Lens proteins block the copper-mediated formation of reactive oxygen species during glycation reactions in vitro

Redox chemistry of lens crystallins: A system of cysteines

The nuclear region of the lens is metabolically quiescent, but it is far from inert chemically. Without cellular renewal and with decades of environmental exposures, the lens proteome, lipidome, and metabolome change. The lens crystallins have evolved exquisite mechanisms for resisting, slowing, adapting to, and perhaps even harnessing the effects of these cumulative chemical modifications to minimize the amount of light-scattering aggregation in the lens over a lifetime. Redox chemistry is a major factor in these damages and mitigating adaptations, and as such, it is likely to be a key component of any successful therapeutic strategy for preserving or rescuing lens transparency, and perhaps flexibility, during aging. Protein redox chemistry is typically mediated by Cys residues. This review will therefore focus primarily on the Cys-rich γ-crystallins of the human lens, taking care to extend these findings to the β- and α-crystallins where pertinent.

Protein posttranslational modification (PTM) by glycation: Role in lens aging and age-related cataractogenesis

Crystallins, the most prevalent lens proteins, have no turnover throughout the entire human lifespan. These long-lived proteins are susceptible to post-synthetic modifications, including oxidation and glycation, which are believed to be some of the primary mechanisms for age-related cataractogenesis. Thanks to high glutathione (GSH) and ascorbic acid (ASA) levels as well as low oxygen content, the human lens is able to maintain its transparency for several decades. Aging accumulates substantial changes in the human lens, including a decreased glutathione concentration, increased reactive oxygen species (ROS) formation, impaired antioxidative defense capacity, and increased redox-active metal ions, which induce glucose and ascorbic acid degradation and protein glycation. The glycated lens crystallins are either prone to UVA mediated free radical production or they attract metal ion binding, which can trigger additional protein oxidation and modification. This vicious cycle is expected to be exacerbated with older age or diabetic conditions. ASA serves as an antioxidant in the human lens under reducing conditions to protect the human lens from damage, but ASA converts to the pro-oxidative role and causes lens protein damage by ascorbylation in high oxidation or enriched redox-active metal ion conditions. This review is dedicated in honor of Dr. Frank Giblin, a great friend and superb scientist, whose pioneering and relentless work over the past 45 years has provided critical insight into lens redox regulation and glutathione homeostasis during aging and cataractogenesis.

Lens Endogenous Peptide αA66-80 Generates Hydrogen Peroxide and Induces Cell Apoptosis

In previous studies, we reported the presence of a large number of low-molecular-weight (LMW) peptides in aged and cataract human lens tissues. Among the LMW peptides, a peptide derived from αA-crystallin, αA66-80, was found in higher concentration in aged and cataract lenses. Additional characterization of the αA66-80 peptide showed beta sheet signature, and it formed well-defined unbranched fibrils. Further experimental data showed that αA66-80 peptide binds α-crystallin, impairs its chaperone function, and attracts additional crystallin proteins to the peptide α-crystallin complex, leading to the formation of larger light scattering aggregates. It is well established that Aβ peptide exhibits cell toxicity by the generation of hydrogen peroxide. The αA66-80 peptide shares the principal properties of Aβ peptide. Therefore, the present study was undertaken to determine whether the fibril-forming peptide αA66-80 has the ability to generate hydrogen peroxide. The results show that the αA66-80 peptide generates hydrogen peroxide, in the amount of 1.2 nM H₂O₂ per µg of αA66-80 peptide by incubation at 37°C for 4h. We also observed cytotoxicity and apoptotic cell death in αA66-80 peptide-transduced Cos7 cells. As evident, we found more TUNEL-positive cells in αA66-80 peptide transduced Cos7 cells than in control cells, suggesting peptide-mediated cell apoptosis. Additional immunohistochemistry analysis showed the active form of caspase-3, suggesting activation of the caspase-dependent pathway during peptide-induced cell apoptosis. These results confirm that the αA66-80 peptide generates hydrogen peroxide and promotes hydrogen peroxide-mediated cell apoptosis.

Alpha-crystallin-derived peptides as therapeutic chaperones

BACKGROUND

The demonstration of chaperone-like activity in peptides (mini-chaperones) derived from α-crystallin's chaperone region has generated significant interest in exploring the therapeutic potential of peptide chaperones in diseases of protein aggregation. Recent studies in experimental animals show that mini-chaperones could reach intended targets and alter the disease phenotype. Although mini-chaperones show potential benefits against protein aggregation diseases, they do tend to form aggregates on storage. There is thus a need to fine-tune peptide chaperones to increase their solubility, pharmacokinetics, and biological efficacy.

