Cytoplasmic phase separation in formation of galactosemic cataract in lenses of young rats

Protein Aggregation and Disaggregation in Cells and Development

Protein aggregation is a feature of numerous neurodegenerative diseases. However, regulated, often reversible, formation of protein aggregates, also known as condensates, helps control a wide range of cellular activities including stress response, gene expression, memory, cell development and differentiation. This review presents examples of aggregates found in biological systems, how they are used, and cellular strategies that control aggregation and disaggregation. We include features of the aggregating proteins themselves, environmental factors, co-aggregates, post-translational modifications and well-known aggregation-directed activities that influence their formation, material state, stability and dissolution. We highlight the emerging roles of biomolecular condensates in early animal development, and disaggregation processing proteins that have recently been shown to play key roles in gametogenesis and embryogenesis.

Pulsed electric fields induce modulation of protein liquid-liquid phase separation

The time-resolved dynamic assembly and the structures of protein liquid dense clusters (LDCs) were analyzed under pulsed electric fields (EFs) applying complementary polarized and depolarized dynamic light scattering (DLS/DDLS), optical microscopy, and transmission electron microscopy (TEM). We discovered that pulsed EFs substantially affected overall morphologies and spatial distributions of protein LDCs and microcrystals, and affected the phase diagrams of LDC formation, including enabling protein solutions to overcome the diffusive flux energy barrier to phase separate. Data obtained from DLS/DDLS and TEM showed that LDCs appeared as precursors of protein crystal nuclei, followed by the formation of ordered structures within LDCs applying a pulsed EF. Experimental results of circular dichroism spectroscopy provided evidence that the protein secondary structure content is changing under EFs, which may consequently modulate protein-protein interactions, and the morphology, dimensions, and internal structure of LDCs. Data and results obtained unveil options to modulate the phase diagram of crystallization, and physical morphologies of protein LDCs and microcrystals by irradiating sample suspensions with pulsed EFs.

Dynamic light scattering study on phase separation of a protein-water mixture: application on cold cataract development in the ocular lens

We present a detailed dynamic light scattering study of the phase separation in the ocular lens emerging during cold cataract development. Cold cataract is a phase separation effect that proceeds via spinodal decomposition of the lens cytoplasm with cooling. The intensity autocorrelation functions of the lens protein content are analyzed with the aid of two methods, providing information on the populations and dynamics of the scattering elements associated with cold cataract. It is found that the temperature dependence of many measurable parameters changes appreciably at the characteristic temperature approximately 16+/-1 degrees C which is associated with the onset of cold cataract. By extending the temperature range of this work to previously inaccessible regimes, i.e., well below the phase separation or coexistence curve at Tcc, we have been able to accurately determine the temperature dependence of the collective and self-diffusion coefficients of proteins near the spinodal. The analysis showed that the dynamics of proteins bears some resemblance to the dynamics of structural glasses, where the apparent activation energy for particle diffusion increases below Tcc, indicating a highly cooperative motion. Application of ideas developed for studying the critical dynamics of binary protein-solvent mixtures, as well as the use of a modified Arrhenius equation, enabled us to estimate the spinodal temperature Tsp of the lens nucleus. The applicability of dynamic light scattering as a noninvasive, early-diagnostic tool for ocular diseases is also demonstrated in light of the findings of the present paper.

Antibodies to a microbial peptide sharing sequence homology with betaA3-crystallin damage lens epithelial cells in vitro and in vivo

Circulating auto-antibodies (Abs) against lens antigens (Ags) are highly prevalent in patients with cataract, but their origin and pathogenic significance are unknown. We hypothesized that Abs raised after exposure to infectious microbes could cross-react with lens Ags. To test this hypothesis, we generated a monoclonal Ab to human betaA3-crystallin. Epitope analysis indicated that the ETQAE sequence in the N-terminus region of betaA3-crystallin was critical for mounting a humoral response. Similar sequences were found in three microbial Ags. Mice injected with a microbial oligopeptide containing ETQAE emulsified with complete Freund's adjuvant (CFA) raised Abs which cross-reacted with betaA3-crystallin and developed lens epithelial cell (LEC) damage in vitro. We also genetically engineered an betaA3-crystallin-expressing E. coli. Mice immunized with the recombinant E. coli developed LEC damage. These results support the hypothesis that exposure to microbes having Ags homologous to self Ags can trigger a humoral immune response that leads to LEC damage in mice.

