The glycation and cross-linking of isolated lens crystallins by ascorbic acid

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.

Vitamin C and the Lens: New Insights into Delaying the Onset of Cataract

Cataracts or clouding of the lens is the leading cause of blindness in the world. Age and diabetes are major risk factors, and with an increasing aging and diabetic population, the burden of cataracts will grow. Cataract surgery is an effective way to restore vision; however, alternatives to cataract surgery are required to reduce the looming cataract epidemic. Since it is well established that oxidative damage plays a major role in the etiology of cataracts, antioxidants have been promoted as therapies to delay and/or prevent cataracts. However, many antioxidant interventions including vitamin C have produced mixed results as anti-cataract therapies. Progress has been made towards our understanding of lens physiology and the mechanisms involved in the delivery and uptake of antioxidants to the lens which may guide future studies aimed at addressing some of the inconsistencies seen in previous animal and human studies. Of interest is the potential for vitamin C based supplements in delaying the onset of cataracts post vitrectomy which occurs in up to 80% of patients within two years. These targeted approaches are required to reduce the burden of cataract on hospitals and improve the quality of life of our aging and diabetic population.

Vitamin C-mediated Maillard reaction in the lens probed in a transgenic-mouse model

Aging human lens crystallins are progressively modified by yellow glycation, oxidation, and cross-linked carbonyl compounds that have deleterious properties on protein structure and stability. In order to test the hypothesis that some of these compounds originate from oxidized vitamin C, we have overexpressed the human vitamin C transporter 2 (hSCVT2) in the mouse lens. We find that levels of ascorbic and dehydroascorbic acid are highly elevated compared to the wild type and that the lenses have accumulated yellow color and advanced Maillard reaction products identical with those of the human lens. Treatment of the mice with nucleophilic inhibitors can slow down the process, opening new avenues for the pharmacological prevention of senile cataractogenesis.

Vitamin C mediates chemical aging of lens crystallins by the Maillard reaction in a humanized mouse model

Senile cataracts are associated with progressive oxidation, fragmentation, cross-linking, insolubilization, and yellow pigmentation of lens crystallins. We hypothesized that the Maillard reaction, which leads browning and aroma development during the baking of foods, would occur between the lens proteins and the highly reactive oxidation products of vitamin C. To test this hypothesis, we engineered a mouse that selectively overexpresses the human vitamin C transporter SVCT2 in the lens. Consequently, lenticular levels of vitamin C and its oxidation products were 5- to 15-fold elevated, resulting in a highly compressed aging process and accelerated formation of several protein-bound advanced Maillard reaction products identical with those of aging human lens proteins. These data strongly implicate vitamin C in lens crystallin aging and may serve as a model for protein aging in other tissues particularly rich in vitamin C, such as the hippocampal neurons and the adrenal gland. The hSVCT2 mouse is expected to facilitate the search for drugs that inhibit damage by vitamin C oxidation products.

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.

Conformational study of N(epsilon)-(carboxymethyl)lysine adducts of recombinant gammaC-crystallin

N(epsilon)-(carboxymethyl)lysine, an advanced glycation end product, is present in the human lens. The effects of CML formation on protein conformation and stability were studied using the recombinant gammaC-crystallin as a model. Conformational change was studied by spectroscopic measurements such as fluorescence and circular dichroism. Conformational stability was determined by unfolding with heat. The results indicated that no conformational change was observed due to CML formation, but conformational stability decreased. These observations can be explained in terms of the relatively stable structure of gamma-crystallin, especially when compared with other crystallins. The lens nucleus is rich in gamma-crystallin and its stable conformation can assist gamma-crystallin sustained insults and remain soluble.