SCOPE OF REVIEW

This review summarizes the properties and the potential therapeutic roles of mini-chaperones in protein aggregation diseases and highlights some of the refinements needed to increase the stability and biological efficacy of mini-chaperones while maintaining or enhancing their chaperone-like activity against precipitation of unfolding proteins.

MAJOR CONCLUSIONS

Mini-chaperones suppress the aggregation of proteins, block amyloid fibril formation, stabilize mutant proteins, sequester metal ions, and exhibit antiapoptotic properties. Much work must be done to fine-tune mini-chaperones and increase their stability and biological efficacy. Peptide chaperones could have a great therapeutic value in diseases associated with protein aggregation and apoptosis.

GENERAL SIGNIFICANCE

Accumulation of misfolded proteins is a primary cause for many age-related diseases, including cataract, macular degeneration, and various neurological diseases. Stabilization of native proteins is a logical therapeutic approach for such diseases. Mini-chaperones, with their inherent antiaggregation and antiapoptotic properties, may represent an effective therapeutic molecule to prevent the cascade of protein conformational disorders. Future studies will further uncover the therapeutic potential of mini-chaperones. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Cataract-linked γD-crystallin mutants have weak affinity to lens chaperones α-crystallins

To test the hypothesis that α-crystallin chaperone activity plays a central role in maintenance of lens transparency, we investigated its interactions with γ-crystallin mutants that cause congenital cataract in mouse models. Although the two substitutions, I4F and V76D, stabilize a partially unfolded γD-crystallin intermediate, their affinities to α-crystallin are marginal even at relatively high concentrations. Detectable binding required further reduction of γD-crystallin stability which was achieved by combining the two mutations. Our results demonstrate that mutants and possibly age-damaged γ-crystallin can escape quality control by lens chaperones rationalizing the observation that they nucleate protein aggregation and lead to cataract.

Identification and characterization of a copper-binding site in αA-crystallin

Previous studies have shown that both αA- and αB-crystallins bind Cu2+, suppress the formation of Cu2+-mediated active oxygen species, and protect ascorbic acid from oxidation by Cu2+. αA- and αB-crystallins are small heat shock proteins with molecular chaperone activity. In this study we show that the mini-αA-crystallin, a peptide consisting of residues 71-88 of αA-crystallin, prevents copper-induced oxidation of ascorbic acid. Evaluation of binding of copper to mini-αA-crystallin showed that each molecule of mini-αA-crystallin binds one copper molecule. Isothermal titration calorimetry and nanospray mass spectrometry revealed dissociation constants of 10.72 and 9.9 μM, respectively. 1,1'-Bis(4-anilino)naphthalene-5,5'-disulfonic acid interaction with mini-αA-crystallin was reduced after binding of Cu2+, suggesting that the same amino acids interact with these two ligands. Circular dichroism spectrometry showed that copper binding to mini-αA-crystallin peptide affects its secondary structure. Substitution of the His residue in mini-αA-crystallin with Ala abolished the redox-suppression activity of the peptide. During the Cu2+-induced ascorbic acid oxidation assay, a deletion mutant, αAΔ70-77, showed about 75% loss of ascorbic acid protection compared to the wild-type αA-crystallin. This difference indicates that the 70-77 region is the primary Cu2+-binding site(s) in human native full-size αA-crystallin. The role of the chaperone site in Cu2+ binding in native αA-crystallin was confirmed by the significant loss of chaperone activity by the peptide after Cu2+ binding.

The evaluation of the oxidative stress parameters in nondiabetic and diabetic senile cataract patients

The aim of the present study was to evaluate the copper (Cu), zinc (Zn), malondialdehyde (MDA), glutathione (GSH), and advanced oxidation protein products (AOPP) levels and superoxide dismutase (SOD) activities in diabetic senile cataract. Ten patients with diabetic senile cataract and ten patients with nondiabetic senile cataract (control group) were included in this study. AOPP, MDA, and GSH levels and SOD activity were measured by a spectrophotometric method. Serum, lens Cu, and Zn levels were measured by an atomic absorption spectrophotometric method. Both the lens and serum Zn and Cu levels between the two groups were not significantly different (p > 0.05). GSH, AOPP, and MDA levels and the SOD activities in the diabetic senile cataract group were significantly increased as compared to the control group (p < 0.05). Oxidative stress is one of the major factors which may lead to the early cataract formation. Oxidative events are of great importance in diabetic complications and, particularly in the lens, may have a role in the pathogenesis of cataract associated with diabetes mellitus as exhibited in this study.

