Zinc in the prevention of Fe²⁺-initiated lipid and protein
oxidation

M. PAOLA ZAGO, SANDRA V. VERSTRAETEN AND PATRICIA I. OTEIZA
 

Instituto de Química y Fisicoquímica Biológicas (UBA-CONICET), Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Buenos Aires, Argentina

ABSTRACT

In the present study we characterized the capacity of zinc to protect lipids and proteins from Fe²⁺-initiated oxidative damage. The effects of zinc on lipid oxidation were investigated in liposomes composed of brain phosphatidylcholine (PC) and phosphatidylserine (PS) at a molar relationship of 60:40 (PC:PS, 60:40). Lipid oxidation was evaluated as the oxidation of cis-parinaric acid or as the formation of 2-thiobarbituric acid-reactive substances (TBARS). Zinc protected liposomes from Fe²⁺ (2.5-50 µM)-supported lipid oxidation. However, zinc (50 µM) did not prevent the oxidative inactivation of glutamine synthelase and glucose 6-phosphate dehydrogenase when rat brain superntants were oxidized in the presence of 5 µM Fe²⁺ and 0.5 mM H₂O₂ .We also studied the interactions of zinc with epicatechin in the prevention of liid oxidation in liposomes. The simulaneous addition of 0.5 µM epicatechin (EC) and 50 µM zinc or EC separately. Zinc (50 µM) also protecte liposomes from the stimulatory effect of aluminum on Fe²⁺-initiated lipid oxidation. Zinc could play an important role as an antioxidant in biological systems, replacing iron and other metals with pro-oxidant activity from binding sites and interacting with other components of the oxidant defense system.

KEY TERMS: aluminum, catechins, free radicals, iron, lipid peroxidation, zinc

INTRODUCTION

Previous studies from our laboratory have demostrated that zinc deficiency imposed on male rats during early postnatal develpment can result in high rates of oxidative damage to testes lipids, proteins and DNA (Oteiza et al., 1995) in alterations of the oxidant defense system (Oteiza et al., 1996) and in an increased susceptibility of the testes to cadmium-induced oxidative damage (Oteiza et al., 1999). Results suggested that this high level of oxidative damage in rat testes could be attributed in part to low tissue concentrations of zinc, as this metal has been proposed to be part of the oxidant defense system, and in part to an increase in tissue iron concentration which occured secondary to the zinc deficiency (Oteiza et al., 1995, Rogers et al., 1987).

One of the proposed mechanisms (Bray and Bettger, 1990) by which zinc could act as an antioxidant is through its capacity to replace redox-active metals (iron, copper) from binding sites in proteins, DNA or membranes. IRon can catalyze one-electron transfer reactions that generate reactive oxygen species, such us the reactive OH·, which is formed from H₂O₂ through the Fenton reaction. Iron also decomposes lipid peroxides, thus generating peroxyl and alkoxyl radicals, which favors the propagation of lipid oxidation.

Catechins are natural antioxidantes present in fruits and vegetables and are the major contributors to the antioxidant capacity of tea and red wines. Catechins can act as antioxidants by directly scavenging reactive oxygen species such as OH·, HOCI, and ONOOH  or by chelating transition metals (see Bravo 1998, for a review). Zinc may inhibit iron-initiated lipid oxidation by preventing iron from binding to the membrane, while catechins could act synergistically with zinc by chelating iron in the hydrophilic milieu. This may constitute an impotant physiological interaction in the oxidant defense system.
In the present study we have characterized the capacity of zinc to act to prevent lipid and protein oxidative damage in the presence of Fe²⁺. We evaluated the possible interaction between zinc and epicatechin (EC) in the protection of liposomes from Fe²⁺-initiated lipid oxidation. We also studied whether zinc could act to protect membranes from the stimulatory effect of aluminum on Fe²⁺-supported lipid oxidation.

METHODS

Materials Brain phosphatidylserine (PS) were purchased from Avanti Polar Lipids Inc. (Birmingham, AL). Ferrous sulfate, zinc acetate and EC were from Sigma Chem. Co. (St. Louis, MO). CIs-parinaric acid was from Molecular Probes, Inc. (Eugene, OR).