Studies on trypsin-modified bovine and human lens acylpeptide hydrolase

Acylpeptide hydrolase removes the N -acetylated amino acids from the peptide substrates but not from intact proteins. Cleavage between amino acid residues 203--204 of the native acylpeptide hydrolase results in the formation of a 55 kDa truncated active enzyme in the bovine lens, in vivo. In this study we explored the hydrolytic properties of the truncated enzyme using lens beta- and gamma-crystallins as substrates. SDS--PAGE analysis indicated that the beta B2-crystallin was cleaved by truncated acylpeptide hydrolase into several protein fragments (10--26 kDa). No cleavage of the gamma-crystallins was observed under similar conditions. Both the acylpeptide hydrolase activity and the protease activity of the 55 kDa enzyme were completely inhibited by diisopropylfluorophosphate, p -chloromercuribenzoate and ebelactone, and moderately inhibited by N -tosyl phenylalanine chloromethyl ketone. SDS--PAGE analysis followed by fluorography of ((3)H) diisopropylfluorophosphate labeled human lens acylpeptide hydrolase preparation showed the presence of the 55 kDa truncated form of the enzyme, as observed in the bovine lens. The peptide (d)-AIKGDQFL-NH(2)--the amino acid sequence 200--207 of the native bovine acylpeptide hydrolase with an in vivo cleavage site of native protein--was hydrolysed by the lens protease(s) suggesting that the in vivo generation of the 55 kDa acylpeptide hydrolase may be mediated through a proteolytic processing. The protease(s) responsible for the cleavage of this peptide was inhibited by diisopropylfluorophosphate and p -chloromercuribenzoate.

Maillard reactions in lens proteins: methylglyoxal-mediated modifications in the rat lens

The nonenzymatic Maillard reaction is thought to contribute to aging and cataract formation in the lens. As levels of methylglyoxal (MG) and glutathione (GSH) affect the reaction, we examined the relationship of these factors and determined the effect of a glyoxalase I inhibitor on the Maillard reaction. Rat lens cultures were maintained for up to 3 days in TC-199 medium with or without 20 m m glyceraldehyde (GLD) and 250 microm S-[N-hydroxy-N-(4-chlorophenyl) carbamoyl] glutathione diethyl ester (HCCG diester). We measured GSH, MG, D-lactate, glyoxalase I activity, immunoreactive MG-derived advanced glycation endproducts (MG-AGEs) and imidazolysine in organ cultured rat lenses. In vitro experiments with isolated rat lens proteins revealed that HCCG alone inhibited glyoxalase I activity in a dose-dependent manner. In organ cultured rat lens protein, GLD increased MG levels 24-fold, and the addition of HCCG diester further increased it by about two-fold. GSH levels fell sharply in the presence of GLD and this was prevented to some extent by the presence of HCCG diester. D-lactate production in the lens was suppressed by HCCG diester treatment. Dialysed lens proteins retained glyoxalase I activity, indicating that the enzyme was unaltered during incubation. MG-AGEs and imidazolysine levels were significantly higher (P<0.05) in GLD-treated lenses, but a combination of HCCG diester and GLD lowered immunoreactive MG-AGEs and imidazolysine levels compared to GLD alone. HCCG had no significant effect on MG-AGE formation in lens proteins incubated with GLD or MG. We conclude that exogenous GLD enhances MG and MG-AGE levels in the rat lens and that this increase is accompanied by a loss in GSH. In addition, inhibition of glyoxalase I promotes MG accumulation.

Chromatographic comparison of the UVA sensitizers present in brunescent cataracts and in calf lens proteins ascorbylated in vitro