Pseudoexfoliation syndrome and trace elements

To investigate the role of zinc and copper in the development of pseudoexfoliation (PSX) syndrome, 34 cataract patients with PSX syndrome and 27 cataract patients without PSX syndrome were included in the study and groups were matched for age and gender. During the cataract surgery, lenses were obtained intraoperatively, frozen under liquid nitrogen, and kept at -70 degrees C until processing. Zinc and copper concentrations were measured by atomic absorption spectrophotometric method after the homogenization (acid hydrolysis) of dried lenses. The mean concentration of zinc in the lens from patients with PSX (20.33 +/- 8.76 microg/g tissue; range 11.04-42.94 microg/g tissue) was significantly lower than that measured in the lens of patients without PSX (28.88 +/- 15.32 microg/g tissue; range 12.02-64.32 microg/g tissue) (P < 0.05). The mean concentration of copper in the lens from patients with PSX (29.51 +/- 10.05 microg/g tissue; range 12.69-59.71 microg/g tissue) and in the lens of patients without PSX (39.72 +/- 25.64 microg/g tissue; range 12.38-92.14 microg/g tissue) was not statistically different. The decreased content of zinc could increase oxidative stress. The results support the role of oxidative stress in the development of PSX in cataract patients.

Renal microvascular injury in diabetes: RAGE and redox signaling

Diabetic nephropathy remains a major cause of morbidity and mortality in the diabetic population and is the leading cause of end-stage renal failure in the Western World. Despite current therapeutics including intensified glycemic control and blood pressure lowering agents, renal disease continues to progress relentlessly in diabetic patients, albeit at a lower rate. It is well recognized that metabolic and hemodynamic factors play a central role in accelerating renal disease in diabetes. However, recent experimental studies have suggested that increased generation of reactive oxygen species (ROS) as a result of the diabetic milieu may play a central role in the progression of diabetic microvascular complications. These ROS appear to be generated primarily from mitochondrial sources and via the enzyme, NADPH oxidase. This review focuses on how ROS play a deleterious role in the diabetic kidney and how they are involved in crosstalk among various signaling pathways, ultimately leading to renal dysfunction and structural injury.

Levels of zinc, iron, and copper in patients with pseudoexfoliative cataract

PURPOSE

To investigate the aqueous humor, lens, and serum concentrations of zinc, iron, and copper in patients with pseudoexfoliative cataract and compare with patients who have senile cataract without pseudoexfoliation.

METHODS

Twenty-five patients with pseudoexfoliative cataract and 25 patients with senile cataract as control group were enrolled in the study. Samples from aqueous humor, serum, and lens materials during extracapsular cataract extraction (ECCE) were collected from all patients. The levels of selected trace elements in three samples in all groups were assayed with atomic absorption spectrometry (AAS) and statistical analyses were performed with t-test for independent samples except lens zinc and serum copper levels. The data weren't normally distributed, therefore Mann-Whitney U test applied for these parameters.

RESULTS

The zinc and copper levels in aqueous humor of PEX group were significantly higher than those of control group (p<0.001). The iron levels in aqueous humor were not significantly different in PEX group and control group (p=0.252). The copper content of lenses was significantly increased in PEX group compared to control group (p=0.029). The iron and zinc content of lenses had no significant differences between the two groups (p=0.248, p=0.719, respectively). The levels of iron and copper in serum were significantly increased in PEX group compared to control group (p<0.001 and p<0.002, respectively). The zinc level in serum had no significant differences between the two groups (p=0.823, p=0.472, respectively).

CONCLUSIONS

Zinc, iron, and especially copper may play a role in PEX syndrome.

Electrophoretic analysis of oxidatively modified eye lens proteins in vitro: implications for diabetic cataract

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) profiles of eye lens proteins showed that both progression of diabetic cataract in rats in vivo and precipitation of soluble eye lens proteins stressed by free radicals in vitro were accompanied by significant protein cross-linking. There was a noticeable contribution of disulfide bridges to protein cross-linking in diabetic eye lens in vivo. In contrast, under conditions in vitro, when eye lens proteins were exposed to hydroxyl or peroxyl radicals, we showed that the participation of reducible disulfide linkages in the formation of high molecular mass products was markedly lower. These in vivo--in vitro differences indicate that the generally accepted role of reactive oxygen species in diabetic cataractogenesis may be overestimated in connection with the processes of protein cross-linking.