Evaluation of the effects of znc on lipid oxidation

Liposome preparation. Phospholipid purity was checked by thin layer chromatography. Phospholipids in chloroform solution were brought to dryness under high vacuum in a Buchi rotavapor for 15 min and further exposed  to an N₂ stream for 15 min. Dried phospholipids were resuspended (2.5 mg phospholipids/ml) in 20 mM Tris-HCI, 140 m M NaCI buffer (pH 7.4), vortexed for 1 min and incubated at 45 ºC for 10 min under N₂. Small vesicles were obtained by three cycles of 45 sec sonication in a Branson 250 sonifier (Branson Ultrasonics Corp., Danbury, CT) at 80 W.
Incubations. To evaluate the effects of zinc and catechins on Fe²⁺-initiated lipid oxidation, 0.5 ml liposomes of PC:PS at a molar relationship 60:40 (0.62 mM phospholipids) were incubated at 37 ºC for 90 min in the presence of FeSO₄ (2.5-50 µM) with or without  the addition of zinc (50-100 µM) or EC (0.05-0.5 µM). FeSO₄ stock solution was prepared immediately before use. Incubations were stopped by the addition of 0.1 ml of 4% (w/v) butylated  hydroxytoluene in ethanol and lipid oxidation was measured as 2-thiobarbituric acid-reactive substances (TBARS).
TBARS reaction. Lipid oxidation products were measured in the incubation mixtures as TBARS (Quinlan et al., 1988). The reaction mixture was added with 0.25 ml of 3% (w/v) sodium dodecyl sulfate. After mixing, 0.5 ml of 1% (w/v) 2-thiobarbituric acid in 0.05 M NaOH and 0.5 ml of 25% (v/v) HCI were added. Samples were vortexed and heated for 15 min in boiling water and TBARS were extracted in 2.5 ml of 1 -  butanol. After centrifugationat 1,000 x g for 10 min, the fluorescence of the butanol layer was measured at 515 nm excitation and 535 nm emission. TBARS values are expressed as malondialdehyde equivalents.
Cis-parinaric acid oxidation. Brain PC:PS (60:40) liposomes (0.62 mM phospholipids) containing 1.6 mol% cis-parinaric acid were preincubated for 5 min at 37 ºC. Zinc (50 µM) was added, and after 2 min of incubation, lipid oxidation was started by the addition of 25 µM Fe²⁺ in the absence or the presence of aluminum (50 µM). Samples were incubated at 37 ºC for 30 min with continuous stirring, and cis-parinaric acid oxidation was followed as the decrease in fluorescence intensity at 410 nm (lexcitation: 325 nm). Results are expressed as the percentage of the initial fluorescence.

Evaluation of the effects of zinc on protein oxidative damage

Adult Wistar rats fed a commercial chow diet (Nutrimento, Argentina) were killed by decapitation. The brains were quickly excised, weighed and placed in ice-cold saline solution. The meninges were re-moved and the brains were homogenized in 10 volumes of 50 mM HEPES buffer, pH 7.4  containing 125 mM KCL. The homogenates were centrifuged at 100.000 x g for 10 min at 4 ºC and the supernatant was decanted. Protein concentration was determined according to the method used by Bradford (1976).
To measure the effect of zinc on the oxidative inactivation of glutamine synthetase (EC 6.3.1.2) and glucose 6-phosphate dehydrogenase (EC 1.1.1.49), aliquots of the supernatants were incubated for 30 nin at 37 ºC in the presence of 5 µM Fe²⁺ and 0.5 mM H₂O with or without the previous addition of zinc (2.5 to 100 µM). Glutamine synthetase activity was measured as described by Miller et al. (1978) and glucose 6-phosphate dehydrogenase was assayed according to the method of Olive and Levy (1975). Results are expressed as the percentage of the activity obtained in the samples incubated in the absence of additions.
Statistics. Data were analyzed using one-way analysis of variance. The Fisher least significance difference test was used to examine differences between group means. A pvalue £ 0.05 was considered statistically significant. Data are shown as mean ± SE.