The water-insoluble (WI) fraction from aged human lenses contains yellow chromophoric sensitizers, which generate reactive oxygen species (ROS) when irradiated with UVA light. The WI proteins from type I to V brunescent cataract lenses were assayed for UVA-dependent superoxide anion synthesis. Rates varied from 8.4-15 nMol h(-1)mg protein(-1), but there was no significant difference in specific activity between cataract types. When calf lens soluble proteins were incubated with ascorbic acid for 4 weeks and dialyzed, they were capable of generating 30-40 nMol h(-1)mg protein(-1)superoxide anion when irradiated with UVA light. Two preparations each of brunescent cataract WI proteins and bovine lens proteins ascorbylated in vitro were extensively digested with proteolytic enzymes and the released amino acids separated by normal phase HPLC. The elution profiles of the digests were very similar based upon the absorbance at 330 nm and fluorescence at 350 nm excitation/450 nm emission. Each peak was pooled and analyzed for the UVA-dependent generation of both superoxide anion and singlet oxygen. Every peak exhibited sensitizer activity, and the UVA-dependent ROS generation was roughly proportional to the absorbance at 330 nm. In addition, the ratio of superoxide anion to singlet oxygen generated was similar with both preparations. These data argue that it is the brown, fluorescent compounds which accumulate during aging and cataract formation that are responsible for the UVA-dependent ROS formation, and that these browning products may be similar to the advanced glycation endproducts produced by ascorbylation of lens proteins under oxidative conditions. This work also presents an initial report of a chromatographic method to separate the UVA-sensitizers present in each of these protein preparations without the use of acid or base hydrolysis.

Spontaneous generation of superoxide anion by human lens proteins and by calf lens proteins ascorbylated in vitro

The proteins isolated from aged human lenses and brunescent cataracts exhibit extensive disulfide bond formation. Diabetic rat lenses similarly contain disulfide-bonded protein aggregates. These observations are consistent with the known link between diabetes, glycation and oxidative damage, and suggest a role for reactive oxygen species (ROS) in this process. To assess whether the glycation-related modifications in human lens proteins spontaneously generate ROS, superoxide anion formation was measured using both cataractous lens proteins and calf lens proteins glycated in vitro with ascorbic acid (ascorbylated). The water-insoluble fraction from aged normal human lenses generated 0.3-0.6 nmol superoxide h(-1)mg protein(-1), whereas the activity increased to 0.5-1.8 nmol h(-1)mg protein(-1)with the WI fraction from brunescent cataracts, and 2.3 nmol h(-1)mg protein(-1)with calf lens proteins ascorbylated for 4 weeks in vitro. The activity in the human lens proteins was observed in both the water-soluble and water-insoluble fractions, and was completely dependent upon the presence of oxygen. The pH optimum curve for superoxide formation increased from pH 6.5 to 10 with both the cataract and ascorbylated proteins. The superoxide-generating activity in human lens was completely bound to a boronate affinity column, but only partially bound with the ascorbylated proteins. The superoxide anion produced by a 5 m m solution of purified N(epsilon)-fructosyl-lysine was barely detectable, and therefore, could not account for the superoxide formed by any of the lens protein preparations. Also, superoxide formation increased 10-fold at pH 8.8 with fructosyl-lysine, but only 1.3-1.8-fold with human lens proteins. The addition of copper-stimulated superoxide formation with glycated bovine serum albumin, but no stimulation was seen with cataractous proteins. Assays of specific compounds showed that catechol, hydroquinone, 3-OH kynurenine and 3-OH anthranylic acid exhibited the greatest activity for superoxide generation, but had a very short halflife. 2,3-Dihydroxypyridine and 4,5 dihydroxynaphthalene were one and two orders of magnitude less reactive. In long-term incubations at 37 degrees, cataractous proteins retained the potential to produce superoxide anion, losing only half of the initial activity after 6-7 days. Therefore, the water-insoluble fraction from aged human lenses and dark brown cataracts are potentially capable of generating >100 nmol mg protein(-1)and >170 nmol mg protein(-1)of superoxide anion respectively, likely due to the presence of advanced glycation endproducts in human lens proteins. This spontaneous generation of superoxide anion in vivo could account for a major portion of the oxidation of sulfur amino acids seen during aging and cataract formation.

Conformational study of N(epsilon)-(carboxymethyl)lysine adducts of recombinant alpha-crystallins

PURPOSE

Lens proteins underwent nonenzymatic glycation, and the advanced glycation end products (AGEs) were detected by immunological assays. One of the major AGE structures is N(epsilon)-(carboxymethyl)lysine (CML). Since the involvement of AGEs in the pathogenesis of diabetic complications is speculated, the effects of CML formation on proteins were studied.