Molecular basis of a progressive juvenile-onset hereditary cataract

In a recent paper, patients with a progressive juvenile-onset hereditary cataract have been reported to have a point mutation in the human gammaD crystallin gene (Stephan, D. A., Gillanders, E., Vanderveen, D., Freas-Lutz, D., Wistow, G., Baxevanis, A. D., Robbins, C. M., VanAuken, A., Quesenberry, M. I., Bailey-Wilson, J., et al. (1999) Proc. Natl. Acad. Sci. USA 96, 1008-1012). This mutation results in the substitution of Arg-14 in the native protein by a Cys residue. It is not understood how this mutation leads to cataract. We have expressed recombinant wild-type human gammaD crystallin (HGD) and its Arg-14 to Cys mutant (R14C) in Escherichia coli and show that R14C forms disulfide-linked oligomers, which markedly raise the phase separation temperature of the protein solution. Eventually, R14C precipitates. In contrast, HGD slowly forms only disulfide-linked dimers and no oligomers. These data strongly suggest that the observed cataract is triggered by the thiol-mediated aggregation of R14C. The aggregation profiles of HGD and R14C are consistent with our homology modeling studies that reveal that R14C contains two exposed cysteine residues, whereas HGD has only one. Our CD, fluorescence, and differential scanning calorimetric studies show that HGD and R14C have nearly identical secondary and tertiary structures and stabilities. Thus, contrary to current views, unfolding or destabilization of the protein is not necessary for cataractogenesis.

Lens cytoplasmic phase separation

Cytoplasmic transparency is a unique feature of lens cells. The cytoplasm is a concentrated solution of crystallin proteins with minor constituents that include cytoskeletal proteins and lens specific intermediate filaments. Under normal physiological conditions, the proteins exist as a single transparent phase. With normal aging, progressive modification of the interactions between lens proteins occurs so that conditions within the lens become favorable for phase separation. These conditions produce intracellular inhomogeneities that approach or exceed the dimensions of the wavelength of visible light (400 to 700 nm) and light scattering from lens cells increases. The resulting opacification is the primary factor in the visual loss experienced in cataract, the leading cause of blindness in the world. We study biochemical factors that regulate the cytoplasmic phase separation and maintain normal cellular transparency.

Lens cytoskeleton and transparency: a model

The function of the cytoskeleton in lens was first considered when cytoplasmic microtubules were observed in elongating fibre cells of the chick lens nearly 40 years ago. Since that time, tubulin, actin, vimentin and intermediate filaments have been identified and found to function in mitosis, motility and cellular morphology during lens cell differentiation. A role for the cytoskeleton in accommodation has been proposed and modification of the cytoskeletal proteins has been observed in several cataract models. Recently, a progressive increase in protein aggregation and lens opacification was found to correspond with the loss of cytoskeletal protein in the selenite model for cataract. In the present report a model is proposed for the role of tubulin, actin, vimentin, spectrin and the lens-specific filaments, filensin and CP49, in the establishment and maintenance of transparent lens cell structure.

Theoretical and experimental basis for the inhibition of cataract

Aggregation of the lens proteins to form high molecular weight clusters is a major contributing factor in age-onset nuclear cataract [Benedek, G. B. (1971) Theory of transparency of the eye. Appl. Optics, 10, 459-473]. This aggregation occurs continually throughout life and contributes to an exponential increase, as a function of age, in the intensity of the light backscattered out of the lens. The time constant deltaT for this exponential increase in human populations is a valuable index, helpful for conducting clinical trials. In-vitro studies have identified reagents capable of inhibiting high molecular weight aggregate formation, as well as the non-covalent interprotein interactions responsible for phase separation. These reagents are also found experimentally to be effective cataract inhibitors in animal model systems in vivo. We believe that the stage is now set for human clinical trials of putative cataract inhibitors. We present rough quantitative estimates of the trial parameters needed to assure an unambiguous determination of efficacy in a trial population. Such a trial simply requires a measurement of the time constant deltaT in the treated population relative to the untreated population. A successful outcome of the trial is indicated if deltaT increases by 20% over that found for the untreated population. Our estimates suggest efficacy could be determined in a two year trial involving about 300 subjects in the treated group.