The protective effect of α-crystallin against acute inflammation in mice

Acute inflammation can activate macrophages or monocytes and subsequently release several inflammatory cytokines and reactive oxygen species (ROS). Oxidative stress triggered by the production of ROS plays deleterious role leading to multiple organ failure. This study was designed to investigate the prophylactic effect of alpha-crystallin, a major chaperone lens protein comprising of alpha-A and alpha-B subunits in inflammation-induced mice. Mice were divided into three groups (n=6 in each): control, inflammation and alpha-crystallin-treated. Results show that ROS was significantly higher in the lymphocytes, hepatocytes and astrocytes (P<0.05) of inflammation-induced mice when compared to control, but no significant changes were observed in the alpha-crystallin-treated group. Increased level of lipid peroxidation (LPO) and decreased activities of antioxidant such as superoxide dismutase (SOD), catalase, glutathione peroxidase and glutathione were observed in the inflammation-induced mice when compared to control, whereas the activities of these were found to be normal followed by alpha-crystallin treatment. We also observed a reduction in reduced glutathione levels in hepatocytes of inflammation-induced mice, which were normalized on alpha-crystallin treatment. The in vitro study has shown that alpha-crystallin treatment not only suppresses the increase in LPO levels but also inhibits the lipid breakdown resulting from autooxidation in mouse cerebral cortex homogenate, and strongly suggests that alpha-crystallin therapy may serve as a potent pharmacological agent in systemic inflammation.

Oxidative modification of rat eye lens proteins by peroxyl radicals in vitro: protection by the chain-breaking antioxidants stobadine and Trolox

In an attempt to model the processes of free radical-mediated cataractogenesis, we investigated the oxidative modification of rat eye lens proteins by peroxyl radicals generated by thermal decomposition of 2,2'-azobis(2-amidinopropane)hydrochloride (AAPH) under aerobic conditions. When incubated with AAPH, the soluble eye lens proteins precipitated in a time-dependent manner. The insolubilisation was accompanied by the accumulation of protein free carbonyls and the diminution of sulfhydryls, yet the processes were shifted in time. The SDS-PAGE analysis of the AAPH-treated proteins revealed the presence of high molecular weight cross-links and, to a lesser extent, fragments. The aggregation and cross-linking of proteins along with the generation of free carbonyls was significantly inhibited by the chain-breaking antioxidants stobadine and Trolox. On the other hand, the AAPH-initiated sulfhydryl consumption was much less sensitive to the antioxidants studied. The results point to a complex mechanism of peroxyl-radical-mediated modification of eye lens proteins with implications for cataract development and they indicate a potentially protective role of antioxidants.

Alpha-crystallin-incorporated collagen matrices as an aid for dermal wound healing

This study evaluated the effects of noncovalently incorporated crystallin into the collagen matrix for dermal wound-healing processes in rats. Crystallin-incorporated collagen matrix (CIC) showed better healing when compared to wounds treated with collagen matrix (CS) and without collagen (CR). Biochemical parameters and histological analysis revealed that increased wound contraction enhanced cell proliferation and efficient radical scavenging in the CIC group. The higher shrinkage temperature of CIC films when compared to CS groups suggested increased hydrothermal stability for the former material. An in vitro release study of CIC has showed sustained and time-dependent release of crystallin from the collagen matrix. These results demonstrate the possibility of using crystallin as therapeutic protein in the wound-healing process.

Activation of protein-bound copper ions during early glycation: study on two proteins

This study proposes several possible pathways by which hyperglycemia could make protein-bound metal ions more redox active. These mechanisms were tested on bovine serum albumin and calf lens protein. Proteins rich in early glycation products were less capable of competing for copper ions in the presence of other ligands (e.g., glycine and calcein), suggesting that glycated proteins might have diminished stability constants of their copper chelates compared to control counterparts. When protein-copper complexes were tested for their ability to cause the oxidation of ascorbic acid, as well as the reduction of molecular oxygen to hydrogen peroxide, glycated and control proteins differed considerably in their redox abilities. Oxidative damage on proteins documented by protein carbonyl content and amino acid analysis indicates the involvement of Fenton chemistry upon metal chelation. The possible biological consequences of the observed activation of metal ions bound to early glycated proteins are discussed.