METHODS

CML adducts were generated in recombinant alphaA- and alphaB-crystallins by incubation with glyoxylic acid and NaBH3CN. SDS-PAGE and size exclusion chromatography were used to detect subunit degradation and high-molecular-weight (HMW) aggregation. Conformational change was determined by fluorescence and circular dichroism (CD) measurements. The chaperone function was studied by DTT-induced aggregation of insulin.

RESULTS

Lysine modification was estimated to be 60-90% depending on the conditions of incubation. No subunit degradation or HMW aggregation was observed. Fluorescence and CD measurements detected a conformational change in CML adducts. Measurements of chaperone-like activity, however, indicated that the formation of CML increased the protein's ability to protect insulin against DTT-induced aggregation.

CONCLUSIONS

Although CML adducts of alphaA- and alphaB-crystallins, the major AGE structures formed in vitro, changed protein conformation, no subunit degradation and HMW aggregation were observed. Moreover, the CML adducts increased chaperone-like activity of both alphaA- and alphaB-crystallins. The results suggest that CML formation alone may not play a major role in protein aggregation and lens opacity.

Characterization of a blue fluorophore isolated from in vitro reaction of N-proportional to-acetyllysine and 3-deoxyglucosone

With the objective to investigate 3-deoxyglucosone (3-DG) mediated lysine crosslinks in vivo, we have isolated a lysine-3-DG-lysine crosslink from in vitro reaction of 3-DG and N-proportional to-acetyllysine (NAL). This crosslink, named as furopyrrolopyridine crosslink (FPPC), has intense blue fluorescence with absorption maxima at 235, 270 and 370 nm and emission maximum at 470 nm. The absorption and fluorescence spectra of FPPC were not altered in pHs ranging from 2-12, but the characteristic spectrum of FPPC (at pH 7.0) disappeared when it was reduced with sodium borohydride. FAB-MS showed that FPPC has a molecular mass of 611, equivalent to the reaction of two molecules each of NAL and 3-DG with the concomitant loss of 5 molecules of water. NMR data showed that FPPC has a pyridinium ring and four free hydroxy groups. Since acid hydrolyzed FPPC can be detected by amino acid analysis, we have determined its levels in the acid hydrolyzates of proteins glycated by 3-DG or in the acid hydrolyzates of normal aged, cataractous, diabetic and brunescent human lens proteins as well as in the acid hydrolyzed glycated hemoglobin, A0.

Identification of 1,1'-bi(4-anilino)naphthalene-5,5'-disulfonic acid binding sequences in alpha-crystallin

The hydrophobic binding sites in alpha-crystallin were evaluated using fluorescent probes 1,1'-bi(4-anilino)naphthalenesulfonic acid (bis-ANS), 8-anilino-1-naphthalene sulfonate (ANS), and 1-azidonaphthalene 5-sulfonate (1,5-AZNS). The photolysis of bis-ANS-alpha-crystallin complex resulted in incorporation of the probe to both alphaA- and alphaB-subunits. Prior binding of denatured alcohol dehydrogenase to alpha-crystallin significantly decreased the photoincorporation of bis-ANS to alpha-crystallin. Localization of bis-ANS incorporated into alphaA-crystallin resulted in the identification of residues QSLFR and HFSPEDLTVK as the fluorophore binding regions. In alphaB-crystallin, sequences DRFSVNLNVK and VLGDVIEVHGK were found to be the bis-ANS binding regions. Of the bis-ANS binding sequences, HFSPEDLTVK of alphaA-crystallin and DRFSVNLNVK and VLGDVIEVHGK of alphaB-crystallin were earlier identified as part of the sequences involved in their interaction with target proteins during the molecular chaperone-like action. The hydrophobic probe, 1,5-AZNS, also interacted with both subunits of alpha-crystallin. Localization of 1,5-AZNS binding site in alphaB-crystallin lead to the identification of HFSPEEK sequence as the interacting site in this subunit of alpha-crystallin. Glycated alpha-crystallin displayed decreased ANS fluorescence and loss of chaperone-like function, suggesting the involvement of glycation site as well as ANS binding site in chaperone-like activity display.