RESULTS

Effects of zinc on Fe²⁺-initiated lipid and protein oxidation

The effect of zinc on Fe²⁺-supported lipid oxidation was evaluated in liposomes PC:PS (60:40) either by measuring the formation of TBARS or by following the oxidation of cis-parinaric acid incorporated into the membrane. Figure 1 shows the effect of 50 and 100 µM zinc on Fe²⁺ (25 µM)-initiated cis-parinaric acid oxidation. In the absence of additions, the incubation of the liposomes at 37 ºC for 30 min caused a spontaneous decay n the fluorescence (60 ± 4% of the initial fluorescence). This is probably due to the presence of lipid peroxides in the liposomes which causes a basal level of lipid oxidation in the presence of contaminating iron. The addition of 25 µM Fe²⁺ caused an acceleration of cis-parinaric acid oxidation, the remaining fluorescence after 30 min of incubation was 38 ± 2% of initial values. The addition of 100 µM zinc significantly prevented (54% inhibition, p <0.05) Fe²⁺-induced cis-parinaric oxidation (50 ± 1% initial fluorescence after 30 min of incubation).
In the presence of 25 µM Fe²⁺, with or without the addition of zinc, cis-parinaric acid oxidation showed bimodal kinetics (Fig. 1). Between 0 and 3.5 min of incubation, the kinetics corresponded to a first order curve with a rate constant of (9.36 ± 0.9) x 10^-9 sec^-1, (9.08 ± 0.63) x 10^-9 sec^-1 and (5.13 ± 0.75) x 10^-9 sec^-1 for the liposomes incubated in the presence of either 25  µM Fe²⁺ alone or Fe²⁺ and 50 or 100  µM zinc respectively. After 4 min incubation, cis-parinaric acid oxidation switched to a zero order kinetic, with a rate constant of (1.20 ± 0.02) x 10^-9 M.sec^-1, (1.45 ± 0.01) x 10^-9 M.sec^-1 and (1.50 ± 0.2) x 10^-9 M.sec^-1 in the liposomes incubated in the presence of either 25  µM Fe²⁺ alone or with Fe²⁺ plus 50 or 100  µM zinc respectively. The magnitude of these rate constants was similar to that found in liposomes incubated in the absence of addition (1.72 ± 0.05) x 10^-9 M.sec^-1 that presented a zero order kinetics between 0 and 30 min of incubation.

Figure 1. Zinc inhibits Fe2+-induced cis-parinaric oxidation. Liposomes of PC:PS (60:40) containing 1.6 mol% cis-parinaric acid (cPNA) were incubated for 30 min at 37 ºC in the absence or presence of either 25 µM Fe2+ alone or Fe2+ and zinc 50 or 100 µM. Graphs correspond to a representative experiment.

The effect of zinc on Fe²⁺-initiated lipid oxidation was also assessed by measuring TBARS production. Figure 2A shows the kinetics (0-90 min) of TBARS formation in PC:PS (60:40) liposomes incubated in the presence of 25 µM Fe²⁺ with or without the addition of 50 µM zinc. Fe²⁺ (25 µM) promoted TBARS generation that had not reached a plateau after 90 min of incubation at 37 ºC. The simultaneous addition of 50 µM zinc partially protecte liposomes from oxidation (55 ±10% inhibition after 90 in of incubation, p < 0.01). The protective effect of zinc was observed from 15  to 90 min of incubation. Figure 2B shows the dependence of the inhibitory effect of zinc on the concentration of added Fe²⁺ (2.5-50 µM). TBARS formation in the presence of 50 µM zinc and Fe²⁺ was significantly lower (p < 0.05) than in the liposomes incubated only in the presence of Fe²⁺ at all the tested Fe²⁺ concentrations (2.5-50 µM). No significant differences were observed in the inhibitory action of zinc (45-60% inhibition) at the various concentrations of Fe²⁺ (2.5-50 µM).

Figure 2. Effects of zinc on Fe2+-initiated TBARS formation. Liposomes of PC:PS (60:40) were incubated at 37 ºC under the following conditions: A-for 0 to 90 min in the presence of (#) 25 µM Fe2+  or (# ) 25 µM Fe2+ and 50 µM zinc, or B-for 90 min in the presence of variable concentrations of ()) Fe2+ or () ) Fe2+ and 50 µM zinc. Lipid oxidation was evaluated as TBARS.

  The capacity of zinc to protect proteins from iron-mediated oxidation was tested by measuring the oxidative inactivation of the enzymes glutamine synthestase and glucose 6-phosphate dehydrogenase in rat brain supernatants. The incubation of the supernatants in the presence of 5 µM Fe²⁺ and 0.5 mM H₂O₂ for 30 min at 37 ºC caused a 68% loss of glutamine synthetase activity (Fig. 3A) and a 20% loss of glucose 6-phosphate dehydrogenase activity (Fig. 3B). The simultaneous addition of zinc (5-50 µM) had no protective effect on the Fe²⁺-mediated inactivation of glutamine synthetase. Zinc not only failed to protect glucose 6-phosphate dehydrogenase from Fe²⁺-induced inactivation, but it caused an additional inhibition of the enzyme.

Figure 3. Effects of zinc on Fe2+-mediated inactivation of glutamine synthetase and glucose 6-phosphate dehydrogenase. Brain supernatants were incubated for 30 min at 37 ºC in the presence of 5 mM Fe2+ and 0.5 mM H2O2 (Fe) with or without the addition of variable amounts of zinc (5-50 mM). Glutamine synthetase and glucose 6-phosphate dehydrogenase actvities are expressed as percentages, considering 100% the activity measured in the samples incubated without additions.