Towards a molecular understanding of phase separation in the lens: a comparison of the X-ray structures of two high Tc gamma-crystallins, gammaE and gammaF, with two low Tc gamma-crystallins, gammaB and gammaD

gamma-Crystallins, although closely related in sequence, show intriguing differences in their temperature-dependent interactions: those that have a high or intermediate Tc for phase separation are cryoproteins whereas low Tc gamma-crystallins are not. To address the molecular basis of phase separation, X-ray crystallography has been used to define the structural differences between high and low Tc gamma-crystallins. A pre-requisite for this study was to clarify the assignment of bovine gene sequences to bovine gamma-crystallin proteins used for biophysical measurements. Based on nucleotide sequence analyses of gamma E and gamma F bovine crystallin genes, gamma F corresponds to the previously crystallised high Tc protein bovine gamma IVa and gamma E corresponds to the high Tc bovine protein fraction previously known as gamma IIIa. The gamma F sequence has enabled the completion of the refinement of the bovine gamma F crystal structure which shows that the molecule has an additional surface tryptophan explaining why gamma F has different spectroscopic properties from gamma B. A high Tc protein from rat lens, gamma E crystallin, has been crystallised and the X-ray structure solved at 2.3 A resolution. Comparison of the X-ray structures of two high Tc proteins, rat gamma E and bovine gamma F, with the structures of two low Tc proteins, bovine gamma B and bovine gamma D, shows that the main conformational change between high and low Tc proteins is in the cd surface loop of motif 3. All four structures have numerous ion pairs on their surfaces leading to a high surface charge density, yet with low overall charge. Comparison of the lattice contacts of the two high Tc proteins with the two low Tc gamma-crystallins indicates that these high Tc proteins utilise more amino-aromatic interactions such as between histidine and arginine. Comparison of the sequences of all the gamma-crystallins which have been characterised for phase separation temperature indicates that only residue Arg/Lys 163 uniquely distinguishes cryo from non-cryo gamma-crystallins and it is close to the altered surface loop. Although this region probably contributes to phase separation, Tc is likely to be a function of an overall global property that is responsive to overall charge distribution. Calculated dipole moments of native gamma-crystallins, low Tc gamma-crystallin sequences threaded into high Tc gamma-crystallin structures, and vice versa, show how both sequence and 3D structure contribute to this overall property. High Tc gamma-crystallins have on average higher Arg/Lys ratios and higher histidine content. It is hypothesised that this increases the proportion of surface static paired charged networks which thus reduces the repulsive hydration force and so increases the attractive interactions of the protein-rich phase in binary liquid phase separation.

Pantethine inhibits the formation of high-Tc protein aggregates in gamma B crystallin solutions

PURPOSE

Solutions of the bovine lens protein gamma B (or gamma II) crystallin at neutral pH in the absence of reducing agents, undergo a slow, partial conversion to a new protein species, gamma IIH. This species is an aggregate composed of an intermolecular, disulfide-crosslinked dimer (approximately equal to 32% of total protein by weight) and loosely associated dimers (approximately equal to 66%). gamma IIH has a phase separation temperature (Tph), at least 40 degrees C higher than that of native gamma II crystallin at any given protein concentration. In this paper we demonstrate that pantethine, a derivative of coenzyme A, inhibits the formation of gamma IIH.

METHODS

gamma II crystallin solutions were incubated at pH 7.1 and room temperature with increasing amounts of pantethine. The Tph of the solutions was monitored as a function of incubation time. Corresponding to each Tph measurement, aliquots of each solution were analyzed by cation-exchange HPLC to determine the amount of gamma IIH formed.

RESULTS

Incubation of gamma II crystallin with increasing amounts of pantethine lowers Tph and suppresses the formation of gamma IIH. With pantethine to protein mole ratios of 0.66, 1 and 2, the Tph of gamma II crystallin is lowered from 8 degrees C in the native protein, to 2 degrees C, -3 degrees C respectively, at a protein concentration of approximately equal to 200 mg/ml. The amount of gamma IIH accumulated decreases from approximately 25% in the native protein to 10%, 1% and 0% respectively in these pantethine-treated protein solutions. For complete suppression of the rise in Tph and inhibition of gamma IIH formation, a 2:1 mole ratio of pantethine to protein is required.

CONCLUSIONS

We suggest that pantethine reacts with two cysteine residues of gamma IIH crystallin by forming a mixed disulfide, and effectively suppress protein aggregation and lowers Tph. This is due to the strong polar character of pantethine which reduces the net attractive interactions between the protein molecules.