Vitamin C as an antioxidant: evaluation of its role in disease prevention

Vitamin C in humans must be ingested for survival. Vitamin C is an electron donor, and this property accounts for all its known functions. As an electron donor, vitamin C is a potent water-soluble antioxidant in humans. Antioxidant effects of vitamin C have been demonstrated in many experiments in vitro. Human diseases such as atherosclerosis and cancer might occur in part from oxidant damage to tissues. Oxidation of lipids, proteins and DNA results in specific oxidation products that can be measured in the laboratory. While these biomarkers of oxidation have been measured in humans, such assays have not yet been validated or standardized, and the relationship of oxidant markers to human disease conditions is not clear. Epidemiological studies show that diets high in fruits and vegetables are associated with lower risk of cardiovascular disease, stroke and cancer, and with increased longevity. Whether these protective effects are directly attributable to vitamin C is not known. Intervention studies with vitamin C have shown no change in markers of oxidation or clinical benefit. Dose concentration studies of vitamin C in healthy people showed a sigmoidal relationship between oral dose and plasma and tissue vitamin C concentrations. Hence, optimal dosing is critical to intervention studies using vitamin C. Ideally, future studies of antioxidant actions of vitamin C should target selected patient groups. These groups should be known to have increased oxidative damage as assessed by a reliable biomarker or should have high morbidity and mortality due to diseases thought to be caused or exacerbated by oxidant damage.

The hydrogen peroxide/copper ion system, but not other metal-catalyzed oxidation systems, produces protein-bound dityrosine

Dityrosine formation leads to the cross-linking of proteins intra- or intermolecularly. The formation of dityrosine in lens proteins oxidized by metal-catalyzed oxidation (MCO) systems was estimated by chemical and immunochemical methods. Among the four MCO systems examined (H(2)O(2)/Cu, H(2)O(2)/Fe-ethylenediaminetetraacetic acid (Fe-EDTA), ascorbate/Cu, ascorbate/Fe-EDTA), the treatment with H(2)O(2)/Cu preferentially caused dityrosine formation in the lens proteins. The success of oxidative protein modification with all the MCO systems was confirmed by carbonyl formation estimated using 2,4-dinitrophenylhydrazine. The loss of tyrosine by the MCO systems was partly due to the formation of protein-bound 3,4-dihydroxyphenylalanine. The formation of dityrosine specific to H(2)O(2)/Cu was confirmed by using poly-(Glu, Ala, Tyr) and N-acetyl-tyrosine as a substrate. The dissolved oxygen concentration in the H(2)O(2)/Cu system hardly affected the amount of dityrosine formation, suggesting that dityrosine generation by the H(2)O(2)/Cu system is independent of oxygen concentration. Moreover, the combination of copper ion with H(2)O(2) is the most effective system for dityrosine formation among various metal ions examined. The addition of reducing agents, glutathione or ascorbic acid, into the H(2)O(2)/Cu system suppressed the generation of the dityrosine moiety, suggesting effective quench of tyrosyl radicals by the reducing agents.

Antioxidant properties of alpha-crystallin

alpha-Crystallin is a major chaperone lens protein to which has been ascribed antioxidant functions. In the present work we have evaluated the antioxidant and free radical scavenging properties of bovine alpha-crystallin in a series of in vitro models: zimosan-induced, luminol-enhanced chemiluminescence response of polymorphonuclear leukocytes, the autoxidation of brain homogenate, bleaching of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)-derived radical cations, trapping of peroxyl radicals, and reactivity toward hypochloric acid. In all these systems, the reactivity of alpha-crystallin is higher than or similar to that of bovine serum albumin. It is concluded that, given the high concentrations of ol-crystallin in the lenses, its capacity to interact with free radicals and to remove hypochlorous acid could contribute to the maintenance of the lens functionality.

