Damaging effects of oxygen radicals on resealed erythrocyte ghosts

Resealed ghosts of human erythrocytes are sensitive to oxidative damage induced by xanthine oxidase acting on xanthine in the presence of iron. Damage was assessed in terms of lipid peroxidation and increased permeation of trapped markers, Na+ and glucose-6-P. Key findings are as follows. (a) Marker efflux from xanthine/xanthine oxidase/iron-treated ghosts accelerated after a lag, Na+ emerging far ahead of glucose-6-P. (b) Both effluxes and lipid peroxidation were stimulated by Fe(III) in a dose-dependent fashion and inhibited by chelating agents. (c) The antioxidant butylated hydroxytoluene effectively halted lipid peroxidation and net glucose-6-P efflux, but slowed Na+ efflux only partially. (d) Lipid peroxidation and marker release could be completely inhibited by superoxide dismutase or catalase, indicating that O2- and H2O2 are both required, possibly as precursors of OH. via the iron-catalyzed Haber-Weiss reaction (O2- + H2O2 leads to OH- + OH. + O2). (e) OH. scavengers, e.g. ethanol, mannitol, choline, had no protective effect against marker efflux and lipid peroxidation. Yet these agents did intercept OH. in the bulk medium, since they inhibited the degradation of 2-deoxyribose added as an extramembranous OH. probe. It is proposed that OH. produced on the membrane at iron binding sites reacts so rapidly with target molecules that scavengers cannot compete. (f) Desferrioxamine abolished all effects, including net egress of Na+. EDTA, while totally inhibitory toward lipid peroxidation and glucose-6-P release, diminished Na+ release partially, changing it to first order, approximately 3-fold faster than background. The latter response was totally inhibited by catalase, but only marginally by superoxide dismutase. This and other evidence suggests that different forms of membrane damage are responsible for enhanced permeation of the two markers; although glucose-6-P depends on lipid peroxidation, Na+ does not, certainly when EDTA is present.

Superoxide and hydrogen peroxide-dependent lipid peroxidation in intact and triton-dispersed erythrocyte membranes

Isolated erythrocyte membranes incubated with xanthine, xanthine oxidase, and Fe(III) underwent lipid peroxidation, as indicated by the thiobarbituric acid reaction and iodometric determination of hydroperoxides. In detergent-free medium (phosphate buffered saline) peroxidation was inhibited by superoxide dismutase, catalase, and EDTA; but was promoted by OH. scavangers, eg. mannitol. Generation of OH. in the system via iron-catalyzed reduction of H2O2 by O-2 was demonstrated by EPR spectrometry using spin trapping. In membranes treated with Triton X-100 lipid peroxidation was stimulated by EDTA and suppressed by OH. traps. This and other evidence suggests that OH. in the medium was an effective initiator of lipid peroxidation in detergent-dispersed membranes, but not in intact membranes.

Photodynamic action of protoporphyrin on resealed erythrocyte membranes: mechanisms of release of trapped markers

Photoactivation of protoporphyrin IX (PP) bound to resealed human erythrocyte (RBC) ghosts results in membrane damage which is manifested by the release of trapped markers Na+ and glucose-6-phosphate (G6P). Efflux of Na+ was rapid, continuous, and virtually complete before the onset of G6P efflux. The sugar phosphate emerged abruptly after a long lag. The antioxidant butylated hydroxytoluene (BHT) had no effect on the permeation of Na+, but greatly suppressed that of G6P. These results suggest that the markers are emitted via different mechanisms. For G6P, disruption of the bilayer by free radical lipid peroxidation appears to be necessary, inasmuch as BHT inhibited peroxidation as measured by thiobarbituric acid reactivity and appearance of phospholipid and cholesterol hydroperoxides on thin layer chromatograms. It is deduced that non-lipid damage is sufficient for Na+ release. This effect is manifested at low light intensities and low PP concentrations. Protein regulators of passive cation permeability may be the primary targets in this case. When sensitive sulfhydryl groups on these proteins were blocked with p-chloromercuri-benzene-sulfonate, Na+ leaked out rapidly, but G6P was unaffected, thereby mimicking the early stages of membrane photodamage.