Interactions of zinc with epicatechin

We investigated the hypothesis that zinc, possibly acting in the interface membrane-solvent by preplacing iron from binding sites, could act synergistically with catechins, whose antioxidant capacity is based on their ability to chelate redox-active metals or to scavenge reactive oxygen species in the hydrophilic milieu.

We tested the effect of EC concentrations (0.05 to 0.5 µM) that did not cause a complete inhibition of lipid oxidation. In liposomes PC:PS (60:40), EC Partially inhibited Fe²⁺-initiated TBARS formation (30-50% inhibition at 0.05-0.5 µM EC). The simultaneous addition of 50 µM zinc increased the protection of liposomes from oxidation and this protection was moderately but significantly higher (p < 0.05) in the presence of both 0.5 mM EC and zinc (76%) than that caused by zinc (54%) or EC (42%) alone (Fig. 4).

Figure 4. Interaction of zinc and EC in the prevention of Fe 2+- mediated lipid oxidation. Liposomes of PC:PS (60:40) were incubated dor 90 min at 37 ºC in the presence of 25 mM Fe2+ (') or 25 mM Fe2+ and 50 mM zinc (8), with or without the addition of variable amounts of EC (0.05-0.5 mM). LIpid oxidation was evaluated as TBARS.

Effect of zinc on aluminum-stimulated TBARS formation.

We have previously shown that aluminum stimulates Fe²⁺-initiated lipid oxidation (Verstraeten and Oteiza, 1995, Verstraeten et al., 1997a). Accordingly, cis-parinaric acid oxidation in the presence of both, 25 mM Fe²⁺  and 50 mM aluminum proceeded at  a higher rate (p < 0.01) than in the presence of Fe²⁺ alone (Inset to Fig. 5). The percentage of initial fluorescence after 30 min of incubation was 38 ± 2 and 29 ± 2 in the liposomes incubated in the presence of Fe²⁺ or Fe²⁺ and aluminun respectively. At 50 and 100 mM concentrations, zinc caused a significant (p < 0.05) reduction in aluminum-stimulated cis-parinaric acid oxidation (Fig. 5). After 30 min of incubation, the remaining fluorescence was 36 ± 2 and 44 ± 1% for the liposomes incubated in the presence of Fe2+, aluminum and either 50 or 100 mM zinc, respectively.

Figure 5.  Zinc inhibits the stimulatory effect of aluminum on Fe2+-supported cis-parinaric acid oxidation. Liposomes of PC:PS (60:40) containing 1.6 mol% cis-parinaric acid (cPNA) were incubated for 30 min at 37 ºC in the presence of aluminum (50 mM) and 25 mM Fe2+ with or without the addition of zinc (50 or 100 mM). Inset: Effect of 50 mM aluminum on 2.5 mM Fe2+-initiated cis-parinaric acid oxidaton. Graphs correspond to one representative experiment.

 Zinc is involved in the protection of cells against free radical-mediated damage at multiple levels (Bray and Bettger, 1990). First, zinc can induce the synthesis of metallothionein, a protein rich in thiol groups that can bind metals with known pro-oxidant activity (Cu⁺, Cd²⁺, Hg²⁺); alternatively, through its thiol groups, metallothionein can scavenge hydroxyl radicals and singlet oxygen (see Sato and Bremner, 1993 for a review). Second, the activities of the antioxidant enzymes, CuZn superoxide dismutase and extracellular superoxide dismutase, strongly depend on zinc (Olin et al., 1995), which acts to stabilize the tertiary structure of the enzymes. Third, zinc can compete for cellular binding sites with redox active metals such as copper and iron (Girotti et al., 1985).

Zinc deficiency has been associated with higher than normal levels of tissue oxidative damage including, increased lipid (Sullivan et al., 1980; Burke et al., 1985;Oteiza et al., 1995; Kraus et al., 1997), protein and DNA oxidation (Oteiza et al., 1995; Olin et al., 1993). We have shown that in male rats fed zinc-deficient diets, high levels of oxidative damage in the testes were associated with alterations in different components, enzymes and substances, of the oxidant defense system (Oteiza et al., 1996). Glutahione depletion, DNA fragmentation and higher rates of lipid oxidation have been reported in liver and brain from zinc-deficient mice (Bagchi et al., 1998). Reduced glutathione levels were also found in red blood cells from zinc-deficient rats (Hsu, 1982, Kraus et al., 1997). The above experimental evidence supports a physiological role of zinc as part of the oxidant defense system.