Phase-separation inhibitors and prevention of selenite cataract

The variation of the phase-separation temperature (Tc) was studied in lenses during formation of cataracts induced by a subcutaneous injection of sodium selenite. In normal control animals, the Tc decreased monotonically with increasing age. Approximately 2 days after administration of the selenite the Tc decreased sharply to a minimum, and then at day 4 the Tc increased dramatically toward body temperature. Mature irreversible cataracts formed approximately 6 days after injection of the selenite. Intraperitoneal administration of WR-77913, a phase-separation inhibitor, prevented the abnormal variation of Tc in vivo. When injected into control animals without selenite, WR-77913 produced no abnormal variation in Tc. The results confirm that Tc is a sensitive measure of early changes in the lens and that opacification associated with abnormal variation in Tc can be prevented in vivo by using a phase-separation inhibitor.

Binary-liquid phase separation of lens protein solutions

We have determined the coexistence curves (plots of phase-separation temperature T versus protein concentration C) for aqueous solutions of purified calf lens proteins. The proteins studied, calf gamma IIIa-, gamma IIIb-, and gamma IVa-crystallin, have very similar amino acid sequences and three-dimensional structures. Both ascending and descending limbs of the coexistence curves were measured. We find that the coexistence curves for each of these proteins and for gamma II-crystallin can be fit, near the critical point, to the function /(Cc-C)/Cc/ = A [(Tc - T)/Tc]beta, where beta = 0.325, Cc is the critical protein concentration in mg/ml, Tc is the critical temperature for phase separation in K, and A is a parameter that characterizes the width of the coexistence curve. We find that A and Cc are approximately the same for all four coexistence curves (A = 2.6 +/- 0.1, Cc = 289 +/- 20 mg/ml), but that Tc is not the same. For gamma II- and gamma IIIb-crystallin, Tc approximately 5 degrees C, whereas for gamma IIIa- and gamma IVa-crystallin, Tc approximately 38 degrees C. By comparing the published protein sequences for calf, rat, and human gamma-crystallins, we postulate that a few key amino acid residues account for the division of gamma-crystallins into low-Tc and high-Tc groups.

Suppression of phase separation in solutions of bovine gamma IV-crystallin by polar modification of the sulfur-containing amino acids

The calf lens protein gamma IV-crystallin, a strong determinant of the net phase-separation temperature of the lens, was chemically modified with N-bromoacetylethanolamine phosphate. The phase-separation temperatures of solutions of the modified protein were measured and found to be dramatically reduced with respect to those of the native protein. At neutral pH the reagent alkylates only the cysteine and methionine residues and introduces a doubly charged phosphate anion at a maximum distance of 10-12 A from the sulfur atoms. At a protein concentration of 38 g/liter, and with 30% of the cysteines and 40% of the methionines alkylated, the phase-separation temperature is lowered from approximately 25 +/- 2 degrees C to approximately 12 +/- 2 degrees C. The ascending limbs of the coexistence curves for the native and modified proteins were determined at two different degrees of modification. The coexistence curve of the protein with 35% of the cysteines and 40% of the methionines modified shows that as protein concentration approaches the critical concentration of 289 g/liter, there is a much larger suppression of the critical temperature, from approximately 38 +/- 2 degrees C in the native protein to approximately 16 +/- 2 degrees C. Incubation of intact calf lenses in vitro with the reagent results in the suppression of the phase-separation temperature by 3-9 degrees C. These results are consistent with the view that the observed suppression in the critical temperature is due to an increase in the hydrophilicity of the protein in the vicinity of the sulfur-containing residues.

Reversal of the limited proteolysis of MP26 during the reversal and prevention of the galactose cataract in rat lenses

The reversal and prevention of the galactose-induced cataract in rats were employed to study their effects on the acceleration of the limited proteolysis of MP26 into MP23-24 previously observed in cataractous lenses of galactose-fed animals. Lenses of rats on a cataract reversal-diet demonstrated the reversal of MP23-24 and MP26 levels to control levels in the clearing cortical areas but not in remaining cataractous nuclear areas. Acceleration of the limited proteolysis of MP26 was observed in the nucleus but not the cortex in the clear lenses of animals on a cataract prevention-diet. The results demonstrated that the limited proteolysis of MP26 may form part of a gradual aging process that although not directly (causally) related to cataractogenesis may at least be accelerated by cataractogenic agents or conditions.