Distribution of ferritin and redox-active transition metals in normal and cataractous human lenses

Previous studies have shown that lenticular levels of Fe and Cu are elevated in age-related cataract. However, it is not known if these metals are present in a state that is permissive for redox reactions that may lead to the formation of free radicals. In addition, there is little data available concerning the concentration and lenticular distribution of ferritin, the major intracellular Fe-sequestering protein, in the lens. The aim of the present work was therefore to determine the distribution of ferritin and the redox-availability of Fe and Cu in healthy and cataractous lenses. Lens ferritin distribution was assessed by ELISA and immunohistochemistry. A modified ELISA detected ferritin in an 'insoluble' lens protein fraction. Ferritin levels were not significantly different in the cortex vs nucleus of healthy lenses. In contrast, ferritin levels in the cataractous lens nuclei appeared to be 70% lower compared to the cortex. This was at least partially due to the presence of ferritin within an insoluble protein fraction of the homogenized lenses. In normal lenses, ferritin staining was most intense in the epithelium, with diffuse staining observed throughout the cortex and nucleus. The redox-availability of lenticular metals was determined using: (1) autometallography; (2) Ferene-S as a chromogenic Fe chelator; and (3) NO release from nitrosocysteine to probe for redox-active Cu. The autometallography studies showed that the cataractous lenses stained more heavily for redox-active metals in both the nucleus and cortex when compared to age-matched control lenses. Chelatable Fe was detected in homogenized control lenses after incubation with Ferene-S, with almost three-fold higher levels detected in the cataractous lenses on average. The Cu-catalysed liberation of NO from added nitrosocysteine was not demonstrated in any lens sample. When exogenous Cu (50 n M) was added to the lenses, it was rapidly chelated. The cataractous samples were approximately twice as effective at redox-inactivation of added Cu. These studies provide evidence that a chelatable pool of potentially redox-active Fe is present at increased concentrations in human cataractous lenses. In contrast, it seems that lenticular Cu may not be readily available for participation in redox reactions.

The effects of hyperbaric oxygen on the crystallins of cultured rabbit lenses: a possible catalytic role for copper

Oxidative effects on lens proteins have been linked with the formation of human age-related cataract, particularly nuclear cataract. This study investigated the effects of hyperbaric oxygen (HBO)-induced oxidative stress on nuclear and cortical alpha-, beta- and gamma-crystallins of cultured rabbit lenses, using high performance liquid chromatography (HPLC). The lenses were treated with 50 atm of either 100% N(2)(control) or 100% O(2)(experimental) for 3, 6, 16 and 48 hr. The levels of reduced glutathione (GSH) and water-soluble (WS) protein decreased more rapidly in the nucleus of the O(2)-treated lens than in the cortex. The first significant loss of WS protein in each of the two regions occurred when levels of GSH had decreased by at least 90% in either the nucleus (at 6 hr) or the cortex (at 16 hr). HPLC analysis of the nuclear WS proteins indicated that beta-crystallins were the first proteins affected by the oxidative stress. Soon after HBO-treatment was initiated (at 6 hr) and prior to insolubilization of protein, nuclear beta- and gamma-crystallins moved to the higher molecular weight alpha-crystallin fraction; 2-D gel electrophoresis and Western blotting indicated the presence of disulfide-crosslinked and non-crosslinked beta- and gamma-crystallins in this fraction. Significantly different HBO-induced effects were observed on lens cortical crystallins compared to those for the nucleus. For example, gamma-crystallins in the cortex shifted very soon after HBO-treatment (at 3 hr) to slightly higher molecular weights, possibly the result of protein/glutathione mixed disulfide formation; however, this phenomenon was not observed in the nucleus. Cortical beta- and gamma-crystallins remained in solution longer than nuclear proteins following HBO-treatment of the lenses, presumably the result of protection from the four-fold higher level of GSH (22 vs 6 m M) present in the lens periphery. Surprisingly, there was no movement of beta- and gamma-crystallins to alpha(H)- and alpha-crystallin fractions in the cortex of the O(2)-treated lens, in contrast to that observed for the nucleus. Cortical crystallins appeared to go directly from being soluble to being insoluble with no high molecular weight intermediate stage. The data suggested a possible chaperone-like function for alpha-crystallin in the nucleus of the stressed lenses, but not in the cortex. HBO-induced effects on lens nuclear supernatants, which mimicked those observed for intact lenses, could be nearly completely prevented by the copper-chelator bathocuproine, but not by the iron-chelator deferoxamine. Overall, the results provide additional evidence demonstrating an increased susceptibility of the lens nucleus to oxidative stress; the greater protective ability of the cortex may be linked to a higher capacity for beta- and gamma-crystallin/glutathione mixed disulfide formation, inhibiting disulfide-crosslinked insolubilization. The data also implicate copper as a catalyst for the autoxidation of -SH groups in the lens, and suggest that alpha-crystallin chaperone-like activity may play a greater role in the lens nucleus than in the cortex in preventing oxidative insolubilization of crystallins.