In vivo acetylation identified at lysine 70 of human lens alphaA-crystallin

Posttranslational modification of protein lysyl residues that change the net charge of the molecule may alter the protein conformation. Such modifications are of particular significance among lens proteins, because conformational changes are associated with the development of cataract. A previously unidentified acetylated form of alphaA-crystallin has been isolated from the water-soluble portion of human lenses. The alphaA-crystallins were fractionated by anion exchange HPLC into seven peaks, each containing more than one form of alphaA-crystallin. The previously reported deamidated and phosphorylated forms were identified by their molecular masses, determined by electrospray ionization mass spectrometry. In addition to these modifications, approximately 5% of alphaA-crystallin had a modification that decreased the charge by one and increased the molecular mass by 42 u. This modification, identified as acetylation, was located uniquely at Lys 70. Like any modification that alters the surface charge, acetylation may affect protein conformation and intermolecular interactions, thereby altering the solubility or chaperone properties of alphaA-crystallin. Acetylation of lysine 70 is potentially significant since it is located in a region that has been implicated in the chaperone activity of alphaA-crystallin.

Post-translational non-enzymatic modification of proteins. II. Separation of selected protein species after glycation and other carbonyl-mediated modifications

There are two strategies applicable to revealing non-enzymatic post-translational modifications of proteins; while assaying of the hydrolytically stable adducts was the subject of our previous communication [1], here we attempted to review separation technologies for the unfragmented modified proteins. There are a few standard procedures used for this purpose, namely Laemmli gel electrophoresis, different modes of gel permeation chromatography and boronate affinity chromatography. The latter approach makes use of the vicinal hydroxy groups present in glycated proteins. Some (but not all) arising adducts exhibit typical fluorescence which can be exploited for detection. In most cases fluorescence is measured at 370/440 nm for the so-called advanced glycation products or at 335/385 nm for the only so far well characterized glycation marker (pentosidine). Some indication exists that, e.g., synchronous fluorescence detection will probably in the future add to the selectivity and allow the distinction of the different adducts arising during non-enzymatic post-translational modifications (glycation). The proteins reviewed are serum albumin, collagen and lens proteins while glycation of hemoglobin is the subject of another review within the present volume.

Glycation mediated crosslinking between alpha-crystallin and MP26 in intact lens membranes

With advancing age, progressive crosslinking occurs between lens crystallin proteins and other lenticular components. This crosslinking may be involved in the development of senile cataracts. Experiments were conducted to determine whether non-enzymatic glycation could be involved in the crosslinking between lens alpha-crystallin and MP26, an abundant lens fiber cell membrane intrinsic protein. In vitro crosslinking of alpha-crystallin and MP26 of bovine lens membranes was observed in presence of two degradation products of ascorbic acid (ASA), dehydroascorbic acid (DHA) and threose. Alkali-washed bovine lens membranes, isolated after glycation with DHA and threose, contained both alpha-crystallin and MP26, as determined by immunoblot and double immunocytochemical labeling studies. In contrast, membranes incubated without these glycating compounds contained only MP26. SDS-PAGE analysis of [125I] alpha-crystallin incubated with lens membranes in the presence of threose showed a higher amount of radioactivity in high molecular weight aggregates than in the aggregates produced when alpha-crystallin and threose were incubated without membranes. A slot-blot immunoassay of alkali-washed human lens membranes showed a higher amount of covalently bound alpha-crystallin in aged, cataractous or diabetic lens membranes than was present in lens membranes from young normal donors. Based on the in vitro results, we hypothesize that non-enzymatic glycation is one of the vivo mechanisms in the crosslinking of alpha-crystallin to lens membrane proteins, such as MP26. This crosslinking may contribute significantly to the development of age-related and diabetic cataracts.