Binary liquid phase separation and critical phenomena in a protein/water solution

We have investigated the phase diagram of aqueous solutions of the bovine lens protein gamma II-crystallin. For temperatures T less than Tc = 278.5 K, we find that these solutions exhibit a reversible coexistence between two isotropic liquid phases differing in protein concentration. The dilute and concentrated branches of the coexistence curve were characterized, consistently, both by measurements of the two coexisting concentrations, c(T), and by measuring the cloud temperatures for various initial concentrations. We estimate that the critical concentration, cc, is 244 mg of protein per ml solution. The coexistence curve is well represented by the absolute value of (c - cc)/cc = 5.2 square root (Tc - T)/Tc. Using the temperature dependence of the scattered light intensity along isochores parallel to the critical isochore, we estimated the location of the spinodal line and found it to have the form (c - cc)/cc = 3.0 square root (Tc - T)/Tc. The ratio of the widths of the coexistence curve and the spinodal line, (5.2/3.0), is close to the mean-field value square root 3. We have also observed the growth of large crystals of gamma II-crystallin in some of these aqueous solutions and have made preliminary observations as to the factors that promote or delay the onset of crystallization. These findings suggest that selected protein/water systems can serve as excellent model systems for the study of phase transitions and critical phenomena.

Phase separation in lens cytoplasm is genetically linked to cataract formation in the Philly mouse

The variation of the phase-separation temperature, Tc, in lenses was studied during the postnatal development of three genetically different mouse strains: Swiss-Webster, Philly, and the (Swiss-Webster x Philly)F1 hybrid. The general behavior of Tc during early postnatal development has two stages: in stage I, Tc increased to a maximum and then, in stage II, Tc decreased. Philly mice are a strain that develops hereditary cataracts about 36 days following birth. In F1 hybrids of Philly and Swiss-Webster mice, cataracts appeared about 49 days following birth, approximately equal to 13 days later in development than in the Philly mice. In the Philly and hybrid mice, stage I and stage II were followed by stage III in which Tc reached a minimum value and then increased toward body temperature. The values of Tc at birth, the slope of the increase during stage I, and the maximum Tc were characteristic for each mouse strain. These results establish that the behavior of the temperature of the phase separation Tc in mouse lens is linked to the genetic strain of the mice and that the value of Tc at birth is an early indicator of lenses that will develop cataracts and lenses that will develop normally.

Controlled modulation of the phase separation and opacification temperature of purified bovine gamma IV-crystallin

In the bovine lens the gamma IV-crystallin fraction is a principal determinant of the phase separation and opacification temperature, Tc (Siezen et al, Proc. Natl. Acad. Sci. USA 82, 1985, 1701). We have now measured the effect on Tc of purified gamma IV-crystallin solutions produced by a variety of reagents which affect protein-protein, protein-water and water-water interactions. Ionic strengths less than physiological increase Tc dramatically, while higher ionic strength has very little effect. Calcium ion concentrations up to 8 mM produce no change in Tc. Glycerol and acrylamide both depress Tc linearly with reagent concentrations; Tc depression of gamma IV-crystallin by these compounds is quantitatively the same as for whole lens. Sulfhydryl reducing agents such as glutathione and dithiothreitol lower Tc, while hydrogen peroxide increases Tc. Changes in opacification temperature of gamma IV-crystallin produced by oxidizing and reducing agents are time-dependent and highly non-linear with reagent concentration. Our results clearly show that bovine gamma IV-crystallin is an important target protein for various reagents which are known perturbants of the opacification temperature of whole lens. The relevance of these findings to human diabetic and senile cataract formation is discussed.

A quantitative microprobe analysis of elements in cortical and nuclear cells of the calf lens

The outer cortical cells in the calf lens remain transparent under conditions that produce opacity in central nuclear cells. The nuclear cells opacify by a mechanism of cellular restructuring that is associated with a cytoplasmic phase separation while cortical cells do not opacify by this mechanism. In this study the differences in elemental composition of nuclear and cortical cells were analyzed using X-ray emission spectroscopy (XES) of tissue that was prepared for scanning electron microscopy. It was necessary to develop special methods of fixation and dehydration to prevent significant distortion of lens tissue and minimize solubilization and redistribution of elements during the histological processing of the tissue. We calibrated the microprobe for the quantitative analysis using gelatin standards which contained known concentrations of sulfur, potassium, phosphorus, chlorine, and cesium. The standard curves were used to determine proportionality constants, which related the intensity of X-ray emission to the molar concentration of each element, and to determine the minimum detectable levels of each element. An important finding is that the intensity of the X-ray emission is dependent on sample density only at low protein concentration. At the high protein concentrations that exist in lens, the intensity is not affected by sample density.(ABSTRACT TRUNCATED AT 250 WORDS)

The molecular basis of cataract formation

The theoretical and experimental basis for the transparency and opacification of the lens is reviewed. The microscopic mechanisms so far established which produce cataract are discussed. The use of quasielastic light scattering spectroscopy to measure in vivo the diffusivity of proteins and to sensitively detect the earliest stages of cataract formation is described.

