2400 M-1 cm-1), and a 17% decrease in the intrinsic protein fluorescence. The multiple component nature of the g' approximately 6 signal indicates that the metal center in its oxidized state exists in more than one but related forms. The g' approximately 6 EPR signal and the yellow color reach a maximum when approximately 1 mol of 13-HPOD is added/mol of iron; the resultant EPR spectrum accounts quantitatively for all of the iron in the protein with a signal at g' = 4.3 representing less than 3% of the total iron in the majority of samples. Addition of a hydroxyurea reducing agent abolished the g' approximately 6 signal and yellow color of the protein and also reversed the decrease in fluorescence caused by the oxidant 13-HPOD. The results indicate that the g' approximately 6 EPR signal, the yellow color, and the decreased fluorescence are associated with the formation of the Fe(III) form of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

X-ray absorption spectroscopy of soybean lipoxygenase-1. Influence of lipid hydroperoxide activation and lyophilization on the structure of the non-heme iron active site

X-ray absorption spectra at the Fe K-edge of the non-heme iron site in Fe(II) as well as Fe(III) soybean lipoxygenase-1, in frozen solution or lyophilized, are presented; the latter spectra were obtained by incubation of the Fe(II) enzyme with its product hydroperoxide. An edge shift of about 2-3 eV to higher energy occurs upon oxidation of the Fe(II) enzyme to the Fe(III) species, corresponding to the valence change. The extended X-ray absorption fine structure shows clear differences in active-site structure as a result of this conversion. Curve-fitting on the new data of the Fe(II) enzyme, using the EXCURV88 program, leads to a coordination sphere that is in agreement with the active-site structure proposed earlier (6 +/- 1 N/O ligands at 0.205-0.209 nm with a maximum variance of 0.009 nm, including 4 +/- 1 imidazole ligands) [Navaratnam, S., Feiters, M. C., Al-Hakim, M., Allen, J. C., Veldink, G. A. and Vliegenthart, J. F. G. (1988) Biochim. Biophys. Acta 956, 70-76], while for the Fe(III) enzyme a shortening in ligand distances occurs (6 +/- 1 N/O ligands at 0.200-0.203 nm with maximum variance of 0.008 nm) and one imidazole is replaced by an oxygen ligand of unknown origin. Lyophilization does not lead to any apparent differences in the iron coordination of either species and gives a much better signal/noise ratio, allowing analysis of a larger range of data.

Inhibition of soybean lipoxygenase-1 by n-alcohols and n-alkylthiols

A series of n-alcohols and n-alkylthiols with carbon chains from 2 to 12 were examined for the inhibition of soybean lipoxygenase-1 (L-1). The alcohol produces a competitive inhibition, the extent of which increases with an increase in the carbon number of alkyl chain up to 8. Whereas the inhibition of the alkylthiol is noncompetitive, the extent of which is almost independent from the carbon number. From the behavior of pKi dependence on the carbon number of the alcohol, the decyl group appears to be optimum to bind to L-1. The thermodynamic analysis for the inhibition based upon van 't Hoff equation indicates positive enthalpy and entropy changes for the binding of the alcohol to the enzyme and negative enthalpy and positive to negative entropy changes for that of the alkylthiol. These observations suggest that the alcohol inhibits L-1 by binding of the hydrophobic alkyl tail to the catalytic site of the enzyme by a hydrophobic interaction. The alkylthiol inhibits by binding of the nucleophilic sulfhydryl head to a polarizable region of the enzyme and the alkyl tail to a hydrophobic region of the enzyme free from the steric hindrance as an anchor.

Formation of free radical metabolites in the reaction between soybean lipoxygenase and its inhibitors. An ESR study

Recent studies showed that soybean lipoxygenase inhibitors like phenidone and nordihydroguaiaretic acid (NDGA) reduce the catalytically active ferric lipoxygenase to its inactive ferrous form. Addition of 13(S)-hydroperoxy-cis-9,trans-11-octadecadienoic acid (13-HPOD) regenerated the active ferric form. In this paper, it is shown that in such a system the inhibitors are oxidized to free-radical metabolites. Incubation of soybean lipoxygenase and linoleic acid with p-aminophenol, catechol, hydroquinone, NDGA, or phenidone resulted in the formation of the one-electron oxidation products of these compounds. Free-radical formation depended upon the presence of the lipoxygenase and 13-HPOD. The free radicals were detected by ESR spectroscopy, and their structure was confirmed by analysis of the spectra, using a computer correlation technique. These data support the proposed mechanism for the inhibition of lipoxygenase by phenolic antioxidants.