The culture of rat lenses in high sugar media: effect on mixed disulfide levels

Lens proteins are long lived proteins with those in the center of the lens predating the birth of the individual. As a result, they are subject to a host of modifications and damage through a variety of mechanisms. Two such modifications have been proposed as primary events which could cause conformational changes potentiating further modifications. These are non-enzymatic glycation and mixed disulfide formation. Human lenses accumulate protein-thiol mixed disulfides of three kinds throughout the lifespan. The presence of one of these, protein-glutathione (PSSG) mixed disulfide has been shown to be intimately involved in protein aggregation. We have utilized ex vivo lens culture and in vitro incubations of purified gamma-crystallin to evaluate the following hypotheses. A) Lenses cultured with a high sugar media will form higher mixed disulfide levels than controls; B) glycation of lens proteins will be dependent on initial mixed disulfide level. Xylose levels in the cultured lens rise rapidly (to 23 mM by 4 h), and the level of glycation after one week is elevated 6-7% over control values. Mixed disulfide levels are also substantially increased but not more than for lenses cultured in control media. gamma-Crystallin modified with 0, 1, or 5 equivalents of GSH was differentially glycated by radioactive fructose. The amount of fructose bound by the protein was found to be inversely related to the extent of mixed disulfide formation. These results indicate that 1) protein modification of one kind may influence further modifications of other types; 2) glycation of lens proteins has no effect on mixed disulfide formation in this system; 3) the sulfhydryl status of lens proteins can affect the potential for protein glycation.

Ascorbic acid mediated alteration of alpha-crystallin secondary structure

Glycation, the non-enzymatic addition of sugar or other carbonyl compounds to the amino groups of a protein, has been shown to occur with a variety of sugars and a diverse group of proteins. This type of alteration is believed to be an important component of aging for lens proteins and perhaps in cataractogenesis. Glycation has been shown to alter function and spectroscopic techniques have shown that in many cases conformational changes have occurred. Circular dichroism spectroscopy has documented modifications to alpha-crystallin tertiary structure induced by glucose and glucose 6-phosphate but generally no change to secondary structure. Ascorbate and is oxidized derivative dehydroascorbate have been shown to be powerful glycating agents as well as forming cross-links between peptide chains. In this study, alpha-crystallin incubated with ascorbic acid for one or two wk shows significant incorporation of ascorbate, non-reducible cross-links between the protein chains and altered CD spectra in the far UV region indicative of secondary structure modification.

Structure and modifications of the junior chaperone alpha-crystallin. From lens transparency to molecular pathology

alpha-Crystallin is a high-molecular-mass protein that for many decades was thought to be one of the rare real organ-specific proteins. This protein exists as an aggregate of about 800 kDa, but its composition is simple. Only two closely related subunits termed alpha A- and alpha B-crystallin, with molecular masses of approximately 20 kDa, form the building blocks of the aggregate. The idea of organ-specificity had to be abandoned when it was discovered that alpha-crystallin occurs in a great variety of nonlenticular tissues, notably heart, kidney, striated muscle and several tumors. Moreover alpha B-crystallin is a major component of ubiquinated inclusion bodies in human degenerative diseases. An earlier excitement arose when it was found that alpha B-crystallin, due to its very similar structural and functional properties, belongs to the heat-shock protein family. Eventually the chaperone nature of alpha-crystallin could be demonstrated unequivocally. All these unexpected findings make alpha-crystallin a subject of great interest far beyond the lens research field. A survey of structural data about alpha-crystallin is presented here. Since alpha-crystallin has resisted crystallization, only theoretical models of its three-dimensional structure are available. Due to its long life in the eye lens, alpha-crystallin is one of the best studied proteins with respect to post-translational modifications, including age-induced alterations. Because of its similarities with the small heat-shock proteins, the findings about alpha-crystallin are illuminative for the latter proteins as well. This review deals with: structural aspects, post-translational modifications (including deamidation, racemization, phosphorylation, acetylation, glycation, age-dependent truncation), the occurrence outside of the eye lens, the heat-shock relation and the chaperone activity of alpha-crystallin.