Lysis of resealed erythrocyte ghosts by photoactivated tetrapyrroles: estimation of photolesion dimensions

1. Resealed erythrocyte ghosts containing Na+ and glucose-6-P (G6P) as markers of membrane integrity were used as a model system for probing the damaging effects of photoactivated tetrapyrroles on cell membranes. 2. Continuous blue-light irradiation of bilirubin (BR)-sensitized and protoporphyrin (PP)-sensitized ghosts made them progressively more permeable to Na+, the cation emerging well ahead of G6P. 3. G6P efflux occurred abruptly after a lag period and resembled an all-or-none process. 4. These and other results suggest that a relatively subtle structural modification (possibly in some crucial protein(s) is sufficient for Na+ release, whereas gross disruption of the bilayer (probably by free-radical lipid peroxidation) is necessary for G6P release. 5. The dimensions of the G6P-releasing photolesions were estimated by density floatation centrifugation, using saccharides of increasing molecular size. Both BR and PP produced pores greater than 11 A but less than 42 A in diam., which is considerable smaller than the size range estimated in hypotonically lysed ghosts.

Photosensitized cross-linking of erythrocyte membrane proteins. Evidence against participation of amino groups in the reaction

Exposure of human erythrocyte ghosts (pH 8, 10 degrees C) to visible light in the presence of the photosensitizer, methylene blue, results in a relatively rapid loss of spectrin (bands 1 and 2 on sodium dodecyl sulfate gel electropherograms) and the appearance of high molecular weight cross-linked derivatives. Isolated spectrin also undergoes photosensitized cross-linking, indicating that the reaction is not lipid-dependent. Extensive cross-linking was neither reversed by dithiothreitol nor prevented by prior blocking of SH groups with N-ethylmaleimide, suggesting that cysteine residues are not crucial bridging sites. The possible requirement for NH2 groups, as suggested by previous model studies (Dubbelman, T.M.A.R., de Goeij, A.F.P.M. and van Steveninck, J. (1978) Biochim. Biophys. Acta 511, 141--151), was tested. Succinylation of spectrin protected against cross-linking, but this effect is attributed to the disruption of quaternary structure, as deduced from sedimentation measurements. However, virtually complete blocking of NH2 groups by amidination perturbed overall structure relatively little, and had no effect on cross-linking. Moreover, exogenous amines such as ethylamine, added in large excess to spectrin prior to irradiation, did not interfere with cross-link formation. These results suggest that NH2 groups are not involved in the reaction.

Purification of calciferol-binding proteins from kidney: physicochemical and immunological properties

The calciferol-binding system of rat kidney cytosol has been purified and is shown to consist of two proteins, each capable of binding either 25-hydroxy-vitamin D3 (25-OH-D3) or 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). The two proteins, designated A and B, have similar sedimentation coefficients (S20w) of 5.2 S. Component A binds 25-OH-D3 with a dissociation constant (Kd) of 10(-7) M while component B binds 1,25-(OH)2D3 with a Kd of 1.6 x 10(-8) M. The estimated molecular weights (Mr) of the two proteins are 105,000 for component A and 250,000 for component B. Amino acid analyses revealed that glutamic acid is the most abundant residue in both proteins, comprising 12% of the total number of amino acid residues. Immunodiffusion test using commercial anti-human serum group-specific protein antiserum gave a precipitin reaction when purified rat serum calciferol-binding protein was used as an antigen, but no reactions could be detected with proteins A and B. This result significantly eliminated the possibility of the presence of the rat serum binding protein in either of the purified kidney proteins. In contrast, anti-rat serum calciferol-binding protein antiserum prepared in rabbits interacted with the rat serum and kidney proteins. This result suggests that the antigenic determinants recognized by the antiserum against the rat serum calciferol-binding protein appear to be similar to those recognized in the kidney proteins A and B. Immunoelectrophoresis of the three rat proteins demonstrated dissimilar electrophoretic mobilities with the serum protein showing the least mobility, a property consistent with its higher lysine content relative to proteins A and B.