Since we found high tissue iron concentrations in association with low zinc levels in the zinc-deficient rats, we hypothesized that the increased oxidative damage found in zinc deficiency could be due to both low zinc and/or high tissue iron concentrations (Oteiza et al., 1995).
In the present study we examined the capacity of zinc to protect lipids and proteins from iron-induced oxidation. In liposomes, zinc partially prevented Fe²⁺-supporteed lipid oxidation as evaluated, both as TBARS formation and cis-parinaric oxidation. The antioxidant capacity of zinc was not strongly dependent upon the concentration of Fe²⁺ (5- 100 µM). This suggests that zinc occupies potential iron binding sites on the membrane surface and that iron cannot displace zinc from these sites, probably due to a higher affinity of zinc for the polar head groups of phospholipids. The kinetics of cis-parinaric acid oxidation suggests that zinc affects the initial steps of Fe²⁺- induced lipid oxidation, since at 100 µM concentration, zinc caused a marked reduction (82%) in the initial rate constant. The constant rate of the second phase of the kinetics corresponding to the propagation of lipid oxidation was slightly modified (25% reduction) by 100 µM zinc.

We have proviously observed that in zinc-deficient rats, iron testes concentration was inversely correlated with glutamine synthetase activity (Oteiza et al., 1995). In the present study we found that although zinc prevented lipid oxidation, zinc added in vitro did not protec either glutamine  synthetase or glucose 6-phosphate dehydrogenase from iron-mediated oxidation. IRon is involved in the oxidative inactivation of glutamine synthetase and several other enzymes. The inactivation implies the binding of Fe²⁺ to the enzyme and offers no protection to glutamine synthetase and glucose 6-phosphate dehydrogenase from oxidation.

Aluminum is a neurotoxic cation that has been shown to stimulate Fe²⁺-initiated lipid oxidation in synthetic and biological membranes (Verstraeten and Oteiza, 1995; Verstraeten et al., 1997a, 1997b, 1998) and to induce oxidative stress in chronically-intoxicated animals (Fraga et al., 1990; Verstraeten el al.,  1997b). We have presented evidence that aluminum, a non-redox metal, acts to stimulate lipid oxidation in the presence of iron by increasing membrane lipid packing and promoting the formation of rigid clusters, thus favoring the propagation of lipid oxidation (Verstraeten et al., 1997a). In the present study, we found that zinc protected liposomes from the stimulatory effect of aluminum on Fe²⁺-initiated lipid oxidation. Zinc could occupy aluminum binding sites in the membrane, preventing aluminum-mediated changes in the physical properties of the bilayer that lead to higher rates of lipid oxidation (Verstraeten et al., 1997a).

Zinc may act synergistically with other antioxidants to protect the cell from oxidative stress. If the mechanism involved in the protection from oxidative damage is through its capacity to replace iron from binding sites, it is reasonable to hypothesize that zinc could interact with antioxidants that act through the chelation of redox-active metals. Catechins can directly scavenge reactive oxygen species and can also chelate metals through their hydroxyl groups. We investigated the possible synergism between zinc and EC in the protection of liposomes from Fe²⁺-initiated lipid oxidation. We tested a concentration range of 0.05 to 0.5 µM, since at 0.5 µM EC we observed an approximate 50% reduction in TBARS formation in the presence of 25 µM Fe²⁺. EC inhibited TBARS formation at all tested concentrations, and the simultaneous addition of EC and zinc caused an additional inhibition when  compared to that caused by zinc or EC individually. These results indicate an additive effect between zinc and EC in the protection of membranes from Fe²⁺- supported lipid oxidation.

In summary, in vitro zinc can protect membranes from Fe²⁺-initiated lipid oxidation, possibly by occupying iron binding sites. Zinc can also protect membranes from the deleterious effect of metals such as aluminum that induce changes in the physical properties of lipid bilayers. Zinc could have an important role as an antioxidant in biological systems and may interact with other components of the oxidant defense system. The characterization of these interactions deserves future research.

ACKNOWLEDGEMENTS

This work was suported by a grant from the University of Buenos Aires, Argentina (TB55).

Corresponding author: Dr. Patricio I. Oteiza. Departamento de Química Biológica, Facultad de Farmacia y Bioquímica. Junín 956. 113-Buenos Aires, Argentina. Phone: (54-11) 4964-8288. Fax: (54-11) 4962-5457. Email: oteiza@qb.ffyb.uba.ar

Received: October 26, 1999. Accepted: October 26, 1999.

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