Lipoxygenase isoenzymes: a spectroscopic and structural characterization of soybean seed enzymes

Applying recent developments in protein purification techniques, a number of lipoxygenase isoenzymes have been isolated in satisfactory quantities for a detailed physical and structural characterization. Four seed isoenzymes from two soybean cultivars have been studied by peptide mapping, free thiol and iron content determinations, and C-terminal analysis as well as by uv-visible absorption and EPR spectroscopy. While differences between the type 1 enzyme and the other isoenzymes were readily detected using proteolytic peptide mapping, digestion with dilute hydrochloric acid and C-terminal analysis both revealed structural features which were similar in all of the isoenzymes. One clear difference between the lipoxygenases was in their free sulfhydryl group content. The uv-visible absorption spectrum of each native isoenzyme was consistent with expectations for the experimental aromatic amino acid content. All of the isoenzymes contained one non-heme iron atom per molecule of protein. The oxidation of each isoenzyme with product hydroperoxide resulted in the conversion of the iron from an EPR silent state into several forms with EPR signals characteristic of high spin iron(III). The EPR spectra of all isoenzymes were remarkably similar. A time course EPR and catalytic activity study revealed that the various EPR active states represent a complex equilibrium between iron atoms in different environments. The pH dependence for the EPR and absorption spectroscopy lends support to the hypothesis that acid/base chemistry represents an important aspect of lipoxygenase catalysis.

Activity of soybean lipoxygenase in the absence of lipid hydroperoxide

Soybean lipoxygenase was assayed under conditions such that the concentration of the enzyme was in excess of the concentration of the substrate, arachidonic acid. Under these conditions, the concentration of lipid hydroperoxides present as contaminants in the substrate was negligible relative to the enzyme concentration, and the concentration of lipid hydroperoxide product could be determined accurately. The ferric form of the enzyme was observed to be fully active and to catalyze the oxidation of arachidonic acid at a near-diffusion-controlled rate, 1.4 X 10(7) M-1 s-1 at 0 degree C, at concentrations of lipid hydroperoxides as low as 5% of the enzyme concentration. From this, it can be concluded that the higher oxidation states that would be accessible by oxidation of Fe(III) by hydroperoxide are not required for catalysis by soybean lipoxygenase. Surprisingly, the activation of the ferrous form of the enzyme was also observed at insignificantly low lipid hydroperoxide concentrations. This activation presumably involves oxidation of the ferrous to the ferric form of the enzyme and must be more facile than has hitherto been reported. This result may rationalize previous reports that the ferrous and the ferric forms of the enzyme are both active.

Iron environment in soybean lipoxygenase-1

The iron coordination in native, Fe(II), lipoxygenase has been studied by Extended X-Ray Absorption Fine Structure (EXAFS). The ligands are 6 +/- 1 nitrogen and/or oxygen ligands at 2.05-2.09 A, with a maximum variance of 0.09 A. The number of imidazole ligands is estimated at 4 +/- 1 using multiple scattering simulations. The remaining ligands are proposed to be carboxylate oxygens.

Inhibition of soybean lipoxygenase 1 by N-alkylhydroxylamines

Micromolar concentrations of N-octylhydroxylamine dramatically increase the induction period in the conversion of linoleic acid to 13(S)-hydroperoxy-cis-9,trans-11-octadecadienoic acid (13-HPOD) catalyzed by soybean lipoxygenase 1. The induction period produced by N-octylhydroxylamine is abolished by 13-HPOD but not by the corresponding hydroxy acid. Addition of a catalytic amount of lipoxygenase to a mixture of 13-HPOD and N-octylhydroxylamine results in consumption of approximately 1 mumol of 13-HPOD/mumol of N-octylhydroxylamine present. These results can be explained by a model in which 13-HPOD oxidizes the enzyme from an inactive ferrous form to an active ferric form, as proposed by previous workers, and N-octylhydroxylamine reduces the enzyme back to the ferrous form. Consistent with this model, the ESR signal at g = 6.1 characteristic of ferric lipoxygenase is rapidly abolished by N-octylhydroxylamine and can be regenerated by 13-HPOD. These results provide additional support for earlier proposals that ferric lipoxygenase is the catalytically active form and also establish a novel method of inhibiting enzymes in this class. The octyl group of N-octylhydroxylamine appears to contribute to binding near the iron, since hydroxylamine and N-methylhydroxylamine do not extend the induction period. In the n-RNHOH series, activity passes through an optimum at R = decyl.