Interaction of inhibitors with muscle phosphofructokinase

The interaction of several inhibitors with muscle phosphofructokinase has been studied by both equilibrium binding measurements and kinetic analysis. At low concentrations of citrate a maximum of 1 mol is bound per mol of enzyme protomer. Tight binding requires MgATP and very weak binding is observed in the absence of either magnesium ion or ATP. ITP at low concentrations cannot replace ATP. In the presence of MgATP and at pH 7.0, the dissociation constant for the enzyme-citrate complex is 20 muM. At 50 muM citrate and excess magnesium ion, the concentration of ATP required to give half-maximal binding of citrate is approximately 3 muM . Both P-enolpyruvate and 3-P-glycerate compete for the binding of citrate and the estimated Ki values are 480 and 52 muM, respectively. Creatine-P, another inhibitor of muscle phosphofructokinase, does not compete with the binding of citrate. Measurement of the equilibrium binding of ATP shows that citrate, 3-P-glycerate, P-enolpyruvate, and creatine-P all increase the affinity of enzyme for MgATP with the concentration required to give an effect increasing in the order given. In kinetic studies, citrate, 3-P-glycerate and P-enolpyruvate each act synergistically with ATP to inhibit the phosphofructokinase reaction. This is indicated by the observation that the three metabolites do not inhibit the enzyme with ITP as the phosphoryl donor and that they inhibit at ATP concentrations that are not themselves inhibitory. Furthermore, the sensitivity to the inhibitors increases with increasing ATP concentrations. Striking differences in the extent of inhibition can be seen by varying the order of addition of assay components. Preincubation of the enzyme with ATP and citrate, 3-P-glycerate, or P-enolpyruvate results in greater inhibition than when the inhibitor is added after the reaction is started with fructose-6-P. Furthermore, the inhibition is reversed partially 10 to 15 min after the addition of fructose-6-P. This phenomenon is particularly striking with creatine-P as the inhibitor. Very high concentrations of this inhibitor are required to show any effect if the inhibitor is added after fructose-6-P. These effects are interpreted as reflecting slow conformational changes between an active form with high affinity for fructose-6-P and an inactive, or less active, conformation that binds the inhibitors. Citrate, 3-P-glycerate, P-enolpyruvate, and creatine-P increase the rate of the phosphofructokinase at subsaturating concentrations of MgITP. The results indicate a common binding site on the enzyme for citrate, 3-P-glycerate, and P-enolpyruvate that is distinct from the ATP inhibitory site. An additional site (or sites) for creatine-P is indicated. All four inhibitors act synergistically with ATP by increasing the affinity of the enzyme for MgATP at an inhibitory site. The inhibitors appear also to increase the affinity of the catalytic nucleoside triphosphate site for substrate.

Photodynamic action of bilirubin on human erythrocyte membranes. Modification of polypeptide constituents

The photodynamic action of bilirubin on isolated human erythrocyte membranes (ghosts) has been studied. When incorporated into ghosts (pH 8.0,10 degrees) the bile pigment photosensitizes in blue light the peroxidation of unsaturated lipids, as evidenced by a positive color reaction with 2-thiobarbituric acid. Accompanying lipid peroxidation was the disappearance of most of the major membrane proteins (Coomassie Blue staining in sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and appearance of polypeptide photoproducts of greater size (mol wt greater than 250,000). The association of membrane proteins (presumably by cross-linking) was insignificant when bilirubin-ghost suspensions were kept in the dark, or when ghosts were irradiated in the absence of bilirubin. Electrophoretic bands 1 and 2 (Fairbanks, G., Steck, T.L., and Wallach, D. F.H (1971), Biochemistry 10, 2606) diminished rapidly during the photoreaction, whereas band 3 and the three sialoglycoproteins disappeared at a much slower rate. Dispersal of membrane consituents by treatment with sodium dodecyl sulfate prior to irradiation resulted in relatively little peroxidation and no noticeable formation of high molecular weight polypeptide complexes. The possibility that malonaldehyde, a product of lipid peroxidation, is involved in cross-linking during irradiation was studied by incubating ghosts with exogenous malonaldehyde. Although the reagent did cross-link membrane proteins (electrophoretic bands 1, 2, 2.1 2.2, and 4.1 diminished most rapidly and high molecular weight bands appeared), the reaction could only be demonstrated with malonaldehyde concentrations several orders of magnitude greater than those detected in irradiation experiments. If malonaldehyde cross-linking occurs, it does not appeare to be the predominant mechanism of polypeptide association during irradiation of bilirubin-containing ghosts.