Photon and fluorescence correlation spectroscopy and light scattering of eye-lens proteins at moderate concentrations

The bovine eye-lens protein, alpha L-crystallin, has been studied with photon correlation spectroscopy to obtain the mutual diffusion coefficient, Dm, with fluorescence correlation spectroscopy to determine the tracer diffusion coefficient, DT, and with light scattering to get the isothermal osmotic compressibility (delta pi/delta c) P,T. The concentration dependence of Dm, DT, and (delta pi/delta c) P,T up to a volume fraction phi of the protein of 2.5 x 10(-2) has been interpreted on the basis of four different interaction potentials: (a) an extended hard-sphere potential; (b) a shielded Coulomb potential; (c) a shielded Coulomb interaction where the effect of counterions is included; (d) a simple mixed potential. The three parameters Dm, DT, and (delta pi/delta c) P,T have also been combined in the generalized Stokes-Einstein equation, Dm = [(delta pi/delta c)P,T . (1--phi) . (DT)]/(kappa B . T). Our results indicate that, in the case that photon correlation spectroscopy gives the mutual diffusion coefficient Dm, the applicability of the Stokes-Einstein equation can be questioned; or that, when one assumes the Stokes-Einstein equation to be valid, there is significant discrepancy between the result of photon correlation spectroscopy and Dm.

Effect of change in concentration upon lens turbidity as predicted by the random fluctuation theory

Theoretical calculations were performed to predict how the light scattering intensity would change with changes in concentration in the gel state. The theory of light scattering was applied to a random distribution of hard spheres. The spherical particles with constant diameter were embedded in a medium having a different refractive index. The light-scattering intensities obtained as a function of concentration showed that in dilute solutions the scattered light intensity increases with concentration. However, in concentrated solution greater than 0.1 or 0.2 volume fraction, the light-scattering intensity decreases with increase in concentration.

Identification of the scattering elements responsible for lens opacification in cold cataracts

Using both quasi-elastic light scattering spectroscopy and angular dissymmetry in the intensity of the scattered light, we examined the onset of turbidity for intact calf lenses and for isolated nuclear cytoplasm. In the case of the nuclear cytoplasm these measurements demonstrate the presence of two kinds of scatterers: small units of approximately 100-A radius and larger elements whose size is distributed around 1,500 A. As the temperature is decreased towards the cold cataract temperature, the intensity of light scattered by the small units stays almost constant while the intensity scattered by the large elements increase very strongly. The opacification of the lens cytoplasm produced by decreasing the temperature results principally from an increase in the concentration of the large scattering elements. For the intact nucleus the situation is qualitatively similar, but the mean size of the large scattering elements shows a more substantial increase than in the isolated cytoplasm as temperature is lowered towards the cold cataract temperature.

The effect of naturally occurring cellular constituents on phase separation and opacification in calf lens nuclear homogenates

We have measured the change in the phase separation temperature, Tc, in calf nuclear homogenate produced by a variety of naturally occurring cellular constituents. For all of the compounds studied, increasing concentrations of test compound were found to lower Tc. Phosphorylated nucleotides had the greatest effect, lowering Tc by 165-305 degrees/mole of test compound. Reduced and oxidized glutathione reduced Tc by 69 and 100 degrees C/mole, respectively. Smaller effects were observed for amino and ascorbic acids and sugars. The pH of the homogenate was also found to affect Tc. In the normal lens, the concentration of each of these constituents is small. Therefore, for each individual component, the cataract associated decrease in concentration is insufficient to produce the large increase in Tc that has been observed during cataractogenesis in some model systems. However, the superposition of the effects of changes in many cellular constituents (including pH and hydration) could possible produce a change in Tc more consistent with the experimentally observed one. In support of this, we have found that combinations of test compound have an additive effect on the Tc of the nuclear homogenate.